[This blog post has been provided by Bec Wolfe-Thomas, an administrator for the Garden Professors blog group on Facebook.]
Woodpeckers (Picidae) frequently get a bad rap from gardeners. It’s often their impression that the birds irreparably damage trees, but this is untrue. Most woodpeckers are insect eaters; they can hear insects under the bark and in the wood of trees. They then target their drilling with uncanny precision to get their meal. This removal of insect pests, such as emerald ash borer, benefits the tree.
And what about the feeding holes left in the tree? This is an exciting bit of tree physiology! Trees are able to compartmentalize or isolate the wounds. After the woodpecker has made a hole to retrieve the insects within, the tree starts compartmentalizing the wound. How long it takes for a tree to compartmentalize a wound and close it depends on species and climate factors.
Below are photos of woodpecker holes in various states of compartmentalization, from freshly drilled to completely compartmentalized and closed holes. Woodpeckers help keep trees healthy by preventing large pest infestations. And while the small feeding holes might be an aesthetic concern to gardeners, they’re only temporary. They will eventually be compartmentalized and closed, and the tree will be healthier in the long run by having fewer pests.
[Please note the larger holes excavated for nesting will compartmentalize but will not close over time.]
California had the worst fires in the last two years of its existence as a state. Hundreds of thousands of acres of brush and forest burned. More importantly thousands lost their homes as fires moved across urban/rural interfaces to destroy communities. The entire town of Paradise, California was burned to the ground. Here in Ventura County, the Thomas Fire was the state’s largest fire by the time it was done, and hundreds lost homes. No other time in history have we been so focused on what will burn, why it will burn, and what we can do to have a “firewise” landscape.
Fire authorities around the world have advocated creating defensible spaces around homes that are clear of ignitable vegetation. Some authorities have mandated by law that mulch, pine needles and other debris be removed as a fire prevention measure near structures. There is a general recognition that any plant can burn. Even well irrigated plants will rapidly desiccate and become flammable in the face of strong wind and a heavy fuel load that is inflamed nearby.
Flammability of landscape around homes is dependent on several factors. Vegetation placement can obstruct or allow for fire fighters ability to reduce damage to a home. While it is natural to assume that avoiding flammable plants is a part of this process, there is no standard method for testing plant flammability. Many lists of firewise plants have unknown origin or are just guesses. Flammability can be assigned four dimensions: ignitability, sustainability, combustibility and consumability. These factors refer to time till ignition; time a material will burn; rapidity or intensity of burn and quantity of material that will burn. The components of combustion are influenced by moisture content, percentage of carbon, percentage of volatile compounds; surface area to volume ratio and other factors. The varied factors are usually not all studied at the same time and are not all equally important to plant flammability. Thus assessing flammability even within the context of a controlled study will only partially assess a material’s likelihood of burning under various conditions. Hence most of the lists are not that helpful.
Behm et. al. showed that variations in flammability between plant species exists, and also that species within the same genus can vary widely in their flammable nature. Thus lists should not assume species in the same genus all have the same flammability. There is some thinking that flammability is an evolutionary trait that some plants exploit to their benefit, i.e. they are made to burn, such as the California Chaparral plant communities. Simply burning fuels in a laboratory setting does not take into account many of the factors associated with fuel burning intensities. Species differences notwithstanding, the amount of dead plant matter (dead twigs and leaves) vs. live matter, the arrangement of leaves, mulch and adjacent species all play a role in the flammability of the landscape itself which cannot be studied in a lab setting. Landscapes are “fuel bed complexes” with multiple elements that are not replicated in studies. For instance, small leaves from some shrubs ignite easily, but when burned as litter, develop low heat release rates because of poor ventilation.
Testing live plant materials alone is misleading because the flammability of an intact shrub is caused by the interaction of live matter with “necromatter”. Dead tissues are thermal catalysts which ignite live material. The ratio of necromatter to live matter influences flammability and is generally not well studied. Fire modelling also has a role in understanding what will burn. Both wind and slope increase the spread rate and the fireline intensity of burnable plants. Fire behavior characteristics on a given plant also are affected by both its physical and chemical characteristic — tissue mineral and water content have impacts on flammability. This bodes poorly for firesafe plant lists because lists do not consider plant physical or chemical attributes and if moisture levels are low it will burn regardless of its structure and geometry or its status on a list. Sometimes though a dense wall of well hydrated vegetation can save homes such as the avocado orchards that held back fire in the Thomas Fire in Montecito, Ca.
While lists don’t satisfy scientific rigor they are great for policy makers and homeowners who want to know what to plant. Unfortunately many lists are just compilations of other lists, none of which were based on research. Sometimes lists confuse one desirable characteristic with another, such as native plant lists that tout drought tolerance. Many drought tolerant plants are not fire resistant especially after a long dry period, indeed they often evolved to burn under such conditions.
For those that live in fire prone areas, fire resistant plant lists will always be an attractive or even required element of landscapes. Lists will not save a structure in the face of high winds and adequate fuel or embers. A defined defensible space around buildings, and maintenance of plantings that removes dead matter, maintains irrigation, and maintains proper distance from combustible surfaces will be more effective than choosing landscape plants from flammability lists.
Fernandes, P.M. and M. G. Cruz. 2012. Plant flammability experiments offer limited insight into vegetation—fire dynamic interactions. New Phytologist 194: 606-609
Behm, A.L., M. L. Duryea, A.J. Long, and W.C. Zipperer. 2004. Flammability of native understory species in pine flatwood and hardwood hammock ecosystems and implications for the wildland –urban interface. International J. of Wildland fire 13: 355-365.
White, R.H. and W.C. Zipperer. 2010. Testing and classification of individual plants for fire behavior: plant selection for the wildland-urban interface. International J. of Wildland Fire 19:213-227.
Dig up dirt. Treat like dirt. Dirt poor. Replace the word “dirt” with “soil” and you get phrases that make no sense. This is a roundabout way of explaining that “dirt” and “soil” are not the same things, either in idioms or in the garden. Yet many of us effectively turn our soils into dirt through poor garden practices.
For the purposes of this post, we’re going to use a single criterion to distinguish between soil and dirt: one is a living ecosystem and the other is not. A thriving soil ecosystem contains sufficient water, oxygen, and nutrients to support bacterial, fungal, plant, and animal life. Regardless of soil type, about half of the volume in a living soil should be pore space and the other half soil particles. Half of the pore space should be filled with water and the other half with air. When we make choices about activities that affect garden and landscape soils, we need to be proactive in preserving both the particle-pore balance as well as connectivity between the soil and the atmosphere.
The only way pore space can be reduced is through soil compaction. So don’t do it.
No driving. If equipment must be brought in, put down a thick layer of wood chips to protect the soil, or at least plywood.
No naked soil. Bare soils are compacted soils. Mulch!
No rototilling. It grinds your living soil into dirt. Disrupt the soil as little as possible when you plant.
No stomping, pressing, or otherwise compacting the soil during planting. Let water and gravity do that work for you.
The only way soil and atmosphere connectivity can be disrupted is by covering the soil with low permeability materials. So don’t do it.
No soil layering. Don’t create abrupt layers of soils with different textures. It interferes with water and gas exchange.
No sheet mulches. I’m sure you’re tired of hearing me say that and I am tired of saying it. Sheet mulches have less permeability than chunky mulches. That means oxygen and water have more difficulty getting through. Period.
Do use lots of groundcovers, chunky mulches, and hardscape in areas where there’s considerable foot traffic. They all protect the soil and are important parts of a well-designed, sustainable landscape.
If you just can’t get enough about soil science for gardens and landscapes, do check out this new publication by Dr. Jim Downer and myself.
Gardeners that read this blog understand that minerals are absorbed mostly by plant roots as ions, and are essential for plant growth and development. Some minerals are required in parts per hundred, and are macro-nutrients while others are only required in parts per million or parts per billion, and are considered micronutrients. As long as enough of the 16 most essential minerals are available, plants grow and reproduce in a healthful way. When not enough of one of the essential elements are available, a deficiency occurs, and plants
may present deficiency symptoms. Mineral nutrient deficiency symptoms are considered abiotic disorders. There are, however, cases where excess or deficiency of elements can be predisposing to disease caused by pathogens. Most some mineral elements do have a role in the development of disease caused by some pathogens but this is largely demonstrated in agriculture and often most home gardens do not suffer nutrient caused plant diseases.
Diseases can be either biotic with a living pathogen driving the disease or abiotic where a physiological condition is caused by the environment and host interactions. Mineral nutrients also are often implicated in abiotic disease.
Perhaps the most famous one is blossom end rot of tomato. This disorder is seen by gardeners around the country and is widely attributed to calcium deficiency. Expanding fruit are a tremendous “sink” for nutrients like calcium and it was thought that if not enough calcium was available in soil the disorder would occur. It is accepted that localized Ca deficiency (in fruit) may play a role in the initiation of blossom end rot, but there are many other factors that lead to the full blown condition, some of which are not fully understood. The fact that blossom end rot (BER) occurs in calcic soils in California underpins the complexity of this disorder. In many cases, simply adding calcium to soils does not correct the problem. Research in California suggest that the plant hormone abscisic acid (ABA) regulates water flow, the development of water conducting tissues, and calcium uptake in tomato. Researchers found that ABA treated tomatoes were cured of blossom end rot. For gardeners, making sure plants are fertilized, and avoiding varieties susceptible to BER is the best course of action.
Soil-borne pathogens are perhaps most affected by minerals dissolved in soil solution. Minerals can act in specific ways (specific ion effects) or as total ion effects (osmotic strength or concentration) having direct impact on pathogenic propagules or on the host itself. In a biological disease relationship there are several possibilities:
• Specific ions harm or favor the pathogen.
• Specific ions harm or support the host.
• Ionic strength changes the root environment making the host weak and susceptible.
• Ions change the pH of the soil solution making it more or less fit for a pathogen or the host.
• Ions change the soil physical environment making it more or less fit for a pathogen or the host.
While it is often espoused that the well “fed” or fertilized plant is resistant to disease, it is rarely borne out in published research on ornamental plants. Keeping a good nutritional level in nursery stock will not necessarily protect plants from many of the virulent pathogens that are capable of causing disease. Excess fertilization may lead to luxury consumption by the fertilized plant and can produce succulent growth that will exacerbate of such diseases as powdery mildew. It is well known that seedling diseases caused (damping off) are more severe with increased medium salinity and it was later discovered that increased soil salinity also increases susceptibility of ornamental plants to Phytophthora root rot diseases. Phytophthora is the most common pathogen associated with rotted roots in most gardens.
Plant mineral nutrition supports plant health in two basic ways (1) formation of mechanical barriers (cell wall strengthening) and (2) synthesis of defense compounds that protect against pathogens. The role of specific elements and their compounds is complicated and unique to each disease/host system. Certainly deficiencies of molecules such as calcium and potassium can interrupt either defense mechanism and if nutrients are supplied enough to prevent deficiencies there is little role of nutrients in preventing disease.
Root rot is a disease of thousands of ornamental plants and a serious problem in many gardens. Root rots caused by Phytophthora spp. occur in a range of nutritional and pH ranges. Nitrogen has been shown to lessen root rots and this is likely due to conversion of nitrogen into ammonia gas in soil which acts as a fumigant. Many studies found no relationship of nitrogen source to root rot disease development. Calcium increases disease resistance to root rot in avocado and other plants. While it is understood that calcium has direct effects on plant membranes, root cell membrane leakage, cell wall thickness, and many other host factors, there are also direct effects on the pathogen in soil. Calcium ions reduce the production of disease spores and disrupt their ability to swim and find susceptible roots. When soils and soil less media are low in soluble calcium, when calcium is easily precipitated out of solution, or when the pH is high and limestone minerals decrease the availability of calcium, root rots will be able to infect. Increases of sodium ions in soils and soil less media can also increase Phytophthora caused diseases.
Some non-essential elements have become popular as disease suppressants. Research has shown Silicon increases resistance of plants to powdery mildew, root rots and to stress in general. Silicon is implicated in strengthening cell walls as well as in defense protein production in plants. But not all plants are capable of utilizing silicon, so its role in plant defense is limited to those species (mostly grasses) capable of metabolizing it. Silicon has been erroneously recommended for widespread disease prevention. Its actual utility is likely very narrow. Much more study is necessary to understand silicon’s role with ornamental plant-pathogen systems. Gardeners will find little use for silicon as a disease prevention too.
Plants extract minerals from container media and garden soils—the process is complicated; it is mediated by the substrate/soil, water chemistry, temperature and the applied minerals (fertilizers) as well as by plants. Gardeners should apply fertilizers that can supply a constant low level nutrient charge or rely on nutrients provided in mulches. Fertilizing decisions are best guided by having a low cost soil analysis by a University lab. Supplying extra soluble calcium may be helpful in managing root rots, especially where heavy rainfall is normal and soils may be highly leached. Preventing salt build up (by leaching irrigation) in high salinity soils (low rainfall places) and that can occur when media dries out, will also help plants avoid infection by root rot organisms. It is good to remember that fertilizers never cure diseases, but there may be a role in preventing disease when plants are nutrient deficient.
Baker K.F. 1957. The UC System Producing Healthy Container-Grown Plants. University of California Division of Agricultural Sciences Agricultural Experiment Station Publication #23.
Cherif M., Asselin A., Belanger R.R. 1994. Defense responses induced by soluble silicon in cucumber roots infected by Pythium spp. Phytopathology 84:236-242.
Datnoff, L.E., Elmer, W.H. and D. M. Huber eds. 2007. Mineral nutrition and plant disease. APS Press The American Phytopathological Society, St. Paul, MN. 278pp.
Downer A.J., Hodel D.R., Matthews D.M., Pittenger D.R. 2013. Effect of fertilizer nitrogen source on susceptibility of five species of field grown palms to Fusarium oxysporum f. sp. canariensis. Palms 57: 89-92.
Duvenhage J.A., Kotze J.M. 1991. The influence of calcium on saprophytic growth and pathogenicity of Phytopthora cinnamomi and on resistance of avocado to root rot. South African Avocado Growers Yearbook 14:13-14.
Faufeux F., Remus-Borei W., Menzies J.G., Belanger R.R. 2006. Silicon and plant disease resistance against pathogenic fungi. FEMS Microbiology Letters 249:1-6.
Kauss H., Seehaus K., Franke R., Gilbert S., Dietrich R.A., Kroger N.. 2003. Silica deposition by a strongly cationic proline-rich protein from systemically resistant cucumber plants. Plant J. 33:87-95.
Lee B.S., Zentmeyer GA. 1982. Influence of calcium nitrate and ammonium sulfate on Phytophthora root rot of Persea indica. Phytopathology 72:1558-1564.
Ma, J.F. 2011. Role of silicon in enhancing the resistance of plants to biotic and abiotic stresses. Soil Science and Plant Nutrition 50:11-18.
Macdonald J.D., Swiecki T.J., Blaker N.S., Shapiro J.D. 1984. Effects of salinity stress on the development of Phytophthora root rots. Cal Ag 38:23-24.
Messenger B.J., Menge J.A., Pond E. 2000. Effects of gypsum on zoospores and sporangia of Phytopthora cinnamomi. Plant Dis 84:617-621.
Powell C.W., Lindquist R.K. 1997. Ball Pest and Disease Manual (2nd ed). Ball Publishing Batavia Publishing. 426 pp.
Span T.M., Schumann A.W. 2010. Mineral nutrition contributes to plant disease and pest resistance. University of Florida Publication #HS1181. http://edis.ifas.ufl.edu.
Tonetto de Freitas, S., K.A. Shackel and E. J. Mitcham. 2011. Abscisic acid triggers whole-plant and fruit specific mechanisms to increase fruit calcium uptake and prevent blossom end rot development in tomato fruit. J. of Experimental Botany 62:2645-2656.
Daylor, M.D. and S. J. Locassio. 2004. Blossom-end rot: A calcium deficiency. J. of plant Nutrition 27: 123-139.
Zentmeyer G.A. 1963. Biological control of Phytophthora root rot of avocado with alfalfa meal. Phytopathology 53:1383-1387.
Last month I shared some basic info on the major techniques for growing plants without soil, namely hydroponics, aquaponics, and aeroponics. With such interest in these topics, I thought it would be good to dive a little further into the technologies used. I’ll provide a bit of basic information about each type of system used for production and provide some resources for further technical reading if you’re interested in learning more. For some simple diagrams of the systems, check out this link (we don’t know if we can “borrow” the images, so we didn’t copy them over).
“Deep” water may be a bit of a misnomer, as it usually brings to mind thoughts of mysterious sea creatures living in the dark depths of the ocean. Technically, the “deep” water can be just a few inches, as it is deep in reference to other methods. This is perhaps the simplest and least expensive of the systems and can be a great entry point for beginners.
For deep water culture, the nutrient solution is held in a large container with some sort of floating support holding the plants. The container is at least a few inches deep and holds a relatively high volume of water. There are some containers that are designed for deep water hydroponics, but repurposed containers will work as long as they are food safe (meaning that they do not leach or break down). Large plastic totes or even plastic buckets can be used. As for supporting structures for plants, Styrofoam is the most common. There are cell trays made of Styrofoam that are commonly used in production of small crops (or for growing transplants, which is a common use of this technique). Foam boards with holes to hold pots can also be used. Back when I was in grad school we developed hydroponic systems in my plant propagation class using foam insulation boards floating in large plastic totes.
One thing that you have to keep in mind for deep water culture is the need to incorporate oxygen into the system. We often talk about the issue of overwatering houseplants and how it can damage roots due to hypoxic, or low oxygen, situations. Imagine how roots growing only in water would create situations for poor root and plant growth. In all the other systems water flow helps incorporate oxygen into the water. In deep water, there is no moving water and therefore no air incorporation. The most common tool used for this, especially for small systems, is an aquarium air pump and air stones that help create bubbles in the system.
One benefit of this system is that it has a low level of risk when it comes to system failure. There are few moving parts to break down and loss of electric doesn’t result in roots drying out due to loss of water flow.
EBB & FLOW
These systems, also called flood and drain systems, are one step of complexity above the deep water systems by introducing water flow. Plants can either float as in deep water culture or be held in media that fills the container. While many containers can be used, the most common are longer channels that promote water flow from one end to the other. This system also introduces a reservoir of some sort that holds excess nutrient solution and a pump to deliver it to the container. The level of water in the container is controlled by a raised drain pipe where solution exits the system back to the reservoir.
The DIY system I build using gutter with the Rwandan students (mentioned in the first installment on hydroponics) is ebb and flow. The drain from the gutters is a few inches high within the channel, so the water raises those few inches before it drains out. Some producers use long channels the width of those floating cell trays to grow plants in a relay fashion, planting them on one end and move them along as new rafts are added until they are harvested on the other end.
This system is common not only in hydroponics, but aquaponics as well. Instead of a nutrient solution reservoir, the water from the tank(s) holding the aquatic stock (commonly fish, but could also be crustaceans like shrimp) is pumped into the plant channels and flows back into the system. Systems may be based on continuous flow into and out of the system, but most commonly a timer is used to have multiple periods of flow and rest mainly as a means to reduce power usage.
NUTRIENT FILM TECHNIQUE (NFT)
This system evolved one more step above ebb and flow by limiting the volume of water used in the system. Here, water is pumped from the solution reservoir to shallow channels where plants are held in pots or blocks of inert media such as rockwool. Roots are not submerged in water, but instead grow within a thin film of solution that flows almost continuously through the system. These channels have a slight slope where the end with the drain is a little bit lower than the end where the water enters. The slope can be adjusted slightly to affect the speed of the water through the system.
This system is becoming common in production of leafy greens and herbs because it uses a much smaller volume of water. But that small volume of water also presents a risk. If there is a power failure or a clog in the tubing that delivers water to the system the roots can very quickly dry out and crops die, especially in situations of high heat and light.
Perhaps one of the most commonly used systems across the world due to their simplicity, drip systems could be compared to a drip irrigation system used in the field. Drip emitters are used to supply nutrient solution to plants in containers containing an inert media such as peat, coir, perlite, or grow stones. The containers can be pots, buckets, or bags/blocks of the media and are most commonly placed on the floor of the greenhouse or growing location with gutters to collect the solution that flows through the containers. A common method is using long, narrow bags filled with coir or other media referred to as the slab method. Another common method, called the Dutch bucket method, uses buckets with drain holes in the bottom, commonly placed on a greenhouse floor. Water trickles down through the media and roots and leaves the system through the bottom of the container.
Systems vary in the collection of the used solution. Some may collect the solution that flows into the gutter and collect it in a reservoir to be reused, however some systems may allow the solution to flow out as waste. These differences depend on the needs of the producer, available resources, and local regulations.
One of the comments that we got on my first article was about people growing container plants could technically consider it a form of hydroponics. That might be a bit of a stretch, but you could technically consider growing container plants in soil-less media as drip or flow through hydroponics if you provide all of the nutrients through soluble fertilizers in the water.
Typically used for small scale production, wick systems are one of the simple ways to grow plants without soil in terms of technology. In this system, a passive wick uptakes nutrient solution from a reservoir and pulls it into the media (usually absorbent itself). This wick could be a true wick, like a string made of absorbent material that inserts into an individual pot or it could be a mat made of absorbent material that pots or trays sit atop.
I’ve seen this commonly used perhaps not strictly in hydroponics, but for watering individual plants like African violets where yarn or twine is inserted into a drain hole in the pot and sits in water. Technically this could be hydroponics if the media doesn’t contribute nutrients to the plant and they are all contained in the water instead.
This is probably the simplest of the methods and is used primarily by small scale producers and home growers. It is similar to the deep water method in that there is no flowing water, but it is even simpler because there isn’t even an air bubbler. In this method, plants are grown in large containers or buckets and the structure that supports them is fixed to the top of the container rather than floating. As the growing solution is used up, the level of solution in the container decreases. This creates a zone where the roots are exposed to air, providing the oxygen that the roots need. The solution is kept at a level where at least the bottom portion of the roots are submerged in the nutrient solution.
Probably the most complex or technical system, aeroponics supplies water and nutrients to plants through a mist or aerosol emitted through pressurized nozzles. The roots hang in a chamber without media and are misted every few minutes with nutrient solution. The excess solution drops to the bottom of the chamber and is reused. This system uses very small amounts of water, which can be beneficial for growing in dry areas but also creates a potential risk if the system or power fails. Just like the NFT system, any prolonged period with out water will quickly result in plant damage or loss. Beside power loss, this systems is also prone to clogged emitters, since the pressurized nozzles rely on very tiny openings to pressurized the solution.
Keep in mind that several systems that are sold for home or small scale production that are labeled as aeroponic, such as AeroGarden and Tower Gardens, don’t technically use aeroponics to grow since the solution isn’t applied as a mist or aerosol. I would say they operate more like a vertical NFT system where water flows over the roots as it travels down the chamber.
It seems so simple to plant a tree. But to grow a tree is more difficult! In many parts of the United States there is enough water for trees and turfgrass, but it is often a bad idea to mix the two. You may have observed that sometimes young trees do not grow as well when planted in turfgrass. Certainly this is a generalized view and tree/turfgrass genetics are very different between their respective species. So it is natural to expect different outcomes when planting different species of trees in any landscape setting, turfgrass notwithstanding. Another factor to consider is time. The day we plant a tree is not the same time reference as ten years later. In ten years, the tree if it is successful, may have modified
its environment significantly, making turfgrass cultivation more difficult. Most tree/turfgrass difficulties begin when the tree is young–as a newly planted tree. If it succeeds in growing a large canopy, difficulties will ensue for the turfgrass. Sometimes turfgrass cultural requirements (frequent irrigation) can predispose trees to root or root-collar diseases such as Phytophthora.
Trees and turfgrass have some similar and very different requirements from their respective landscape settings. Both trees and turfgrass require sunlight to photosynthesize and grow. Both would usually prefer full sunlight without shade. As trees grow they shade the turfgrass sward beneath their canopies. Turfgrass can lose density, and become a thinner sward that is more susceptible to diseases such as powdery mildew. Trees grow roots near the soil surface and as they become larger, some trees may even proliferate roots near the mowing height of turgrass and suffer repeated injury from mowers, also increasing the risk of pest invasion into the tree. Both trees and turfgrass need water and soil minerals to grow. While soil minerals are usually abundant enough for both, water is often limiting for one or the other in this landscape combination.
The maintenance practices required for turfgrass often injure trees, especially young trees. Mowing near trees can injure the bark on the lower stem especially if the mower comes to close and actually scrapes the young stem. Since grass will grow longer where the mower can’t reach right near a tree stem there is a temptation to use a string line trimmer or weed whip to maintain the grass that has shot up around the tree stem. The repeated use of string line trimmers around trees removed young bark and can “girdle” the tree stem. While trees can survive these practices their growth rates are slowed considerably.
One approach to having trees growing with turfgrass is to remove a ring of turf away from the tree and replace it with mulch. This eliminates the need to maintain the turfgrass near the trees stem and root flare. Richard Harris and others (1977) found many years ago that a one foot circle removed around the stem of newly planted trees would increase their establishment rates compared to trees with turfgrass growing right near the stem. Whitcomb (1979) also recognized that turgrasses are competitors with newly planted shade trees. Whitcomb’s earlier research (1973) showed reduction in root density when trees were planted in a sward of Kentucky bluegrass.
As trees grow it is important to widen the ring around them giving more room for mulch and reducing the competing turfgrass underneath their expanding canopies. This is a general concept; some trees can live in turfgrass without problems as long as resources are not limiting. Riparian trees such as sycamore can grow well in swards of turfgrass, but other species such as Peruvian pepper (Schinus mole) tend to languish.
Trees are adapted to drop leaves, this is termed litterfall and it becomes part of their natural mulch. Litterfall tends to prevent annual plants such as grasses from developing. Fallen leaves, fruit and twigs are recycled by fungi providing nutrients back to the tree. Turfgrass cultivation interrupts this process and while trees obtain some of the nutrients supplied to turfgrass, as Whitcomb observed, turfgrasses are fierce competitors for nutrients so young trees are especially susceptible to nutrient deprivation in turfgrass swards. For the best results in your
garden, it is best to maintain some distance between young trees and turfgrass. It is optimal if the mulched (no turf) area under a tree can expand to its dripline as it grows.
It seems that as interest in gardening grows, especially among younger generations, interest in different techniques that home gardeners use and different plants they grow are also on the increase. You see the old standbys like straw bales and containers emerge. Terraria, succulents, and air plants are having their moment. And all kinds of technology driven indoor growing systems are all over the web, mostly hydroponic, but some aeroponic and aquaponic as well (we’ll talk about the difference in a bit – if you’re just here for that, skip the first 2/3 of the article).
I had been thinking about getting one of those new techno aeroponic growing systems as a demo for my office as a discussion starter for those interested in controlled environment growing whether on the homework commercial scale. There is a general interest and need for basic education for hydroponics and aquaponics in the area that I hope to build extension programming around, so having something at the office could provide some interest from walk-in and social media clients. I had dusted off a first generation AeroGarden that I found in the storage shelves in the office storage catacombs and set it up in my office. It is a far cry from the new models I saw in those online ads that are outside of my budget for “toys to show off at the office.” It doesn’t have nice LED lights or connect to my phone via Bluetooth like the fancy new models. Given its age, it produces more noise and heat than the lettuce and herbs I’ve tried to grow in it. Maybe I’ll be able to get one of the fancy models one day.
Then I remembered a book that an urban ag friend of mine had written on building DIY hydroponic systems from common building materials and resolved to not only build a system, but incorporate it into my programming somehow. The book, appropriately titled “DIY Hydroponic Gardens: How to Design and Build an Inexpensive System for Growing Plants in Water” by Tyler Baras shares plans for building a variety of types of hydroponic systems using basic building materials like gutters and lumber, drip irrigation tubing and fittings, and various other bits and bobs. Tyler had been a featured speaker for the West Virginia Urban Agriculture Conference that I started and hosted when I worked for WVU Extension, so the book was on my radar – I placed an order. (Note: I don’t get a kickback for sharing the book – just sharing a good resource that happens to be from a friend.)
Teaching Hydroponics to an Unlikely Audience
As luck would have it, I had an opportunity to put the book, and my DIY hydroponic skills, to the test. Our university does quite a bit of work with and in Rwanda and in May I had the opportunity to travel to Rwanda as part of a study abroad program with my Ph.D. advisor. Rwanda is a very small country, just under the size of Massachussets, with a very big population by comparison – 12 million vs 7 million! Feeding that many people is a struggle, and even though Rwanda produces a lot of produce (and more lucrative export crops like coffee and tea), it still imports a lot of its fruits and vegetables. We were studying how innovation spreads in rural areas, and just before our trip I found a news article sharing that there would be an upcoming $8M USD ($8B RWF) investment in hydroponics in the country in order to increase production on the limited amount of land available.
In June I was scheduled to teach a group of Rwandan exchange students that are part of a sponsored program at the university, and remembering the planned investment in hydroponics I planned to add DIY hydroponics to the curriculum. This is fitting, since most small-scale operations would rely on finding what materials would be locally available. While the operations started by the investment would likely bring in “real” hydroponic systems, if small scale producers want to use the technology or if individuals want to build skills, they’re going to have to use what is at hand.
It was interesting teaching an audience who were interested in learning about the new technology, but have little experience or general knowledge on the subject. Even more interesting was the fact that many of the students had not used or even seen some of the basic power tools we used in building the system. I’m no shop teacher, but in the end the students not only learned a little bit about hydroponics and hydroponic systems, but also some skills using tools that they can apply in other applications.
Hydroponics, Aeroponics, & Aquaponics – Oh My!
Earlier I mentioned that there are differences between hydroponics, aeroponics, and aquaponics. In some ways, they use similar basic setups. All are based on soil-less growing using an inert media to support plants, supplying nutrients and water directly to the plant roots, and providing light to the plants using either natural sunlight or supplemental lighting. Differences come from the source of plant nutrients and from how they are delivered to the plant. I thought I’d take a few minutes to talk about the basics of each of the techniques so you can understand the differences just in case you want to buy or build your own system. If there’s interest, I hope to focus on hydroponics and controlled environment agriculture over my next few blog posts – tell me what you’re interested in learning.
Most people are familiar with the concept of hydroponics. This technique relies on roots being submerged in a nutrient-rich solution where the nutrient content is engineered from a variety of mineral sources. There are a variety of different systems (that will hopefully be the subject of an upcoming blog) where the root zone interacts directly with the solution. In some cases, roots are submerged in a large volume of solution while in others a film or shallow stream of water flows through the root zone. Systems where roots are submerged in the solution may simply be a large reservoir where the plants float on top where systems relying on flow may involve a pump. Movement of water adds another plant need -oxygen, which is required for respiration by the roots. In systems where there is no flow, air is often pumped in to provide oxygen.
Most flowing systems are recirculating, where the solution returns to a reservoir and is pumped back into a reservoir to be reused. While it may seem counterintuitive, these recirculating water based growing systems have been touted as production methods that conserve water. That’s why some of the leading hydroponic production and research comes from areas of the world where water is scarce. Less common are flow through systems where water and nutrients are not recaptured but discarded after initial use.
Aeroponic systems have much of the same basic setup but instead of the roots interfacing directly with water solution it is applied as a pressurized mist. These systems generally use a much smaller volume of water, but there are some drawbacks. Failure of the system, such as an electric outage or clogging of the nozzles that pressurize the mist (which is a common occurrence) can quickly result in plant failure since roots can dry out quickly. Several systems that are sold commercially that market themselves as aeroponic, such as the AeroGarden or Tower Gardens, are more similar to a flowing hydroponic system than a pressurized mist aeroponic system.
The plant growing structures of aquaponics are similar to those of hydroponics, with the addition of larger reservoirs to accommodate the addition of aquatic livestock such as fish (or sometimes crustaceans). The waste produced by the stock provide the nutrients needed by the plants rather than an engineered nutrient solution. These systems require having the technical knowledge to meet the needs of the aquatic stock and balancing those with the needs of the plants. The addition of the aquatic stock also introduces a microbiome of bacteria and fungi, many of which are required for animal health but can also introduce pathogens that can negatively affect human health.
Are you interested in learning more about these systems? What do you want learn about in hydroponic or other systems? Let me know in the comments and I’ll try to base some future articles on what our readers are interested in.
Arborists, Professional Credentials, and Designating Bodies
The Merriam-Webster dictionary defines arboriculture as “the cultivation of trees and shrubs especially for ornamental purposes” (2019a) and arborists as the specialists that care for those trees (2019b). In the US there are two primary certifications for arborists. Arborists can become a Registered Consulting Arborist (RCA) through the American Society of Consulting Arborists (ASCA) (ASCA, 2019a) and/or become an International Society of Arboriculture (ISA) Certified Arborist (ISA, 2019a). The ISA also offers a range of associated certifications, including ISA Certified Arborist Utility Specialist, Arborist Municipal Specialist, Tree Worker Climber Specialist, Tree Worker Aerial Lift Specialist, Board Certified Master Arborist, and ISA Tree Risk Assessment Qualification. For the sake brevity, the focus of this post will be on the flagship programs from each organization, ASCA’s Registered Consulting Arborist and the ISA Certified Arborist credentials.
Relevance for Gardeners
Many gardeners who have trees are likely familiar with arborists, or at least with a local “tree guy”. While trees are beautiful, they can also suffer from disease, nutrient deficiencies, and other plant health issues. In addition, trees often require maintenance and pruning to ensure safety, avoid damage to houses or other property from falling branches, or to allow more light through the tree canopy to reach the ground and garden. Maintenance and pruning of trees requires experience and expertise to ensure that trees aren’t damaged in the process, and that pruning is done safely. When their services are needed, hiring an arborist with professional credentials can be an excellent way to ensure that trees are properly cared for and that nobody is hurt in the process.
In my personal experience, certified arborists in my community charge more per hour. However, the extra cost in exchange for ensuring the 60 year old pin oak that is the same age as my house lives on for another 60 years is worth it.
Other Considerations in Choosing an Arborist
Whether an arborist is certified or not, it is a good idea to check that the arborist is insured. That is because if they or one of their crew is injured on your property, you as the property owner may be liable for any injuries that occur while tending to the trees on your property.
Also, as with hiring any professional for home repairs or improvements, it’s important to check their references or get recommendations from colleagues and friends. You should also check whether your local government has any regulations on tree management within city limits. If so, be sure that who you hire has experience with following those regulations, and has all of the business registrations and approval from the local government.
It is also important what your needs are. If you need a large specimen tree pruned and the potential for tree injury or death would be devastating, then hiring someone with documented expertise is important. If you’re instead just trying to get the half-dead tree in the back yard chopped down and there’s no risk of falling a tree onto a power line or structure, then documented expertise might be less important (though insurance might be, see above).
In addition, if you are getting a large tree taken down, consider the value of the wood in your tree. Large hardwood trees may have significant value. Some tree guys will offer to take it down for free in exchange for the wood. While this may sound like a good deal, I know people who were shorted thousands of dollars from such transactions when considering the value of the wood and the per hour cost of an arborist.
Type of Credential: Professional Certificate
Both the RCA and ISA Certified Arborist credentials are considered professional certificates. This means that these are optional credentials. Arborists are not required by law to be certified. This means that an arborist can operate without certification. Arborists that are certified have had their credentials reviewed, and meet or abide by the criteria described below.
Education and Professional Experience Requirements
Registered Consulting Arborists must be a current ASCA member, and be a graduate of the ASCA’s Consulting Academy (ASCA, 2019b). No additional education or work experience is required.
ISA Certified Arborist must have three or more years of experience in arboriculture and/or a degree in the field of arboriculture, horticulture, landscape architecture, or forestry from an accredited institution of higher education (ISA, 2019b).
Registered Consulting Arborists must pass an open-book exam as part of the ASCA’s Consulting Academy, which also includes other assignments before, during, and after the academy (ASCA, 2019c).
ISA Certified Arborists must pass a qualifying exam (ISA, 2018).
Code of Ethics
The ISA Certified Arborist program has a code of ethics that all certified arborists must abide by (ISA, 2019c). The RCA program does not have a code of ethics.
Registered Consulting Arborists must complete 420 continuing education units (CEUs) to be eligible for the RCA credential (ASCA, 2019b). Their website does not specify if additional CEUs are required in order to maintain the RCA credential.
ISA Certified Arborists are required to complete 30 CEUs in a three-year period in order to maintain their certification (ISA, 2019d).
ASCA. 2019a. The RCA. American Society of Consulting Arborists. https://www.asca-consultants.org/page/RCA (accessed 25 September 2019).
ASCA. 2019b. Eligibility/Fees. American Society of Consulting Arborists. https://www.asca-consultants.org/page/EligibilityFeesRCAs (accessed 25 September 2019).
ASCA. 2019c. ASCA’s Consulting Academy. American Society of Consulting Arborists. https://www.asca-consultants.org/page/ConsultingAcademy (accessed 25 September 2019).
ISA. 2018. ISA Certified Arborist Application Guide. https://www.isa-arbor.com/Portals/0/Assets/PDF/Certification-Applications/cert-Application-Certified-Arborist.pdf.
ISA. 2019a. Types of Credentials. International Society of Arboriculture. https://www.isa-arbor.com/Credentials/Which-Credential-is-Right-for-You (accessed 25 September 2019).
ISA. 2019b. ISA Certified Arborist. International Society of Arboriculture. https://www.isa-arbor.com/Credentials/Types-of-Credentials/ISA-Certified-Arborist (accessed 25 September 2019).
ISA. 2019c. Code of Ethics. International Society of Arboriculture. https://www.isa-arbor.com/Credentials/ISA-Ethics-and-Integrity/Code-of-Ethics (accessed 25 September 2019).
ISA. 2019d. Maintaining Credentials. International Society of Arboriculture. https://www.isa-arbor.com/Credentials/Maintaining-Credentials (accessed 25 September 2019).
Merriam-Webster. 2019a. Definition of Arboriculture. Merriam-Webster. https://www.merriam-webster.com/dictionary/arboriculture (accessed 25 September 2019).
Merriam-Webster. 2019b. Definition of Arborist. Merriam-Webster. https://www.merriam-webster.com/dictionary/arborist (accessed 25 September 2019).
Gardeners are assaulted with marketing campaigns nowhere better than in the fertilizer aisle of a garden center. There are so many choices and the labels suggest that fertilizing garden plants is a complicated process that requires specialized products.
Laws require that fertilizers list the proportion of the most important macronutrients on the front of the bag. Nitrogen, Phosphorus and Potassium abbreviated N, P, K, respectively, will always be shown with the ratio of their concentrations such as 5-10-15. This indicates the bag contains 5% nitrogen, 10% phosphorus and 15% potassium. The bag will also specify the weight of fertilizer contained in it, usually in pounds here in the United States. Of course, labeling requirements vary in other countries. Since fertilizers are not drugs or pesticides the labeling requirements are relatively lax and thus are open to extensive marketing. With fertilizers, as long as the contents are labeled and the proportion of N P and K are somewhere on the bag, any other claim can seemingly be made.
For years fertilizers have been designed specifically for certain plants. You can commonly purchase citrus food, camellia and azalea food, vegetable fertilizer, and of course turfgrass fertilizers which have been widely marketed for decades. Some fertilizer blends are based on research. Turfgrasses have a high requirement for both Nitrogen and Potassium and you often see elevated percentages of these in turfgrass fertilizers. Palms also require more potassium than Phosphorus and products have been developed that are “palm special” fertilizers. Despite all the research, manufacturers still throw in some phosphorus even though phosphorus is abundant in the United states in most soils. It is not clear to me what makes citrus food different from any other fertilizer, although claims that it is best for citrus can usually be found on the product. We grow more citrus in Ventura County, particularly lemons than anywhere in California, but none of the growers apply “citrus food”.
Some fertilizers are marketed for acid loving plants. Acid forming fertilizers have the ability to temporarily reduce pH in media or soil to which they are applied. This is because they have a high amount of ammonium or urea as the nitrogen source. Microbes in soil oxidize the nitrogen to nitrate and release hydrogen ions that make the soil more acid. Continual use of acid forming fertilizers can drop soil pH to dangerously low levels and make nutrients unavailable to many plants. This is especially the case in high rainfall climates where mineral nutrients are easily leached from soil. Another product often used to lower pH is aluminum sulfate. Often marketed as “hydrangea food” it helps to promote blue flowers in this plant. Special care should be given here as aluminum can easily be applied to toxic levels.
For as long as fertilizers have been sold they have been marketed as products that make plants grow better. When the importance of mycorrhizae were discovered, manufacturers found a new marketing angle that could be used to sell fertilizers. Now countless manufacturers include mycorrhizal inoculants as part of their fertilizer blend. Not only are we able to feed the plant but we can also feed the soil with beneficial micro-organisms. This all sounds great but often the inoculant, if present in the bag, is not viable. In many cases garden soils already have plenty of mycorrhizal fungi in them so they really are not needed in fertilizer products. Other fertilizers include growth stimulants such as humic acid, fulvic acid or humates. Research does not support their efficacy in horticulture.
Fertilizer manufacturers feel that it is important that we gardeners use things that are “all natural”. I don’t know what is natural to you but for me it is natural to challenge claims that have no scientific basis. The very practice of fertilizing plants is NOT NATURAL! But the products are often purported to be. Sometimes natural is synonymous with organic. There are an amazing number of organic fertilizers, so much so that it becomes bewildering as to which one to choose. Organic fertilizers may or may not be ‘certified’ by an agency such as OMRI as meeting some standard. Generally speaking organic fertilizers are made from some carbon based source. They can be sourced as manures or as plant or animal based meals or products. The N-P-K ratios vary widely.
I am always careful to avoid manure-based organic fertilizer products as they can be unnecessarily salty. While many organic fertilizers may be rather “slow release” as they need to mineralize from organic to soluble forms, this is often not the case with manure based products which can easily burn plants if over-applied. Some organic fertilizers are made from rock like substances such as leonardite which are very slow releasers of nitrogen. These products are mined and are similar to coal but have fertilizer value. In trials I have conducted on containerized plants some of these products were top performers.
Another common type of organic fertilizer are the biosolids products such as Milorganite. These are processed sewage sludge products that are de-watered and made into fertilizer. They are very effective nitrogen sources. Some biosolids fertilizers have also been sources of metals, such as zinc, lead and cadmium. Metal contents are closely monitored by manufacturers but since these products come through municipal systems, zinc, copper and other metals such as lead are often elevated. Slow release and organic fertilizers are useful if they are applied with understanding of the mineral needs of the plants they are applied to.
Most plants grow fine without fertilization and the main fertilizer element that plants respond to is nitrogen. So despite all the marketing claims seen on fertilizer bags, a fertilizer with an adequate source of nitrogen will likely benefit plants in need of that element. Specialized fertilizers that promote flowers or roots are not substantiated by research. Elements other than nitrogen are usually not required and ratios of N P and K are not tuned for more blooms or more roots. Adding phosphorus to your fertilizer does not promote flowering unless your soil is deficient in phosphorus (a rare condition). Gardens in high rainfall areas will likely need more potassium and nitrogen, but Phosphorus is hardly ever limiting to plant growth. Most plants do not need or require special fertilizers but will respond to fertilizers that contain an element they and soils they are growing in are lacking.
For gardens that have a loam soil texture, little fertilizer will be needed. Soil types often determine fertilizer needs for plants. Sandy soils likely need the most fertilizer because they do not hold nutrients well. They are also the most likely soils to lead to pollution because fertilizer elements will leach easily. Plants growing in sandy soils are also at greatest risk from injury of overfertilization. Plants growing in clay soils are least likely to require fertilization because clays hold and retain cations so well. An informed way to fertilize your garden is to obtain a soils test and base your fertilizer program on the results you obtain. For more on technical details of fertilizing see John Porter’s column in this blog archive.
When you think you need to fertilize garden plants follow these suggestions:
-Base your fertilizer program on a soils test
-Fertilize sandy soils more frequently than clay soils but with smaller amounts
-Most gardens require some nitrogen but not Phosphorus or Potassium so look for NPK ratios with X-0-0 as these products will only supply nitrogen.
-Some plants such as palms and turfgrass benefit from potassium so use a product with X-0-X
-Do not fertilize at planting time, wait until plants establish
-Always apply water after applying soluble fertilizers so they are dissolved
-If using Organic fertilizers chose one with a higher N content
-Never over-fertilize. Landscape fertilization can impair natural waterways resulting in algal blooms that kill fish and other aquatic life.
I was taught in horticulture school that the ideal soil is composed of 50% solids and 50% voids or spaces which are themselves composed of a variable amount of water from small amounts to as much as 25% water when the soil is at field capacity or the amount of water left in soil after gravity has pulled all the free water down in the profile. So the “ideal” soil always has 25% pore spaces or more depending on how much water is present. These conditions are vital for root growth since roots go through the chemical process of respiration which involves absorbing oxygen and giving off carbon dioxide. For gas exchange to happen in this ideal soil, spaces or voids are important, and necessary. A well-structured soil has micro aggregates (pea sized or smaller clumps of soil) in high concentrations which creates many of these spaces and is said to have high porosity.
Porosity – the amount and types of voids – is determined by two major factors: 1) The size and distribution of the soil particles; and 2), how those particles are arranged. Sand, silt, and clay particles can be arranged and formed into pathways that help move air and water. These paths are formed by past root channels and the movement of organisms like worms and insects. These channels are glued together by exudates from roots, bacteria and fungi. This organic, both living and dead, soil fraction also add and stabilizes porosity. All together, soil particles, plant residues and microorganisms create a fragile structure that adds more porosity than just the pores and voids created by spaces between the soil mineral particles.
Structure and porosity can be physically destroyed or crushed. The soil can be squished by heavy equipment or constant foot traffic of animals, such as humans or horses, or others that constantly tread over the same soil. Compacted soil can be near the surface (the worst for trees since their roots are mostly near the surface), or lower down in the profile. A “plow pan” is actually a compacted zone at the depth of plow or ripping agricultural implements where soil structure is constantly destroyed at the same depth over and over. As soon as roots and worms create a new pathway and reinforce it with micro-aggregates and glues, it is destroyed again, creating a zone of loosened soil where the implement has traveled, but a zone immediately below what which has been compacted by the pressure of the implement.
Most horticulturists and many gardeners know that compacted soils are bad for plants growing in them. Shade trees frequently have restricted growth in these kinds of soils. This can happen at a young age when trees are just planted or on large specimen trees, such as in parks that have the soil compacted around them by visitors. Footpaths, picnic tables, playground equipment or any publicly attractive park feature will often have compacted soils in the area.
Deprivation of litterfall and mulch layers, either through wearing out (grinding of organic matter by foot traffic) of the mulch or by mulch/litter removal through raking will promote compaction by removing the cushioning effect of that mulch layer. Sadly, the tree itself can be the feature that attracts people to it, resulting in compacted soils all around its base that limits its health.
What is not so well known is why growth is slowed in compacted soils. The effects of compaction are multi-fold. Compacted soils are less porous because the compaction literally reduces the air pockets in the soil, making it more dense with lower oxygen diffusion rates. Soil with destroyed structure becomes less permeable to water infiltration and holds less water. Under these conditions tree roots may not be adequately hydrated, and cannot physically penetrate the highly compacted soil. Thus, they are not able to develop and expand and explore enough to supply the needs of the tree. Reduced soil oxygen, along with other site, soil, and tree variables such as water and nutrient uptake, are all reasons for restricted tree growth.
There is compelling evidence that different species of trees can exert greater pressure at their root tips to break through compacted soils. Different tree species also have different root architectures – finer, deeper, shallower, etc. Thus, there is a genetic factor in a tree’s ability to deal with this soil problem.
Soils are more or less compactable depending on their texture, structure and moisture status. Generally a dry soil is harder to compact that a moist one. Dry soils resist compaction (but still can be compacted) because the soil aggregates stiffen as they dry. Wet soils are easily compacted but people and machinery also easily sink in very wet soils. Waterlogged soils may or may not have structure, but the water in the pores, prevents further collapse of the soil structure. Soils that are moist (at field capacity) are just right for growing plants, and are also perfect for compacting and thus must be protected from compaction.
Soil compaction is measured by calculating what’s called bulk density (Bd). Bulk density is the weight of soil in a given volume, and is measured in grams/cubic centimeter. In order to measure bulk density a special soil sampling device called an “intact soil core sampler” is used. This device extracts a core of soil while preserving its structure. The volume of the sample is a constant. The soil sample is removed and dried to drive off all the water and then the weight of dry soil is divided by the known sample volume giving the bulk density.
Bulk densities vary depending on the soil texture (%Sand:Silt:Clay) and to a smaller extent on the organic matter content. Sands generally have very large particles, more pore spaces and lower bulk densities than silts, loams and clays which will
A Comparison of Root Limiting Bulk Density for Different Soil Types (NRCS 1998 in Dallas and Lewandowski, 2003)have the highest bulk densities. Thus compaction is determined by measuring both bulk density and soil texture. Generally, pure rock has a bulk density over 2.65 g/cc. Uncompacted sands may have bulk densities of 1.2-1.4, while loams and clays may have Bd from 1.5-1.8 g/cc. A sand may be compact at 1.4 but a clay may have a higher Bd of 1.5 and not be considered compacted. Organic soils can have Bd that are much lower – 0.02-.9 g/cc. Generally, soils (average of all textures) with bulk densities over 1.5 can be suspected to be compacted and will limit tree growth.
Bulk density for a given soil is not a fixed property, it can change depending on the history of what has happened to the soil. For instance, in an annual color bed or vegetable garden bed, the soil may be turned or tilled by the gardener, amended, and replanted. During this process structure is destroyed, but the organic matter, growth of the crop, and time foster a new soil structure, perhaps even more porous than the soil was previously. This can happen in one growing season. In the case of compacted soils around trees, it can take years for a mulch laid over a compacted soil to correct the compaction.
Another way of looking at this is: if you can get the sampler into the soil, its likely not compacted, but if you have to use a hammer to get it into the soil it might be compacted (or dry). Pressure required is going to depend on the soil moisture, as well as the state of compaction. Compaction can also be measured by a device called a penetrometer which quantifies resistance to penetration. We as gardeners can use a screwdriver, if you can push it into soil; it is less compacted than if you can’t. The screwdriver test is also used to test for moisture content–when soils dry out, they resist penetration. The depth of water penetration in an irrigated soil is the depth to where the screwdriver stops when pushed in. So, it is easy to confuse a compacted soil with a dry soil. Also, if a soil is compacted, water will not easily enter, so many compacted soils are also perennially dry soils since irrigation does not easily penetrate them. This can be seen as increased runoff when you try to irrigate or ponding if the compacted zone does not drain away.
How do we fix compacted soils with high bulk densities? I was always taught that chemical fixers like gypsum, soil penetrants, or other chemical means will not affect a structurally damaged and compacted soil. The only way to fix them is to physically un-compact them. So, further destroying structure by ripping, drilling, trenching, air spading, or in some other way breaking up the compacted layers is the thing to do. Basuk (1994) cites cases where soil modified to treat compaction actually re-compacts (bulk density increases) over time (2-3 years after a compaction relieving treatment is applied). Numerous studies indicate that breakdown of arborist chip mulches will lead to reduced bulk density, but little is known about actual bulk density reductions with mulch applications over time. I am confident mulches will reduce bulk density, but given the diversity of soils, textures and compaction levels, I can only imagine this is a variable response. Removing the cause of the compaction, (foot traffic, machine usage etc) is the first step. Mulching following some kind of “soil fluffing” procedure should begin the process of increasing soil porosity and reducing bulk density. It may take years to relieve compaction passively through the action of mulching. If soil can be mechanically broken up, the compaction issue is solved and soil structure will slowly be reformed in time depending on what is grown thereafter.
Bassuk, N. 1994. A review of the effects of soil compaction and amelioration treatments on landscape trees. Journal of Arboriculture 20:9-17.