Fertilizers — a cautionary tale

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.

Fertilizers can be marketed for many reasons; here one brand offers many 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.

Laws require that fertilizer manufacturers list the N-P-K ratio in % by weight on the bag in a prominent place

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”.

There is no such thing as “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.

Technically aluminum sulfate is not a fertilizer: note the 0-0-0. It can be toxic if applied too much and can make soil too acid.

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.

Most gardens do not require mycorrhizal inoculants.

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.

There are many organic fertilizers. They usually have low N-P-K values; some are good slow release sources of nitrogen.

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.

Biosolids are made from treated human waste. They are an excellent source of nitrogen, but may contain unwanted metals.

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.

Phosphorus does not promote flowering. “Bloom maker” fertilizers are a marketing gimmick.

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.

GPs at the Tradeshow: Looking for snake oil and finding…..the dirt on tillage

The Annual Meeting and Professional Improvement Conference of the National Association of County Extension Agents is that one time of year where extension agriculture professionals gather to share ideas, give talks, network, and let their hair down. The name of the organization is a bit outmoded: many states no longer call their extension personnel agents, but rather educators, experts, professionals, area specialists, and the like. Most aspects of agriculture are included: from the traditional cows and plows of animal science and agronomy to horticulture and sustainable agriculture (I’m the outgoing national chair of that committee). There’s also sharing on agriculture issues like seminars on engaging audiences about genetic engineering, teaching and technology like utilizing social media and interactive apps, and leadership skills.

It is the one time every year or so that Linda Chalker-Scott, grand founder of the Garden Professors, and I get to hang out. If we’re lucky we’ll meet up in some sessions, chat in the hallways, or grab a drink. But one of our favorite conference activities is taking a turn around the trade show floor. This is where companies and organizations are vying for the attention of extension educators to show them their newest equipment and products….we are, after all, the people that share growing and production information with a great number of potential clients across the country.

Since the organization runs on money, almost no company that comes calling with the money for a trade show spot is turned away. This means that the products may or may not stand up to the rigors of scientific accuracy. In years past we’ve found snake oil aplenty, like magical humic acid that is supposed to be this natural elixir of life for plant growth. The only problem is that humates don’t exist in nature and there’s little documentation of any effect on plant growth. The product that was supposed to be this magic potion was created from fossil fuels and no actual peer-reviewed research was offered by the company – hardly convincing. There were magic plastic rings that supposedly acted as protective mulch around mature trees and could slowly release water, except that mature trees don’t really need protective mulch and the amount of water would be negligible to a tree that size. So will we be smiling or scowling when we’ve made our way through the trade show.

Right off we set our sites on a company starting with “Bio”, which can be a good indicator of questionable rationale. That lit up the first indicator on our woo-ometer. Beneficial bacteria you apply to plants/soil: woo-ometer level two. So LCS and I engaged the representative. Asking about the product and what it does. We learned about their different products that could help increase the rate of decomposition of crop residues in farm fields, of turfgrass improvement, increased crop production, and treatment of manure pits on dairy and hog farms (which, if you’ve ever experienced one, you’d know would benefit from any help they can get in terms of smell).

Most of the products like this give vague descriptions of the beneficial bacteria it contains. They’re akin to compost teas that can have any number of good, bad, and downright ugly bacteria and fungi in them.  Since you don’t know what’s in these products, any claims on soils or plants are suspect at best. However…our rep went on to tell us that the company created blends of bacteria from specific strains that had been researched for their effects on decomposition, soil nutrient availability, and plant growth. There was a brochure with the specific bacteria listed, along with studies the company had conducted.

We asked about peer-reviewed research, which is our standard for evidence here at the GP, and while he had no results to share he assured us that university-led research is currently in the works. And as we’ve stated in regards to applying of beneficial bacteria to soil – while there’s little evidence showing the effectiveness of applying non-specific bacteria to plants, using directed applications of specific bacteria which have been tested for specific functions are supported by research. So our woo-meter didn’t fully light up. We reset it and continued the hunt.

We scoured the rest of the trade show and found one other soil additive that lit up the first lights of our woo-meter, but the rep must have been out for lunch so without anyone to talk to we couldn’t confirm woo or no-woo.

However…..we did find something spectacular! The local employees of the USDA NRCS (Natural Resources Conservation Service) had an interactive demonstration of soil, specifically showing the benefits of reducing or eliminating tillage. The NRCS works with many farmers to incorporate conservation practices on farms, including no-till production, by providing technical assistance, farm plans, and even grants, cost-share, and easement programs. Many farmers have benefitted from their grant for season extension high tunnels (which are seen as a soil conservation technique, since they shelter soil). We were so enamored with the demonstration, we asked them to do it again…so we could record it. So, for your viewing pleasure check out the video below where you can see how well no-till soil holds its structure while tilled soil falls apart. This effect is from the exudates from all the beneficial microbes in the soil that act like glue to promote good soil structure. We’ll let the video speak for itself……

So not only does the trade show get a smile instead of a scowl from us, but also two thumbs up! Either there has been some weeding out of the trade show sponsors, maybe the snake oil salesmen didn’t get the traction they were hoping for at the conference, or hopefully some of these companies have failed to reach an audience with their pseudoscience.

 

Landscape fabric – a cautionary tale

This isn’t the first time I’ve ranted about bad mulch choices and it certainly won’t be the last. But this pictorial cautionary tale is too important to pass up.

We already know that sheet mulches can be death to microbes, plant roots and animals living in the soil underneath. Our newly published research shows that landscape fabric reduces carbon dioxide movement between the soil and atmosphere about 1,000 times more than wood chip mulches do: plastic mulches are even worse. Oxygen movement will be likewise affected.  And while gaps and holes in these barriers can lessen the impact, the question remains: why would you use ANY mulch that reduces gas movement? Yet people persist in using fabrics and plastics, usually to “smother” weeds (and that verb should set off alarm bells for anyone thinking about collateral damage to soil life). But weeds are weeds for a reason, and they will eventually colonize the surface of sheet mulches as soil, organic matter, and water collect over time.

So without further ado, here is a case study of what happens when sheet mulch is used for landscape weed control.

These irrigated landscape beds are in Wenatchee, Washington, which has hot, dry summers. As you can see, bark mulch has been used to hide the shame of sheet mulching. And from a distance it looks…okay.

Upon closer inspection, you can see the shroud of death emerging from the bark mulch (which has no means of staying in place, especially on a slope).

And even close you can see the soil that’s blown in, along with the bark and other organic matter. Just add water, and you get weeds!

Weeds, weeds, weeds! Lots of weeds. Sunny weeds!

And shady weeds!

Border weeds!

Rocky weeds!

The weeds are thriving – but the trees are not. The crowns are dying…

…and the trunks are suckering.

But you’ll note that the trees in the first photo outside of the beds are thriving.

And it’s all because of that “weed control fabric.” Which is working so well that this landscape had to be treated with herbicide the day I was there – to control the weeds.

Soil compaction–the urban stress of death for shade trees

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 in soil is created by the action of roots, fungi and soil fauna developing channels, micro-aggregates and incorporation of organic matter so that soil becomes highly “structured”.

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.

Soil structure is physically crushed and destroyed by cars driving over this tree planting area. As the surface compacts, runoff increases, the soil holds less water and oxygen as a result tree roots die.

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.

Tree growth is limited when soil is compacted around the trunk. Turf loss, and a dry soil are symptoms of a compacted zone around this tree

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 willhttps://i1.wp.com/www.deeproot.com/blog/wp-content/uploads/stories/2014/04/Soil-organic-matter-soil-texture-table1.png?resize=636%2C326

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.

References:
Bassuk, N. 1994. A review of the effects of soil compaction and amelioration treatments on landscape trees. Journal of Arboriculture 20:9-17.