Compost in Seed Starting Mix: Recipe for Success….or Failure?

A recent question posted to the Garden Professors blog Facebook group (a place where you can join and join in conversation of garden science) asked about the potential for compost added to seed starting media to cause failure in germination.  It is a good question, and one that seems to have several different camps – from garden hero author folks swearing by it in their (non-peer reviewed) books, to fact sheets saying it isn’t a good idea.

I’ve always promoted that the best practice for seeds starting is using a sterile media to avoid such problems as damping off.  Many of the problems I’ve heard associated with compost and seed starting are that improperly finished compost can introduce disease microorganisms to the media or cause phytotoxicity, it can make the mix too heavy and thus create anaerobic conditions that starve emerging seedlings of oxygen or cause decomposition, and there is the potential for residues of herbicides in composts using farm waste, manure, or lawn clippings as a feedstock. But does compost really pose a risk to seed starting?  I decided to take a very quick spin through the literature to weigh the possibilities.  Here are some of the potential issues and what a quick glance at the literature says.

Keeping the Germs out of Germination

Compost, even finished compost, has a high microbial activity.  For the most part, the fungi and bacteria in compost are good guys that pose no threats to plants, they decomposers or neutral.  But incorrectly managed compost can also harbor fungi such as Pythium and Rhizoctonia that cause damping off or even other diseases such as early and late blight if diseased plants were added to the compost and sufficient heat levels weren’t maintained.  Composts that don’t reach 140°F and maintain that temperature for several days to kill off potential pathogens run the risk of introducing diseases into seedlings.

Many promote the use of compost and compost products for potential antagonistic effects on bad bacteria.  We’ve discussed compost tea and the lack of conclusive evidence that it has any effect on reducing disease here many times before, and this article found that there is no significant effect of compost tea on damping off.  Some other articles, such as this one, did find that commercially prepared composts added to media did suppress damping off.  However, it is to be noted that these are commercially prepared composts, which have a strict temperature requirement and often require testing for pathogen and bacterial populations.  Many home composters aren’t as proficient at maintaining temperatures suitable for pathogen elimination.

Even if the compost is pathogen free, introduction into a germination media could potentially increase the population of pathogens already present in the media (or that land on it from the air) by providing a source of food for bacterial and fungal growth.  The sterile mixes aren’t just sterile from a microorganism perspective, they’re also sterile from a nutrient perspective as well to help inhibit potential pathogen growth.  The seeds come with their own food, so it isn’t needed for initial germination – the seedlings should be moved to a more fertile mix once they’ve established their first set of true leaves.

Image result for damping off
Damping off, source hort.uwex.edu

You may be saying- “but we also direct sow seeds outdoors, where there’s lots of pathogens present in the soil.”  While this may be the case, damping off is still a definite problem in direct sowing and the loss of investment in materials, lights, and time is generally much lower (and less painful) than in indoor seedling production.  This is especially the case for large operations or for home gardeners who grow lots of stuff from seed.

This is the main issue that leads to the best practice recommendation to use a sterile seed-starting mix that doesn’t contain compost.  If a mix contains compost, it should be from a commercial enterprise that follows best practices or  pasteurized.

Maturity isn’t just for wines, cheeses, and people

Continuing to talk about proper composting, improperly finished compost that hasn’t properly matured (finished composting) can also lead to problems with seed germination.  Unfinished compost can still have woody material included, which has a high C/N ratio and also contain/release phytotoxic compounds during the decomposition process. The presence of decomposition microorganisms in a high C/N ratio means that there is still decomposition happening, which requires nitrogen for the process.  With absence of nitrogen in the media, the nitrogen from the seed or the seedling can be leeched out, effectively causing mortality after or even before germination.  The tender seedling serves as a source of N for the decomposing fungi.

We’ve had this discussion before when it comes mulch.  While mulch is perfectly fine on top of the soil, if it gets mixed into the soil there could be potential implications on N availability.

A germination bioassay is one tool commonly used to test for compost maturity.  Quickly germinating (and inexpensive) seeds are germinated on the compost (or on filter paper soaked with an extract from the compost in some commercial operations).  The rate of germination vs germination failure can give some insight into the maturity of the compost.  This paper discusses the use of the technique for commercial sawdust compost used for potting media.

You can use a bioassay of your own to test for compost maturity (or herbicide persistence, discussed later) for applications in your garden.  Sow an equal number of inexpensive, fast-germinating seeds like radish or lettuce sown on the compost with a control sown on moist paper towel in a bag.  Compare the number of germinated seeds and thriving seedlings after several days to see if there is an issue with the compost.

Keeping Things Light

One other quality required for seed starting media is a good level of porosity (pore spaces) for the media to hold air.  Air (oxygen) is important as it is needed by the roots for respiration.  If the media is too heavy or holds too much water you run the risk of hypoxia, or lack of oxygen, in the roots.  This can result in root die off and subsequent seedling failure.  Most seed starting media are composed of very light materials such as peat moss, coir, vermiculite, or perlite for this very reason.  Compost, by nature, is a more dense material with less porosity and has a higher water holding capacity.  Therefore incorporation of too much compost can create the potential risk of compaction or excessive water holding in the mix.

When Persistence Doesn’t Pay Off

Most herbicides break down during the composting process through a variety of physical and biological interactions.  However there have been reports of some herbicides that are persistent after the composting process, resulting in a residue that could damage plants grown using the compost (see this paper for some examples).  Many of the reports show the damage manifesting in mostly large applications of compost to gardens.  However, the more fragile nature of germinating seeds and young seedlings make them especially susceptible to herbicide residue damage.  For further discussion (and examples of bioassays used to detect herbicide residues), check out this paper.

So the potential for pathogens, risk of improperly matured compost, effect on porosity, and potential for herbicide persistence present some significant risks to germination if they are incorporated into seed starting media.  These are the risks that cause many sources to promote using sterile seeds starting media, and I think the advice is well founded.  While some may not experience these possible issues, the potential is still there.

Prepping Your Garden for The Next Growing Season

William H. McCaleb, Blog Contributor
Program Assistant for Agriculture and Natural Resources, Halifax County, VA. and Master Gardener

For gardeners in the eastern U.S., last year was a better than normal gardening season. Better than normal yield, better than normal precipitation, and in our case in Virginia cooler than normal which yielded excellent spring cool season crops as well as early summer crops.

But all good things must come to an end; that being the result of several heavy frosts.   With that said, I am looking forward to next year’s challenges and what I want to grow for our family. Oh, for the taste of one more summer ripened tomato, but for now, that is a dream and it is time to reflect on what grew well in the garden as well as what didn’t do so well.  Hopefully you have kept a garden journal to help you in this task. I find that writing down details of what is planted, the orientation, spacing, fertilization/liming rates and frequency, weekly rainfall amounts, production amounts, etc. is helpful as you start planning for the next season.

Like me, you should start thinking about what you want to grow in 2015. Take time to reflect on your 2014 garden production, care, and location. Also, evaluate what went right and what went wrong with the plants and varieties you planted and harvested. This will start you off in the right frame of mind in preparing for the next growing season. Good planning and preparation for next year gives you the tools to have an even better gardening season. A successful vegetable gardener is a happy well fed gardener!

I know, you too are already missing those fresh tomatoes, potatoes, peppers, squash, okra, and other great home grown vegetables we treated ourselves to this year, but the next season is ‘just around the corner’ so to speak. After all the days are getting a little longer. Spring can’t be far away!

If you just happen to live in an area that hasn’t had frost yet, take your prompt from your plants: when annuals and seasonal vegetables turn brown and begin to die back, it is time to clean up your garden.

Clean up the Garden
Your best action is to remove any spent or failing plant materials. Experienced gardeners know that many of the bacteria, fungi, and other disease-causing organisms that caused those diseases. Pathogens that are sources of those diseased plants this past season can survive over the winter in dead leaves, stems, roots, and dropped fruits that get left in the garden. Much like a piece of bread that is kept too long and looks like it has penicillin growing on it, garden debris also will carry the pathogens that can come alive with those same problems when the temperatures begin to rise in the spring. Prevention of diseases and insect infestation now, will keep you from a repeat of problems in next year’s garden.

A good leaf rake, given enough ‘elbow grease’, works well in getting the bulk of dead plant material out of your garden. If you experienced early or late blight or other tomato related diseases this past growing season, you want to make sure you reduce, to the best of your ability, the risk of repeating that problem again next year. Yes, there are many new varieties of vegetables available today that are ‘resistant’ to some of these diseases, but ‘resistant’ does not mean they are immune to them. You don’t want to take the chance of returning pathogens, so do a good job, cleaning and ‘sanitizing’ your garden now. Make sure, when removing the plant debris, that you totally destroy that debris so that no pathogens are left behind.

To Compost or Not!
Can you compost this dead plant material and use it next spring? Information that you find from Extension offices across the U.S. will recommend that you do not. The reason being is that most people do passive composting i.e. put it in a pile, and then using what compost develops, put the compost back in the garden for the next season. It is best to burn the plant material; this will destroy the pathogens and weed seeds as well and return some carbon back into the ground when you spread it out. Please check local/state laws prior to burning. Many states and/or localities have burn bans especially this time of the year. Another method, if your local law allows it you can bag the material and send it to the landfill. Each year there are more localities that ban yard waste from their landfills. If you are not sure, check with your locality to learn more about your local waste and recycling laws.

If you do decide to go with active composting; composting at a temperature 140°F, or higher, will destroy many of the disease organisms as well as many weed seeds. You will need a temperature probe to monitor compost temperatures.   It’s really not hard to source a compost thermometer either through the internet or local retail outlets such as garden centers or nursery supply stores. If in doubt about your compost pile reaching these high temperatures, it is best to side with caution and discard the material by properly bagging it or by burning based on your local ordinances.

Preventing Overwintering Pathogens
Some of our most notorious insects of the garden such as Mexican bean beetle, squash vine borers, European corn borer, cabbage loopers, can also overwinter in garden debris. Larvae will use debris as a safe harbor. Flea beetles and spider mites, as well, can find food and winter shelter in spent plant material and weeds.

After you have finished cleaning up the debris from your garden, it is time to turn over the soil to both aerate and break up any remaining debris into smaller pieces that will be turned under. A good rototiller will help make this job easier. Once buried, any plant material left will decompose more rapidly.

For some pests and pathogens, turning over the soil after removing spent plant materials is recommended as the main line of defense against overpopulation next year. Consider this information from “Home and Horticultural Pests: Squash Bugs and Squash Vine Borers,” from Kansas State University,

“A vigorous autumn… rototilling can physically destroy cocoons and larvae (of the squash vine borer). Brought to the surface, cocoons and larvae are more susceptible to predation by birds and exposed to cold winter elements, leading to their demise. Deep plowing physically destroys cocoons and larvae burying them deep beneath the soil surface so pupated moths become entombed underground.”

Steps to a Healthier Garden
If you haven’t done a soil test in three years or more, it is time to retest and determine the needs of your garden soils based on what you will be growing in the next season. Soil test kits and instructions are available from your local Extension Office. Also, in planning next year’s garden, rotation of your crops is a must do item. This simple action will help keep disease issues down.  If your soil test(s) recommend liming, you can go ahead and put down lime this time of year, allowing it to start adjusting the pH. If the ground is frozen already, wait until spring. As you add lime, you can also help build soil structure by incorporating compost or shredded leaves. These soil additives will also add beneficial micro-nutrients and beneficial organisms. If you want to further build the soil, you may want to consider putting in a cover crop that will not only hold soil, but when tilled in early spring, will further build a healthier garden soil. A legume such as white or red clover would be something to consider. Check with your local Extension Office for best cover crop recommendations for your area.

Prepping Your Garden for the Next Growing Season (pdf)

 References:

http://pubs.ext.vt.edu/426/426-334/426-334.html
http://www.ksre.ksu.edu/bookstore/pubs/mf2508.pdf

image sisters
“Three Sister’s Garden-Fall Clean-up “Southern Virginia Botanical Gardens” Photo by W. McCaleb 10/28/14 Corn, Beans, and Squash was grown here as the native Cherokee have done for centuries. Cleaned up and ready for spring 2015!

 

 

 

Mortal Kombat – garden version

Soil solarization is regarded as an environmentally friendly alternative to pesticides for controlling nematodes, weeds and disease.  Sheets of plastic (generally clear) are spread over the ground and solar energy heats the soil underneath to temperatures as high as 55C (or 131F).  Since the soil environment is usually insulated from temperature extremes, the organisms that live there are unlikely to be resistant to heat stress.

This is a practice best suited to agricultural production, where monocultures of plants have attracted their specific diseases and pests.  Decades of research have shown success in controlling pests in greenhouses, nurseries, and fields.  But there’s a down side to this chemical-free means of pest control.

It shouldn’t be surprising that beneficial soil organisms, in addition to pests and pathogens, are killed by solarization.  Studies have found that soil solarization wipes out native mycorrhizal fungi and nitrogen-fixing bacteria.  One expects that other beneficial microbes, predacious insects, and parasitoids living in the soil (but so far unstudied) would be eliminated as well.

This may be an acceptable loss to those who are producing crops; soil can be reinoculated with mycorrhizal fungi, for example.  But for those of us caring for our own gardens and landscapes, this is literally overkill.  (And consider that most of us probably have trees and shrubs whose fine roots extend over our entire property.)

So this spring, instead of solarizing your soil, consider some less drastic measures of pest and disease control. Minimize soil disruption to preserve populations of desirable microbes. Plant polycultures (more than one species) in your vegetable garden, or at least practice crop rotation.  Protect and nourish vegetable gardens with compost.  Use coarse organic mulches, which provide habitat for beneficial insects and spiders, in landscaped areas.  Above all, try to treat your soil as the living ecosystem it is, rather than a war zone.

Parking tickets, compost tea, and pseudoscience in the Ivory Tower

Back in November 2009, Jeff posted an educational and amusing commentary about Harvard’s use of compost tea. Much vigorous discussion followed, and we’ll return to that topic in a moment. But first, I’d like to tell you about my morning yesterday.

In September of 2010, I received a ticket for parking longer than 2 hours in a restricted zone. Now, there was no way I committed this infraction; I had hard core proof that could not be rationally challenged. So, armed with my husband’s affidavit as to my whereabouts, as well as a dated receipt showing I was at the post office at the time when I was apparently parked several miles away, I went to court to challenge the ticket. During our briefing, the sitting magistrate told us we would need to provide a “preponderance of the evidence” to win our respective cases. For me, it was an anticlimactic turn of events, as the citing officer (whom I’d subpoenaed) did not show up, so the ticket was dismissed for lack of evidence.

And thus we return to today’s subject – use of compost tea without a “preponderance of the evidence.” Jeff took Harvard to task for buying into this “bullpucky”, I think he called it, and now Berkeley has decided to drink the Kool-Aid. One of my dear colleagues at University of Washington forwarded me a link announcing that Berkeley Botanic Gardens was adopting compost tea as an “eco-friendly fertilizing method.”

As the article reports, compost tea is being used

1) as a disease suppressant
2) to provide nutrients, and
3) to reduce the amount of water needed.

I’ve written a lot about compost tea, and I’ve reviewed journal papers on the topic as well. In a scientific nutshell, there is no solid evidence to support use of compost tea, particularly aerated compost tea, in disease suppression. Likewise, there is no evidence to support a nutritional role (I just finished reviewing a manuscript on this topic and the data were unconvincing). Finally, I cannot understand why spraying compost tea onto the leaves of a plant would reduce its water requirements. The “preponderance of evidence” is truly lacking.

Students at Berkeley have the dubious honor of supporting this nonsense through their student fees: $11,000 has been spent on a 300 gallon tank, worm composting bins, and a spray tank.

Whatever happened to using good old compost, and letting nature create its own “tea?” (Compost used as a mulch also helps reduce irrigation needs.)

Building healthy soils?

I love living in Seattle…but I’m getting increasingly impatient with the City’s “Building Healthy Soils” propaganda.  For years I’ve questioned their recommendation to perpetually amend landscape soil with organic material to no avail.  Let’s see what you all think of their “fact sheet” (which you can read here in its entirety).

“The best way to improve the soil is to add plenty of compost or other organic matter throughout the entire planting area before planting. Thoroughly mixing these materials deep into the soil helps provide water, air and nutrients to plant roots.”

Hmm.  No mention of how to determine IF your soil needs improving; without a soil test, you have no idea what your baseline organic matter level is.

But perhaps this recommendation is only for vegetable gardens and annual beds?  Nope.  In the next paragraph, we’re told to “Mix in organic material before planting lawns, perennials, trees and shrubs.”  We’re given helpful how-to instructions:  “Use a shovel or digging fork to mix amendments into the top 6 to 12 inches of soil. It is important to amend the entire planting bed — not just small holes for each plant. When planting individual trees and shrubs in lawns or existing beds, amend an area at least 3 feet wide, or 3 to 5 times as wide as root balls over 12 inches in diameter. Rototill large areas where digging is impractical.”

Now we’ve got a serious problem.  This practice is recommended for existing beds.  Not only will extensive digging or rototilling destroy any soil structure you might have, it will also take out the roots of any desirable plants in the vicinity).

But let’s continue to ignore reality and go on to the annual recommendations for adding compost to soils.
“Clay soils: 16 cu. feet (.6 cu. yard) = 2 inch layer of compost for new gardens. Use 1 inch per year in established gardens.”
“Sandy soils: 24 cu. feet (.9 cu. yard) = 3 inch layer of compost for new gardens. Use 1 – 2 inches per year in established gardens.”

Is the compost used as a mulch in these existing gardens?  No – the guidelines are prefaced with this instruction:  “Gardens: mix compost to 10- to 12-inch depth.”  (Can’t say this does much for promoting root growth either.)

This document shows a breathtaking lack of understanding of how landscapes function, especially over the long term.  It takes an agricultural practice (annual organic amendment of crop fields) and misapplies it to permanent landscapes.  It is devoid of the research which continues to show that improper soil amendment can cause serious problems such as soil subsidence, perched water tables, and nutrient overloads.  This last point is especially important to anyone living near aquatic ecosystems, since excess nutrients always end up in the water.

Before you plant this year, find out what your soil needs before amending it.  And remember that mulching is the natural (and sustainable) way to add organic matter to the soil.