Spring 1998

From the Farmer to Farmer Conference • November, 1997

Spring comes. You transplant your seedlings into a potting mix made with your own (or purchased) compost. Your seedlings, which were healthy, green and growing before being transplanted, turn yellow and may even shrink. What went wrong?

As participants at MOFGA’s Farmer to Farmer Conference learned from Will Brinton, owner of Woods End Lab in Mount Vernon, Maine, the problem could have been caused by excess salt in the potting mix (from the compost) or from using a compost that hadn’t completely matured. These and other problems are quite common in composting operations, Brinton explained.

He began his talk with a slide show of composting and growing operations in the United States and Europe to make the point that “compost quality determines the success or failure of a compost mix.” He was hired by Longwood Gardens in Pennsylvania, for example, which recycles “huge amounts” of potting mixes, grass, plant debris, and so on. Longwood was “amazed that we could be successful [at making a compost] without steam sterilization or fungicides.”

Brinton then showed EPCOT – which, he explained with quiet humor, stands for Experimental Prototype Community of Tomorrow – where highly technology-dependent hydroponic growing systems are used. The prototype community is, ironically, not sustainable, said Brinton. “There’s so much control required, so much wastage. The big greenhouses are polluting the groundwater with nitrates.” Instead of this kind of high-tech, energy dependent, polluting kind of operation, “we want a system that controls pests and provides fertility all around the clock.”

He showed a slide of a biowaste facility in Europe that recycles leaves, grass and food scraps. While these operations are quite common there, “organic farmers are not using [the compost] – they’re skeptical about the quality.”

Also in Europe, Brinton has seen a very strong anti-peat movement developing and a movement to coir (coconut fiber) instead. Coir can be mixed 50% with leaf and yard waste compost to make soil blocks, Brinton explained. The coir works well, “but it doesn’t have as much water holding capacity as peat.”

Another slide showed a composting unit made by a Vermont company; it is put in the basements of high rises to compost kitchen wastes from people in the apartments, and the residents can then use the compost in their houseplant mixes or rooftop gardens.

Regarding municipal compost, he said that it varies tremendously in quality because no quality control is exercised. “Let the buyer beware,” he said. “Commercial growers don’t know what they’re getting [when buying municipal compost] and have moved away from taking free, community compost.”

One of the quality problems relates to salts. “Biosolids composts or chicken manure compost are not generally suitable to potting mixes unless they’re heavily cut with inerts,” Brinton explained. “You need [a maximum of] 10% compost of this kind to avoid salt problems – but then you don’t have enough other nutrients to meet the plants’ needs.”

Another quality problem concerns the stability and maturity of compost. “One that swells up a plastic bag [because it is not mature and is still composting] is not good for potting mixes,” Brinton said. “Its off-gassing – including ethylene – can be very damaging to seedlings.” He told about one company whose compost was drying out and therefore cooling, making it seem as if the composting process were finished. “But the drying was just inhibiting the biological process,” Brinton went on. “Then it continued composting after it was sold and put in a pot. Basically, compost has to be watered, just like a garden – with an inch a week; it should be moistened just to the point where you can almost squeeze water from it when it’s made into a tight-fisted ball.”

At Woods End, Brinton and his colleagues test potting mixes made with compost by growing garden cress in them. Garden cress “is very sensitive and fast growing. It’s good to compare potting mixes.” For a control, they use ProMix BX, which has a chemical starter fertilizer solution in it. “Your seedlings in compost mix should grow as well as those” in the ProMix, he said. “If not, investigate your compost.”

He also told of a closed jar cress test developed by Jacques Fuchs of Switzerland. Put some compost in a jar, he said, then put some cress seeds in; seal that jar tightly. Do the same with another jar that is not sealed. If the compost is not mature, it may give off gases that will harm the plants. Brinton recommended mixing 25% compost, 75% peat and 2% lime, letting the mix sit for a week, then doing this test. “If [the cress] does well, then try pushing [the compost component] to 40 to 50 percent.” He warned, however, that “it takes a special compost to go that high without problems.”

In Germany, said Brinton, material can be sold as “finished compost” only if it heats less than 20 degrees C. above the ambient temperature when put in a Dewar flask, which is like an elaborate thermos. The compost takes one to seven days to heat in the flask. “The hotter they get, the less mature they are,” he said.

Another way to test the maturity of a compost is to grow a plant in a pot of it, then drop the plant out of the pot. If its roots are proliferating at the edge of the pot, the compost is immature; the roots are “growing away” from it because of the oxygen stress. If the compost is mature, you’ll have a nice rootball.

Brinton described three possible uses for compost. First, it can be used as a fertilizer substitute – not a potting material – if it holds nutrients well, is easy to handle, and has a high N-P-K value; second, it can be used in a potting mix if it is porous, has a high water holding capacity, is mature, has a high concentration of available nitrate-nitrogen, and is low in salts; third, it can be used to control diseases if it is mature, has a good microbial content and if fungal or bacterial targets are present. Regarding this third possibility, he said that it is amazing “how much money is being spent on fungicides at nurseries. You can go in with compost and take that whole line out of their budget.

“Once I get a good potting mix, I test it for disease control,” he continued. “I inoculate the mix with Pythium or Rhizoctonia. Some [composts] will totally stop that infection immediately. Maturity really determines how well a compost will control disease.” The worst case, he said, is when a compost is excessively wet, is stored in a cold place over the winter, then is brought into the greenhouse in the spring. Disease can become rampant then. The best case is when compost is made in the summer, is stored in a root cellar over winter and is not allowed to get overly wet. “Moisten it and stir it with your hand during the winter to make sure it stays moist,” he advised.

Commercial, chemical blends of potting mix have no disease control potential, said Brinton. “Usually with 30 to 40% compost, you get disease control below the economic threshold.” He projected a dramatic slide showing that 10% compost stopped take-all disease in wheat grown in pots, and said that the same results had been obtained for almost all vegetable crops, for two reasons: microorganisms in compost compete with pathogens for food; and microorganisms in compost create “antibiotic” metabolites against fungi.

Brinton then showed a slide of a Swiss man who makes compost from grass and hay piled in windrows that are just 1-1/2 feet high. The grass is allowed to grow to 10 inches to a foot high before it’s cut so that it has some cellulosic structure before it’s put in the compost. The windrows are turned, then the material is put in small piles. “He has amazing compost in six or seven months,” said Brinton. “It’s pretty rich in nitrogen at the end. He puts it in the root cellar that he built just for compost, where it continues to mature. He uses it the next spring. His plants are amazingly healthy.”

He also told of an organic/biodynamic grower in Massachusetts who makes compost potting mix from leaves and grass and won’t use peat. The point, he said, is to “give your compost piles value. Pay attention to them. Visit them once a week or at least once every two weeks.”

Brinton warned that some organic growers “are at the edge of the salt problem now from heavy compost applications.” He cited as an example a greenhouse where compost was applied regularly but from which water was never lost. “They needed an irrigation system like nature itself – oversupplying water now and then.” Brinton told the grower to remove the plastic from his greenhouse for six weeks so that rain could leach the salts.

In Maine, Longfellow’s Greenhouses in Manchester “has really worked on bringing compost into its operation,” said Brinton. “It’s a good place to visit.”

Regarding specific ingredients and specific brands of compost, Brinton said to “watch out for chicken manure and fish wastes: You’re introducing a big charge of salts. If you’re using chicken manure, reduce the amount of compost used in your potting mix.” A Maine company that is actually selling a compost-based seed starting mix is using chicken manure, he continued. “They dilute it with peat and are aware of the salt problem, but check it occasionally and ask them for salt numbers.” (These are explained below.) Another Maine company hopes to have a new dairy manure and fish waste based compost in the spring. The company is working with Brinton and with Eric Sideman on the product. A Vermont company was “mortified when they got some complaints” about salt and maturity problems.

Some of the measures of a quality compost include the salt concentration, pH, carbon to nitrogen ratio, and density. The salt concentration should be 1 to 3 millimhos per cm or ds/kg. “You can get a salt testing kit – it’s like a small pen and is not even as complicated as a pH meter,” said Brinton. “It costs $80 to $90 and lasts years. You just shove it in with added water. Create a salt consciousness for yourself.” While a value of 3 is considered high, “3 in a chemical based mix is worse than 3 in a compost based mix,” because the salts in the chemical based mix are chloride salts, and chloride is worse for plants than phosphates and sulfates, which are the main salts in compost based mixes. “We can push our mixes to 3 and get good growth and germination,” he said.

The pH should be 6.0 to 6.5. Raw sphagnum peat has a pH of 3.8 to 4.5, which can be brought to about 6.2 by adding 2 to 3% limestone by weight at least five days before mixing the media, in order to give the lime time to react. If you’re using wood ashes to raise the pH, you should be aware that the salt levels in this material “are unbelievable,” said Brinton. “More than 1/2% in a potting mix will be too high. I gave up working with ashes: If you make one wrong calculation, you’re out of the range. People take their ashes out and dump them on their compost. You have to watch out. I would use [ashes] in the garden and till them in, but not in a potting mix.”

The carbon to nitrogen ratio of the compost that’s going into the soil mix is important, said Brinton. After the compost is mixed with peat, you can’t interpret the C:N ratio. The finished compost should have a ratio of 15 to less than 25. “That’s being generous,” he continued. “At a ratio above 20, it’s not releasing nitrogen into the medium.” A test of the C:N ratio isn’t necessary, however, if you know that your compost is mature and the salt concentration is okay.

The density of the mix at field capacity should be 10 to 30 pounds per cubic foot. If it’s over 30, “you’ll have complaints. It will be so dense, it would never work in a speedling. You’ll have problems with water and air infiltration.” He warned that compost gets heavier as it ages. “I’ve seen it go from 10 to over 50 pounds as it loses organic matter.”

The concentration of ammonium-nitrogen in the compost going into the soil mix should be less than 100 parts per million, while that of nitrate-nitrogen should be greater than 500 parts per million. These amounts satisfy the needs of both microbes and plants.

Currently, you can get growing media tested at the University of Maine, the University of Massachusetts at Amherst or Cornell at Ithaca. Even there, “they may not understand these compost type mixes. I have seen them warn that the salt is off the chart, but it was okay. When you get state tests on compost, interpret them with a grain of salt.” Woods End tests composts and potting mixes for about $100 each. It also sells a hand-held respirometer that tests the maturity of a compost. (The U. of Maine test, which costs $10, checks pH, salt concentrations, nitrate-nitrogen, major and minor nutrients, and some potentially toxic nutrients. Send 2 cups of your sample (in a plastic bag and not dried) to the Maine Soil Testing Service, Deering Hall, Univ. of Maine, Orono 44469-5722 – ed.)

Responding to a question about leaf mold, Brinton said that this could be substituted for peat but should be at least a year old. “You don’t have to handle it very much. Keep it moist. Watch out for hidden lime – they’re sometimes sweeping up carbonates when they’re taking leaves from the streets.”

Also, whenever people put vegetable wastes in their compost, he encourages them to add lime, “otherwise the pH will be too low throughout the process.”

– Jean English

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