Good Fruit Grower

April 15

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www.goodfruit.com GOOD FRUIT GROWER APRIL 15, 2015 33 Bair recommends that growers learn the following about any finished compost they plan to use: —pH (a typical range is 5.5 to 8.5) —Moisture (15 to 40 percent range) —Electrical conductivity or soluble salts —Carbon-to-nitrogen ratio (if using as a fertilizer, should be less than 25:1, if as organic matter, should be 25-40:1) —Plant-available nitrogen —Other nutrients, such as phosphorus, potassium and sulfur —Micronutrients, such as boron, zinc, copper, and manganese Additionally, tests can be run on carbon dioxide evo- lution and ammonia (NH 3 ), which are compost maturity indicators. Nutrient analysis, carbon-to-nitrogen ratio, and pH determine quality and how to best use the compost. "As a general rule, the closer in contact with growing plants, the higher quality and more stable you want the compost to be," said Carpenter-Boggs. "A long curing phase is important because that's when humic acids build and new microorganisms come back into the material." Lower quality or less mature composts still have value, but should be used as large-scale field amendments, top dressing, or mulch. "You'll still get some benefits, like retaining soil moisture, protecting superficial roots, and building organic matter, but if it's going to be in direct contact with roots, the compost needs to be really stable," she said. One of the last things to occur during the curing phase is conversion of ammonium to nitrate. A range of at least 400 parts per million of nitrate is a good indication of mature compost. Slow release When using compost as a fertilizer, be ready for a long wait. Nutrient release from compost happens in years, not weeks or months like inorganic fertilizers. "Nutrient release happens very slowly and is usually around 10 to 15 percent of the amount applied in the first year," said Carpenter- Boggs. Mineral fertilizers like greensand, rock phosphate, lime, azomite, or other rock powders can be added to the compost to increase mineral bioavailability. Compost can also be turned into compost tea for a faster way to extract the nutrient benefits. In a best-case scenario with a high-nitrogen content compost of 5 percent and a 25 percent release rate, 200 pounds applied per acre will release about 2.5 pounds of nitrogen the first year, 1.9 pounds of nitrogen the second, and so on, she says. "It takes three years to get half of the nitrogen out of the application and about nine years to get 90 percent of the nitrogen out of the application." Common application rates of compost are typically between 1.5 and 10 tons of dry compost per acre for annual application, but rates of 50 tons of dry compost per acre are not uncommon if applied sporadically. Carpenter-Boggs said studies have shown that the peak of nutrient release from compost occurs in early to mid-summer, which is also the time of peak nutrient uptake by many plants. The slow nutrient release can be good or bad, depending on your plant needs. Slow release is not necessarily bad, but has to be factored into your nutrient management, she said. • "As a general rule, the closer in contact with growing plants, the higher quality and more stable you want the compost to be." —Lynne Carpenter-Boggs

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