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The Garden Plot for June 06, 2008
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: “Compost Options”
We have been getting a lot of questions about composts: which are and are not organic? The confusion is because there are two “organic” definitions. The legal definition for “certified organic compost” is defined at the federal level by the National Organic Program. For information about allowable certified organic fertilizers, your can call the Montana Department of Agriculture Organic Certification Program. The biological definition of organic compost is: anything made from animal or vegetable substances, such as manure, wood bark, or grass clippings.
Compost is a good fertilizer for the garden no matter what definition of organic it carries. All composts increase soil organic matter and provide food and shelter for soil microorganisms. All composts provide a slow-release, stable form of plant nutrients. The best vegetable garden plant-producing compost has a carbon-to-nitrogen ratio of 20:1. The lower the ratio, the more nitrogen that is available for our plants. Heavy nitrogen feeders, such as vegetable gardens, want composts with lower c-to-n ratios. Compost pH should be between 6.0 and 7.5. Montana soils often start out with a high pH (greater than 7.5), so checking the pH of composts and soil amendments before applying them can make a big difference to your plant’s growth. Good compost should also contain a balanced level of plant nutrients. Commercial composters usually supply a nutrient analysis. Ask for one if necessary. In general, nitrogen, phosphorus, and potassium in a ratio of 1:1:1 is best, depending on your garden’s soil fertility. Composts can be applied earlier in the spring and later in the fall than synthetic fertilizers and uncomposted manures, because the nitrogen in them is more stable and less easily leached.
Composts differ based on the materials of which they consist. Wood products in a compost, such as composted wood bark, provide good aeration and a lower pH for high pH soils. Wood products provide calcium and phosphorus, but little nitrogen.
MANURES are good sources of nitrogen, but may have excessively high pH. In long-term studies, manures applied to vegetable gardens slowly increase soil pH over the years. POULTRY and LLAMA MANURE, have low C:N ratios and are very concentrated. Their nitrogen is more rapidly available; thus, the chance of burning young seedlings is higher. SHEEP MANURE is the next most nitrogen-concentrated manure. HORSE MANURE has lower levels of nitrogen and phosphorus than other manures, but the highest number of weed seeds. COW MANURE, Like horse manure, is a balanced soil amendment, but cow manure provides higher levels of nitrogen and phosphorus. The more composted a manure is, the more stable the nitrogen. This means that it takes longer for the nitrogen to become available to plants, but there is less chance of groundwater contamination and burning of young seedlings. If the manure still smells and is sticky, like mud, it is probably not well composted. Composted manure has a lower pH than uncomposted manure, usually 7 to 7.5. A good pH and nutrient balanced compost is a mix of manures and wood product materials.
There are several new Montana-made commercial composts available now. They are composed of manures and wood product materials. In our tests, they were found to have high nutrient levels and a relatively high pH. As long as your garden soil pH is not too high, these composts would be a good amendment choice. There are also several local commercial composts made from wood products and biosolids (human manure). These composts have a lower pH and relatively high nutrient levels. Biosolid-containing composts are organic if you apply the biological definition, but not the legal definition. Biosolid-containing composts are not allowed in certified organic production.
For more information on manures, composts, and fertilizers, talk to your County Extension agent.
Helen Atthowe's new short program of gardening tips
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