Soil fertility is a function of the biological, physical and chemical characteristics of soil. An organic fertility program should consider all of these interrelated factors in order to optimize and sustain crop production.
Soil tests are useful for monitoring soil organic matter content, which influences the physical and biological quality of soil. Soil tests also estimate the level of chemical nutrients in the soil that are available to plants. This helps determine the quantity and type of soil amendments needed for good crop yields.
Organic matter management is an essential part of organic agriculture. Generous additions of compost, animal or green manures are needed to feed soil microbes, but organic growers need to carefully monitor soil test P levels when adding organic amendments to the soil (see discussion below Building Soil Organic Matter). Organic matter management is essential because the by-products of decomposition of organic amendments bind soil particles to improve the physical condition, or structure of soil, and also because organic matter is the storehouse of nutrients in the soil. Many nutrients, especially N, P, S, Cu, and Zn, are released when organic matter decomposes. The good structure promoted by organic matter results in enhanced root growth, which increases plant retrieval of soil nutrients, which is a classic synergistic effect. Decaying soil organic matter releases nutrients unevenly during the growing season. In the late spring after the soil warms there is usually a flush of nutrients, and the rate declines after that with the rate during the season dependent mostly on soil moisture. When the release of nutrients, or mineralization, is low, as when soils are cool in the early spring, fertilizing with soluble forms of nutrients may benefit crops. This is why some relatively available phosphorus and nitrogen should be banded, or placed near the roots of crops early in the growing season. For example, use bone meal and dried blood or a seed meal like peanut or soybean to provide some available P and N, respectively, or use a commercial organic fertilizer blend. Information on the nutrient content of various organic soil amendments can be found at MOFGA Fact Sheet #11(link is external).
Nitrogen is the most common limiting nutrient on organic farms. The most common sources of nitrogen are seed meals, fish meal, blood meal and livestock manures. Most sources of nitrogen used by organic farmers are expensive and that explains why most growers turn to livestock manures, compost or crop rotations with legumes. When using manure or immature compost, remember that only up to half the N becomes available to plants during the season following incorporation. Each ton of compost containing 1% N can provide a crop with 5 to 10 lb of N per acre. When calculating N needs remember, as discussed above, that there is a release of about 20 lb/acre or more of N for each 1% soil organic matter. These releases of N vary with drainage and other soil conditions, and may not be well timed to crop needs, especially early, short season crops. Annual crops need N most intensely about three to four weeks after emergence or transplanting. Therefore, sidedressing, or spreading soluble N along the crop row, at this time is most efficient. Because soluble organic N fertilizers are expensive, it is advisable to use lower rates than recommended for synthetic fertilizers. A sidedressing of 25 lb/acre of actual N is reasonable for many crops growing in a fairly fertile soil. This requires 200 lb dried blood, 400 lb soy or cottonseed meal, or the equivalent from other sources of N.
Phosphorus is low in many New England soils, and can limit crop growth, especially early in the season. Soils testing less than 10 lb/acre available phosphate (P2O5) usually require substantial applications of phosphate. Hard rock phosphate contains about 2% available P2O5; soft, or colloidal, rock phosphate contains 3% available P2O5. Thus, a ton of these materials provides only 40 to 60 lb available P2O5/acre. Bone meal contains about 20 times more available P2O5 by weight, but is more expensive. Bone Char contains 16% available P2O5 and is less expensive than bone meal. With soils low in P, it can help crops to place proportionally more P fertilizer in the crop row than to broadcast it evenly. Maintain a pH of 6 to 7 with limestone to maximize P2O5 availability. Compost and manures tend to contain P2O5 as well as N or K2O. Repeated applications will raise P levels substantially and care must be taken to avoid building excessive P levels in the soil that could lead to contamination of ponds and lakes. Monitor P levels and adjust compost or manure applications accordingly.
Sul-Po-Mag is the Potassium fertilizer of choice when Mg is also needed. Potassium sulfate is commonly used when no Mg is needed. Potassium becomes very slowly available from granite dust or greensand, which may be applied at 3 to 5 tons to the acre to build up K reserves. Wood ashes contain soluble K, but must be used with caution because they will raise the pH rather rapidly and can be caustic. The liming effect of 1 pound of ashes is roughly equal to 2/3 of a pound of limestone. No more than 1/2 ton of ashes per acre should probably be applied at once, and only then if called for by low pH, low K and sufficient Mg.
Magnesium is best applied as dolomitic lime, but when liming is not required, other Mg sources are Sul-Po-Mag or Epsom salts. Sul-Po-Mag is the better choice if potassium is also required, as it is less expensive than Epsom salts. However, Epsom salts can be applied as a foliar spray to alleviate Mg deficiency. Dissolve 1.5 lb per 10 gal water and spray at weekly intervals.
Micro-nutrients are generally sufficiently supplied to plants by regular additions of organic matter to the soil. Wood ash is another excellent source of micronutrients. Some seaweed extracts may also supply trace minerals. In soils low in boron (B), remedial applications are widely recommended for crops that readily suffer from B deficiency. In this case, 1 to 2 lb/acre of B is applied to the soil with other fertilizers. It is difficult to apply such a small amount uniformly, but boron can be ordered as part of a fertilizer blend. Most boron products are soluble and can sprayed evenly over the soil. Several forms of B are OMRI listed, including Solubor, Fertibor and Biomin Boron. It is advisable to monitor B levels with soil tests and tissue tests (for perennial fruits). Excess levels of B are toxic to plants, and some crops are quite sensitive to boron.
Rock powders can be used, along with organic matter, to build up and balance soil reserves of plant nutrients. However, these are not very soluble nutrient sources, and are not effective for treating short-term nutrient deficiencies. Using some soluble fertilizers may be advisable while building soil reserves of plant nutrients with rock powders and organic matter.
Limestone is a widely used rock powder. It raises the soil pH and provides calcium (Ca) and varying amounts of magnesium (Mg). When Mg tests below about 100 lb/acre, high-Mg limestone, or dolomite, should be used for liming. If Mg is above about 150 lb/acre, use calcite, or low-Mg lime. Choose your fertilizer materials considering the desired 20:4:1 base saturation ratio of Ca:Mg:K in the soil, but remember, this goal is only a ballpark figure and is definitely secondary to establishing the proper pH of 6 to 7 for most crops and supplying nutrients shown to be deficient by a soil test.