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.

In general, the goal should be to maintain nutrient elements within the high or optimum range as reported on the soil test. When nutrient levels are within this range, the needs of most crops will be met. If levels are low or medium (below optimum), most crops would benefit by increasing levels to high or optimum. However, if levels are at above optimum or very high levels, there will be no additional benefit and excess levels may reduce crop yield or quality and may cause environmental harm. This has happened on some organically managed fields where soil testing was not used to monitor fertility levels. When a nutrient is above optimum levels it should not be included in any amendments until the excess is used up by crops. It may be wise to temporarily stop applying compost until nutrient levels are in the desired range. Standard soil tests can be performed at any time, but testing for nitrogen should be done in the summer when this nutrient is normally at its highest level. Seepre-sidedress soil nitrate test (PSNT) for sampling and handling information. It is important to keep in mind that factors other than nutrients may limit crop potential, and simply adding more nutrients will not solve such problems.

Organic matter management is an essential part of organic agriculture. Generous additions of compost, animal or green manures are needed to feed soil microbes, the by-products of which bind soil particles together to improve the physical condition, or structure of soil. Good structure promotes root growth and thus enhances plant retrieval of soil nutrients.

Decaying organic matter releases a slow, steady supply of nutrients to a crop so long as soil temperature, moisture, pH, and aeration support microbial activity (as when soil is properly drained and well warmed). When this release of nutrients, or mineralization, is low, as when soils are cool, fertilizing with soluble forms of nutrients may benefit crops. This is why some soluble phosphorus (P) and nitrogen (N) should be banded, or placed near the roots of crops early in the growing season. For example, use bone meal and dried blood to provide soluble P and N, respectively, or use a commercial organic fertilizer blend. See Table 7, for the nutrient content of various organic materials.

Nitrogen Up to half the N contained in manures and immature compost can become 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. Similarly, 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. Many 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. That requires 200 lb dried blood, 400 lb soy or cottonseed meal, or the equivalent from other sources of N.

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. CAUTION: Many growers use large amounts of rock powders such as rock phosphate to try to build fertility. Over time this can lead to levels way above optimum. To allow P levels to come down, no amendments containing P should be applied including compost. This is unfortunate since compost is an important part of good soil management.

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 low to medium, high-Mg limestone, or dolomite, should be used for liming. If Mg is high/optimum, use calcite, or low-Mg lime. Choose your lime and fertilizer materials with the goal of attaining the desired percent base saturation of 2-5% K, 5-15% Mg and 60-85% Ca, or a 20:4:1 ratio of Ca:Mg:K in the soil. This is a ballpark figure, but is important along with establishing the proper soil pH of (6 to 7 for most crops) and supplying nutrients shown to be below optimum or high by a soil test. (see base saturation on page 3).

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 temporarily alleviate Mg deficiency. Dissolve 1.5 lb per 10 gal water and spray at weekly intervals.

Phosphorus is low in many New England soils, and can limit crop growth, especially early in the season. Soils testing low to medium in 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 slowly available P2O5/acre. Becauserock phosphates provide P very slowly, growers have often applied high amounts over a number of years. When the P eventually became available,soil levels have become excessive.Bone meal contains about 20 times as much available P2O5 by weight. Phosphorous from bone meal is more rapidly available than it is from rock powders, but it is more expensive. With soils low in P, it can help crops to place proportionally more P fertilizer in the crop row than to broadcast it evenly. Manure is an excellent source of readily available P2O5. Maintain a pH of 6 to 7 with limestone to maximize P2O5 availability. Compost and manures tend to contain less P2O5 than N or K2O, but repeated applications will raise P levels substantially. Repeated use of these materials has resulted in excessively high soil levels. Nutrient levels should be monitored with regular soil tests.

Potassium is very slowly available from granite dust and greensand, which are 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. Sul-Po-Mag is the K fertilizer of choice when Mg is also needed. Potassium sulfate from natural sources is an excellent choice when K is needed but Mg is not.

Minor elements are generally sufficiently supplied to plants by regular additions of organic matter to the soil. 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, such as crucifers. In this case, 1 to 2 lb/acre of B should be applied to the soil. 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, such as beans and peas are quite sensitive to boron (see Table 2).

Table 7: Fertilizers Approved for Organic Production

Typical Nutrient Content (by Weight) of Organic Materials (Nutrient content varies)

Typical Nutrient Content (by Weight) of Some Mineral Deposits (Availability varies with soil pH, origin, and depends largely upon fineness of grind)