Guidelines for Organic Fertility Management

A strong organic fertility program considers the interrelated factors of a given soil’s biological, physical, and chemical characteristics to optimize and sustain crop production. Organic production emphasizes practices such as cover cropping and mulching to build soil fertility and to improve the physical and biological quality of soil. Bagged organic amendments also play an important role in organic production to supply essential plant nutrients to meet crop needs.

Organic matter management is the core of good soil fertility. Generous additions of organic materials, such as compost or green manures, are needed to feed soil microbes, which in turn leads to improved soil structure, aeration, and drainage. Improved water infiltration also indirectly supports healthy crops by promoting better root growth and helping plants access more nutrients and water. In addition, organic matter is the storehouse of nutrients in the soil. Many nutrients, especially N, P, S, Cu, and Zn, are mineralized and released when organic matter decomposes. 

Soil amendments used in organic cropping systems are typically complex, whole nutrient sources (e.g., compost, manure, seed meals, and rock powders). Since many of these amendments provide multiple plant nutrients, it can be challenging to maintain nutrient balance over time. As a result, excessive levels of certain nutrients (especially phosphorus) may build up, especially when compost- or manure-based materials are repeatedly applied to meet crop nitrogen needs. Soil testing provides a way to monitor these trends over time, enabling growers to adapt their nutrient management strategies to optimize yield, reduce costs of unnecessary nutrients, and minimize environmental impact. If a nutrient is rapidly accumulating, then adjustments should be made in the fertility program to provide only those nutrients needed. 

In general, the goal should be to maintain nutrient elements within the optimum range as reported on a soil test. When nutrient levels are within this range, the needs of most crops will be met. If levels are below optimum (very low or low), most crops will benefit by increasing levels to optimum. However, if levels are above optimum, there will be no additional benefit, and excess levels may reduce crop yield or quality, attract pests, and may cause environmental harm. This frequently occurs on organically managed fields where large amounts of manure or compost have been applied over the years. When a nutrient is above optimum its application should be minimized until the excess is taken up by crops. See Soil Testing. 

Calculating nutrient contributions for organic materials can be difficult because nutrients are released unevenly during the growing season and their release may not match the timing of crop uptake needs. The rate of release is dependent on the type of organic material, and largely determined by the C:N ratio of the material (lower C:N ratio = faster release rate). For example, compost, which primarily decomposes during the composting process and has a higher C:N ratio, is slower to release nutrients than manures, which have lower C:N ratios (see Tables 10 and 10a below). The rate of release during the season is also dependent on soil moisture and temperature, both of which impact microbial activity. Cool, wet or dry soils typically slow decomposition and mineralization. In the late spring after the soil warms there is usually a flush of nutrients released, and the rate of release usually declines after that. When the release of nutrients is slow, applying fertilizer with higher levels of plant-available nutrients may benefit crops. An example of this is applying available phosphorous and nitrogen in a band near the roots of crops early in the growing season, when the soil is still cold. See Tables 10 and 10a for the nutrient content and speed of nutrient release of several common amendments.

Nitrogen (N). Anywhere from 10%–90% of the N contained in compost, manure, and plant and animal byproducts may become available to plants during the season following incorporation (Tables 7, 7a). On average, there is a release of about 10–20 lb N per acre for each 1% soil organic matter over a full season. These releases of N vary with drainage and other soil conditions, and may not be well timed to crop needs, especially for early, short season crops. A crop that is in the ground for 3 months will get at most 10–15 lb N per acre from each percent of organic matter since it will only be in the ground for half of the time that soil microbes are active. Many annual crops need N most intensely about 3–4 weeks after transplanting, or just before the period of maximum growth. Therefore, sidedressing, or spreading a rapidly available source of N along the crop row so it will release nutrients at this time is most efficient. Examples of appropriate sidedressing materials include feather meal, blood meal, seed meals, and dehydrated poultry litter. These materials are relatively expensive, so it is advisable to prioritize their use on high-value crops. A PSNT collected at the right time can help estimate the most appropriate rate. In many cases, leguminous winter cover crops are an affordable N source for summer vegetables and are especially important in organic fields where N is needed but other nutrients (such as phosphorus) are above optimum. See Nitrogen and Nitrogen Management. 

Calcium (Ca) is typically supplied in sufficient quantities by lime applied to manage soil acidity. When liming is not required and soil Ca tests below optimum, the best alternative source of Ca for organic producers is gypsum.

Magnesium (Mg) 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. However, Epsom salts can be applied as a foliar spray to temporarily alleviate Mg deficiency. Dissolve 15 lb per 100 gal water and spray at weekly intervals.

Limestone is widely used to raise soil pH and provide Ca and varying amounts of Mg. The appropriate rate of limestone should be determined by soil testing and adjusted based on the calcium carbonate equivalence of the material. The selection of dolomitic or calcitic lime should be based on soil test levels of Ca and Mg. When Mg tests below optimum, dolomitic, or high-Mg limestone, should be used for liming. If Mg is optimum, a calcitic (low-Mg) lime may be used. Pelletized lime (pulverized limestone that is re-formed into pellets) may contain binders that are not approved for use in organic systems. See Soil Acidity, pH, and Liming section.

Phosphorus (P) occurs at naturally low levels in many New England soils, but has been increased to above optimum levels on many farms through historical applications of dairy manures. If P levels are low, it can limit crop growth, especially early in the season. Maintain a pH of 6–7 to maximize P availability. Compost and manures are an excellent source of readily available P. Compost and manures tend to contain less P than N or K, but repeated applications of moderate rates will raise P levels substantially. Repeated use of these materials may result in excessive soil P levels, so nutrient levels should be monitored with regular soil tests. If P levels are much above optimum, minimize applications of amendments containing P, including compost.

Potassium (K) is most often applied as potassium sulfate, or, when Mg is also needed, as Sul-Po-Mag. It can also be applied as granite dust or greensand, both of which release K very slowly over many years and can be applied at 3–5 tons per acre to build up K reserves. Wood ashes contain soluble K, but must be used with caution because they can raise pH rapidly and can be caustic. The liming effect of wood ash can be variable, though is often estimated as roughly half that of limestone. If large amounts are to be used, the best practice is to have the material analyzed for both K content and calcium carbonate equivalence (i.e., liming potential).

Sulfur (S) fertilization has historically not been necessary due to S deposition from acid rain. However, environmental regulations have dramatically reduced acid rain and it is now often necessary to add S for ideal crop performance. Brassicas, alliums, corn, and potatoes are especially sensitive to S deficiency. Sul-Po-Mag, gypsum, and organic amendments such as compost and manure are good sources of S. Elemental S fertilizer lowers pH and should only be used when this effect is desired.

Micronutrients are generally sufficiently supplied to crops by regular additions of organic amendments. Wood ash is another excellent source of micronutrients. Some seaweed extracts may also supply micronutrients. In soils low in boron (B), especially sandy soils, remedial applications are widely recommended for crops that readily suffer from B deficiency, such as brassica crops. In this case, 1–2 lb per acre of B should be applied to the soil. It is difficult to apply such a small amount uniformly, but B can be ordered as part of a custom fertilizer blend. Alternatively, most boron products are soluble and, once dissolved, can be 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. Excess levels of B are toxic to plants, and some crops, such as beans and peas, are quite sensitive to high boron levels (see Table 3).

Table 10: Typical Nutrient Values for Common Fertilizers Approved for Organic Production

Table 10a: Typical Nutrient Values for Common Mineral Materials Approved for Organic Production