Fact Sheet FS1388
Organic growers, like all farmers, can use targeted plant nutrition to grow healthy crops with better yield and protection from pests and disease. Soil testing, soil type, plant tissue analysis, and careful observations of crop health should guide nutrient applications. Translation of soil test reports into organic-compliant recommendations is important for certified organic farmers. General guidelines are provided in this document for this purpose.
For organic growers, it is important to keep certain crucial details in mind. Based on USDA's National Organic Program (NOP) standards, organic farming is an ecological system that requires continuous improvement in soils and avoidance of materials that may negatively impact soil health. Crop rotation, cover cropping, composting, and livestock integration are some of the strategies used on organic farms. Such cultural practices are typically developed into an approved organic farm system plan.
When soil amendments are needed, organic farmers can look to the Organic Material Review Institute (OMRI) to see if a certain material is allowed for use in a certified organic operation. However, some materials such as naturally occurring limestone mined from the earth do not have an OMRI listing but still may be permitted to be used as an amendment on a certified organic farm. In such cases, the organic farmer should check with their certifying agency before applying the material.
Also, organic farmers may import materials such as horse manure or community collected shade tree leaves onto their farm for producing compost. In such cases, the organic farmer needs to document certain details about the source of the material and confirm that nothing synthetic or prohibited has been added. Organic growers also need to be mindful to avoid use of any materials on the USDA-NOP prohibited lists. For example, sewage sludge and biosolids are prohibited for application to USDA Certified Organic farmland. Find more informatio about allowed/prohibited substances on the USDA publication titled Substances for Organic Crop + Livestock Production (PDF).
Figure 1: Applying limestone based on soil testing is standard practice for all farming systems, including USDA Certified Organic. Photo by Joseph Heckman.
Soil pH and Liming
The first consideration to remember is soil pH targets and liming practice for organic farming are very similar to other farming systems. Limestones are calcium carbonates (calcite) or calcium magnesium carbonates (dolomite) mined from the earth and are approved for use in organic agriculture. These naturally occurring minerals are the most widely used liming materials in all of agriculture. In general, natural materials that are mined from the earth are permitted. Physical changes to the material by grinding are acceptable, but chemical alteration is not. Liming materials typically come from local quarries and may not be OMRI-listed.
Amendments that are not OMRI-listed can sometimes be used on organic farmland, but farmers should check with the organic certifying agency to be sure. Organic growers should note that calcium oxide and calcium hydroxide, sometimes called burnt lime or quick lime, are prohibited materials for certified organic farms.
Another thing to keep in mind is that some growers are particular about Ca to Mg saturation levels on the soil's cation exchange capacity (CEC). Calcium saturation levels near 68% and Mg levels near 12% are generally good soil fertility and agronomic targets. This results in a Ca:Mg ratio of about 6.
Organic growers may also use wollastonite, a mined calcium silicate. Wollastonite is a naturally occurring mineral that functions much the same as limestone to supply Ca and raise soil pH. The benefit of using wollastonite is that it also supplies plant-available silicon, which strengthens crops and protects against powdery mildew disease. Additional information about silicon nutrition can be found in FS1278, Silicon Needs of Soils and Crops. Information on suppliers of OMRI-approved wollastonite is available through publicly accessible online resources.
Nitrogen
The next focus should be on nitrogen (N). A regular soil fertility test report does not directly determine N availability. However, soil organic matter content can be measured by soil test, and this factor, along with the type of crop, can be a useful consideration in deciding how much N to recommend.
As a rule of thumb, each percent of organic matter in soil may convert organic N into 20 pounds per acre of plant-available N during the growing season. For example, a soil with 4% organic matter content may supply about 80 pounds of available N per acre. But there are many exceptions to this rule that depend on other agronomic factors such as crop rotation, previous organic amendments, types of mulch, soil temperature, and weather conditions that influence the N cycle.
An organic grower must adopt an effective organic system plan (a requirement to become certified as organic) that will, over the years, build organic matter content and improve soil health. This should increase the ability of the soil to supply N to crops with minimal supplemental N fertilizer. Because organic-approved N fertilizers are expensive, it is to the farmer's advantage to minimize the need to use supplemental N fertilizers. But, if some N fertilizer is needed, the organic grower can utilize fertilizers such as dried blood (13%N), fish emulsion (4% N), or other approved N sources.
In the case of corn and most types of annual vegetables crops, organic growers can use the special soil test called the pre-sidedress soil nitrate test (PSNT). More details about this special soil test for N can be found in Rutgers Cooperative Extension fact sheet FS285, Soil Nitrate Testing as a Guide to Nitrogen Management for Vegetable Crops.
Use of the PSNT during an early crop growth stage helps to determine whether an organic farm plan is working to effectively supply N to the crop. Note that the PSNT soil test is only useful for annual crops, not perennials. Also, the PSNT is only useful when a farm employs good production practices that build soil organic matter content, such as planting legume cover crops, and applications of manure or compost. Attempts to use the PSNT on soils with low organic matter content are normally not useful; in this case, it is safe to assume that N amendments are needed.
In the case of woody perennials and tree crops, the need for N should consider the results of plant tissue analysis.
Phosphorus
Soil tests for plant available phosphorus (P) provide a useful guide for determining the need for P fertilizer. Many agricultural soils in New Jersey are already well supplied with P. Organic growers who may have applied compost or manures regularly over a period of years will often have very high levels of soil test P. In such case, a zero-P fertilizer recommendation may be appropriate. However, if soil test P is optimum, a nominal maintenance application of P should be recommended. If the soil test is below the optimum range, a higher build-up P fertilizer application should be recommended.
Soil temperature can influence P availability. When planting into cold soil, it may be necessary to supplement with P fertilizer even when soil test levels are optimum.
Compost is generally a good source of plant-available P. Another P fertilizer source organic growers can use is bone meal (10 to 15% P2O5). Rock phosphate (25 to 30% P2O5) is a widely used fertilizer for organic farming. However, the P is only slowly available. When soil tests low in P, rock phosphate may be applied at 500 to 1000 pounds per acre. Legume cover crops such as red clover should be grown to make the P from the rock phosphate more available. The P availability to crops from rock phosphate varies by source. For example, rock phosphate mined from North Carolina is more soluble than rock phosphate mined from Florida.
Potassium
Soil testing for potassium (K) determines the need for K fertilizer. In general, sandy soils are more prone to K deficiency than loamy soils. Potassium fertilizer recommendations for organic growers are similar in practice to other farming systems. Organic growers can use certain potassium sulfate fertilizers or potassium magnesium sulfate (langbeinite) fertilizers so long as the products are approved for organic production. The OMRI listing can be used as a guide to approved materials. Potassium magnesium sulfate is a good choice for soils that need both K and Mg. Potassium sulfate is the better choice for soils that need K but already have high levels of Mg. Other organic-approved K fertilizers may include materials such as granite dust or other rock powders. Where manures and composts are being applied, the K rate recommendation should credit the K content of those materials. Potassium chloride and potassium nitrate fertilizers are not approved for organic farming. If in doubt about organic compliance, contact your certifying agency.
Sulfur
Organic farms that make regular applications of composts and manures and build soil organic matter content are not usually deficient in sulfur (S). However, if S fertilizer is needed, consider that it might already be supplied as part of the K fertilizer application; for example, if langbeinite or potassium sulfate are used. Otherwise, gypsum (23% S) products that are OMRI-listed may be used to supply S. Elemental sulfur is also allowed as a soil amendment or to lower an elevated soil pH.
Micronutrients
Soil testing and plant tissue analysis are very important to organic growers and other farming systems. Certified organic producers must use diagnostic tests to document that micronutrients are needed before they can be applied as fertilizer. But once organic growers have diagnostic tests to show a need for a micronutrient, they may apply it and use many of the same micronutrient fertilizers as other growers. There may be a few exceptions to types of micronutrients fertilizer products. Organic growers should check with their certifying agency to be sure that the material is approved.
Micronutrient soil tests for Boron (B), Copper (Cu), Manganese (Mn), Iron (Fe), and Zinc (Zn) are based on the soil test levels rating described as either "Low," "Adequate," or "High," on the Rutgers Soil Test Reports. Besides the soil test extract levels, soil pH should also be taken into consideration. Most micronutrients become less plant available as soil pH increases. The exception, however, is molybdenum (Mo) which becomes more available as soil pH increases.
When making product choices for organic farms, growers should always be prepared to check with their certifying agency to be sure the application follows the USDA National Organic Program standards.
References
February 2026
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