Soil Test Report

Written By - Joseph Heckman, Extension Specialist, Soil Fertility, Department of Plant Biology, Cook Campus - Rutgers, The State University of New Jersey

Authors note: Requests for the translation of soil test reports into organic-compliant recommendations are becoming frequest. For this purspose, I write these general guidelines.

When working with organic growers, it is important to remember certain crucial details. Organic farming is an ecological system that, by the nature of the standards, 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 ways an organic farm may work good cultural practices into an approved organic farm system plan. When inputs are needed, organic farmers can look for the OMRI (Organic Material Review Institute) listing to see if a material is permitted for use in a certified operation. However, some materials, such as naturally occurring limestone mined from the earth, might not have an OMRI listing but still be permitted to be used as an amendment on a certified organic farm. In such cases, the organic farmer should check with their certifier before applying the material. Also, organic farmers may import to their farm materials such as horse manure or community-collected shade tree leaves, for purposes of making compost. In such cases, the organic farmer needs to document certain details about the source of the material and ensu that nothing synthetic or prohibited has been added. Organic growers also need to be mindful to avoid the use of any materials on the USDA-NOP prohibited list.

The first place to start is to remember that soil pH targets and liming practices for organic farming are very similar to other farming. 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 check with the organic certifier to be sure. An important thing to remember when making liming recommendations for organic growers is that calcium oxide and calcium hydroxide (sometimes called burnt lime or quick lime) are prohibited materials. Another thing to keep in mind when working with organic operations is that some growers are particular about calcium-to-magnesium saturation levels on the CEC. Calcium saturation levels near 68% and magnesium levels near 12% are generally good soil fertility and agronomic targets. Organic growers may also use wollastonite, a mined calcium silicate. This naturally occurring mineral functions like a liming material, much the same as calcium carbonate. The benefit of using wollastonite is that it also supplies plant-available silicon, which strengthens crops and protects against powdery mildew disease. OMRI-approved wollastonite can be found by web search.

The next focus should be on a recommendation for nitrogen. A regular soil fertility test report does not directly determine nitrogen availability. However, soil organic matter content can be measured by soil tests. This factor, along with the type of crop, can be a useful consideration in deciding how much nitrogen to recommend. As a rule of thumb, each percent organic matter in soil may convert into 20 pounds of plant available N during the growing season. For example, soil with 4% organic matter content may supply about 80 pounds of available N. However, many exceptions to this rule depend on other agronomic factors such as crop rotation, previous organic amendments, types of mulch, soil temperature, and weather conditions that influence the nitrogen cycle. An organic grower must adopt an effective organic system plan (a requirement to become certified organic) that will over the years build organic matter content and soil health. This should increase the ability of that soil to feed crops with N with minimal supplemental N fertilizer. Because organic-approved N fertilizers are expensive, it is in the farmer’s interest to minimize the need to use them. But, if some N fertilizer is needed, the organic grower can supplement with 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 vegetable crops, organic growers can use the special soil test called the pre-sidedress soil nitrate test (PSNT). Use of the PSNT during an early crop growth sage helps to determine with confidence that their 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, such as legume cover crops, and applications of manure or compost, that build soil organic matter content. Attempts to use the PSNT on soils with low organic matter content are normally a waste of time. In the case of woody perennials, the need for N should consider plant tissue analysis.

Soil test for phosphorus (P) is a very 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 the years will often have high levels of soil test P. In such a case, a zero P fertilizer recommendation may be appropriate. However, if soil test P is below the optimum range, a P fertilizer should be recommended. 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 (contains 25 to 30% P) 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 of rock phosphate. Legume cover crops such as red clover should be grown to make the P from the rock phosphate more plant available. The P availability to crops from rock phosphate varies by source. Rock phosphate from North Carolina is more soluble than that mined from Florida.

Soil test for potassium (K) is a useful guide for determining 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. 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 approve 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 already have high levels of Mg but need K. Other organic approved K fertilizers may include materials such as rock powders, granite dust, and greensand. 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.

Organic farms that make regular applications of composts and manures and build soil organic matter content are not generally deficient in sulfur (S). However, if S fertilizer is needed consider that it might already be supplied as part of the K fertilizer application. Otherwise, gypsum (23% S) products that are OMRI-listed may be used to supply S.

Soil testing and plant tissue analysis are very important to organic growers because they must have diagnostic tests to document that micronutrients are needed before they can be applied as a fertilizer. But once organic growers have the 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. So organic growers should check with their certifier 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 level rating described as “Low”, “Adequate”, or “High” on the Rutgers Soil Test Report. Besides the soil test extract level, soil pH should also be taken into consideration. Most micronutrients become less available as soil pH increases.

When making product recommendations for organic farms, growers should always be prepared to check with their certifier to be sure the application follows the USDA National Organic Program standards.

Organic growers, like all farms, can use targeted plant nutrition to grow healthy crops with better protection from pests and disease. https://njaes.rutgers.edu/soil-profile/pdfs/sp-v29...

References:

Heckman, J.R., R. Weil, and F. Magdoff. 2009. Practical Steps to Soil Fertility for Organic Agriculture. 33 pages. In C. Francis. (ed.). Organic Farming: The Ecological System. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, 677 South Segoe Road, Madison, WI 53711, USA. Agronomy Monograph 54. Madison, WI.

Joseph Heckman, PhD, is a Rutgers University Professor of Soil Science and teaches courses in Soil Fertility and Organic Agriculture. For nine years, he served on the board of Northeast Organic Farming Association-NJ. He currently serves on the board of the Raw Milk Institute. On the family farm in Ringoes, NJ, he raises grassfed Jersey beef, black locust trees, and USDA Organic Hay Certified by PCO. Email: heckman@njaes.rutgers.edu

Image caption: Soil test report for a farm field in need of soil fertility recommendations for USDA Certified Organic Farming. The book on display, Organic Farming the Ecological System, is a good reference. This best seller was published by the American Society of Agronomy.