Soil and plant tissue analysis

Table 1. Critical soil test values for four diverse field crops common to Nebraska (based on soil samples taken to an 8-inch depth). Values are derived from EC155, Nutrient Management for Agronomic Crops in Nebraska

1. Bray-1 P values; 2. Olsen-P values; dry bean production regions in Nebraska have high pH, calcareous soils where Olsen-P test is more appropriate (however, values can be multiplied by 1.5 to estimate Bray-1 values); 3. Only applicable for sandy soil with very low soil organic matter; * = not limiting in soils of typical production regions

Nutrient management on organic farms should economically meet crop nutrient needs and avoid soil nutrient depletion, while maintaining or improving soil productivity without excessive nutrient losses. Soil nutrient availability is dependent on diverse soil chemical, physical, and biological properties, their interactions, and their interaction with the cropping system. While measurements can be made for many soil properties, crop performance is the best indicator of soil productivity. Farmers typically manage to minimize soil physical and chemical constraints to sustainable productivity through practices such as:

Most crop essential soil nutrients are adequately supplied by agricultural soils in Nebraska. Application of nitrogen and phosphorus is most common for high crop productivity in Nebraska. Deficiencies of potassium, sulfur (with sandy soil), iron (with calcareous soil), and zinc can occur and require attention in some fields. Yield increases due to application of other nutrients are rare in Nebraska. Management to avoid acidic soil conditions (Nebraska Extension NebGuides G1503 and G1504) and soil aggregation, and to avoid compaction (NebGuide G896) are also important for ensuring sufficient nutrient availability to crops.

Soil sampling every four years is advised to test for soil organic matter, pH, and availabilities of phosphorus, potassium, and zinc, as well as other properties of interest. Soil sampling practices and methods, such as grid versus management zone sampling, are addressed in NebGuide G1740. Soil pH 6 is the desired minimum pH for most organic cropping systems in Nebraska. Iron deficiency is commonly associated with calcareous soils with pH > 7.2. Alkaline soils on organic farms can be acidified over time by applying OMRI-approved mineral sulfur (e.g., sulfate of potash) or acidic mulches (e.g., pine needles).

Applying agricultural lime to raise soil pH is allowed in organic production. Beef feedlot manure has a liming effect, typically the equivalent of about 60 pounds of agricultural lime per ton of manure dry weight. However, most other manures and composts, and biological nitrogen fixation, have a net acidifying effect. Plant and soil testing laboratories commonly do recommend nutrient application rates, but Nebraska Extension EC155, Nutrient Management for Agronomic Crops in Nebraska is especially valuable for interpretation of soil test results and determination of nutrient and lime application rates.

Critical levels for soil nutrient availability represent thresholds above which no yield increase is expected due to application of the nutrient, although application of organic materials may have other positive effects on the crop. See Table 1 for a sample of critical soil test values for several essential plant nutrients for common field crops in Nebraska. These values can be used as a starting point for making annual decisions about nutrient sources and application rates; however, developing a long-term nutrient management plan and proactive (instead of reactive) management of soil fertility is advised for organic farms.

Soil nitrate-nitrogen is highly variable throughout a field and over time is not always a reliable predictor of fertilizer need. Many farmers use a late-spring soil nitrate test to determine sidedress application rates in corn, but sidedress fertilizer applications are rare in organic farming. Crop nitrogen demand and annual removal should guide fertilizer or amendment application rates in organic farming, but rates can be reduced if nitrate is detected in soil samples taken to the rooting depth (2–4 feet for most crops; see EC155 for interpretation of deep soil nitrate values in agronomic crops) and when a legume is the preceding crop (see section below for further information).

In addition to increased productivity, using organic materials to fertilize crops can improve broader measures of soil “quality” or “health.” Changes to soil physical and biological properties that may result from long-term organic nutrient management include reduced bulk density, increased aggregate size and stability, increased water infiltration, and increased water-holding capacity; increased microbial biomass, diversity, and respiration; and increased nitrogen mineralization potential, reduced soil erosion and soil crusting, and increased soil carbon sequestration (NebGuide G2283). Soil physical and biological tests are available at some soil testing labs. Other “soil health” do-it-yourself procedures also are available, including the USDA NRCS Soil Quality Test Kit (https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/health/assessment/?cid=nrcs142p2_053873).

Plant tissue analysis every three to five years to diagnose potential nutrient deficiencies of perennial crops, such as fruit tree crops, is advised. See New Mexico State University Cooperative Extension Service Guide A-123, Sampling for Plant Tissue Analysis, for crop-specific sampling techniques.

Nutrient sources

Biofertilizers

Biofertilizers are products designed to provide enhanced nutrient availability and uptake, stimulation of crop growth, biological nitrogen fixation, and protection against insect pests and disease. Depending on the purpose, biofertilizer products can be applied to soil, seeds, or foliar tissue. Evidence to support product claims is often limited or mixed. Recent research suggests biofertilizers may be most beneficial in soils of low to moderate soil organic matter and nutrient availability or with foliar application. In contrast, biofertilizer applications to soils of >3 percent organic matter rarely result in measurable crop or soil benefits. Introduced microorganisms often fail to compete and survive with already well-established and resilient microbial communities.

Several common categories of biofertilizers include nitrogen-fixers, phosphorus-solubilizers, phosphorus-absorbers, and humic acid. Nitrogen-fixers such as Rhizobium (in symbiosis with legumes), Azospirillum, and Azotobacter convert atmospheric nitrogen into ammonia. Bacillus and Pseudomonas are examples of microbes found in phosphorus-solubilizing biofertilizers that lower the soil pH to dissolve soil-bound phosphate for plant availability and may be most effective for calcareous soils. Arbuscular mycorrhizal fungi take up soil phosphorus, zinc, and copper and transfer these to plant roots, but these are typically abundant in Nebraska agricultural soils. Humic acid is important to plant growth but is already abundant in most soils. A soil with 3 percent organic matter may have as much as 10–15 tons of humic acid in the surface soil layer; thus, adding a few more ounces per acre is unlikely to enhance crop growth and yield.

While biofertilizers may be beneficial in some situations, it is important to note that biofertilizers, unlike chemical fertilizers, are not regulated. Product labels often lack detailed information about the ingredients. If they contain living organisms, the shelf life is often short. Given the number of products on the market with unproven claims of crop and soil benefits, growers are urged to include a non-treated check or control in the field when using biofertilizers to test the claims of soil or crop benefits and evaluate their cost-effectiveness. See the Nebraska Extension “Grower’s Guide to On-Farm Research” (http://cropwatch.unl.edu/nofrnzmag) for tips on conducting simple, but effective and scientifically valid, experiments on your own farm. Evaluations of many biofertilizers and other nontraditional soil additives are documented but others are too short lived to be evaluated well (http://extension.agron.iastate.edu/compendium/index.aspx).

Cover crops

Cover crops can improve soil physical properties, nutrient cycling, and soil microbial activity. In addition, cover crops can scavenge residual nitrogen mineralized from soil and organic amendments before it is lost to volatilization, runoff, or leaching. However, like other sources of organic nitrogen, nitrogen contained in cover crop biomass is not entirely available to the next crop. It is important to consider the C:N of the cover crop residue. Species with high C:N (>20:1; e.g., grasses) result in net immobilization of soil nitrogen in the short term, whereas nitrogen will be more readily available following decomposition of species with low C:N (<20:1; e.g., legumes).

Nitrogen fixation by legume cover crops can be especially useful for increasing soil nitrogen availability and balancing the abundant phosphorus and potassium supplied through manure or compost application. Legume cover crops high in nitrogen, such as hairy vetch and field pea, can provide up to 150 pounds of plant-available nitrogen per acre to the following crop, depending on the amount of growth. However, the cover crop needs to be terminated early enough to avoid increased C:N as the plant matures, and to allow sufficient time for residue decomposition, mineralization of organic nitrogen, and soil water recharge (farmers in western Nebraska should carefully consider trade-offs between cover crop nitrogen contributions and soil water use during fallow periods). To this end, it is recommended that legume cover crops are terminated at the flowering stage and at least one to two weeks prior to planting.

Table 5. Nutrient analysis and approximate release speed (time required for mineralization of guaranteed nutrients) of common products for organic production

Source: Pacific Northwest Extension PNW 636, Estimating Plant-Available Nitrogen Release from Cover Crops.

When using cover crops in nitrogen management, it is important to consider the total nitrogen content of the cover crop residue. If a cover crop has 3 percent nitrogen concentration at the time of termination, the resulting soil input is 60 pounds of organic nitrogen per ton of dry matter. Subsequent ammonium-nitrogen release from that organic nitrogen is estimated to be 19 pounds of nitrogen per ton after four weeks and 28 pounds of nitrogen per ton after 10 weeks (Table 5). Thus, if a grower can produce 3 tons of dry matter per acre of a 3 percent nitrogen cover crop, approximately 84 pounds of plant-available nitrogen per acre could be mineralized within 10 weeks after termination. Combined with other organic amendments to meet plant phosphorus, potassium, and micronutrient demand, cover crops can be a sustainable and cost-effective nutrient source for organic production.

Source: Nebraska Extension EC117, Fertilizer Suggestions for Corn.

Crop rotation

Table 6. Predicted ammonium-N (Am-N) release from cover crop residues with differing N content and C:N ratios

Crop rotation can contribute to improved soil physical properties, pest management, nutrient availability, nutrient use efficiency, and crop yield. Legumes in the rotation (e.g., beans, alfalfa, or clover) can result in a nitrogen credit for subsequent crops due to biological nitrogen fixation and less nitrogen immobilization compared with a non-legume as the preceding crop (Table 6). However, the nitrogen credit for forage legumes is greater than for grain or vegetable legumes because most of the nitrogen (whether scavenged from the soil or biologically fixed) is removed during harvest of the grain.

In contrast, forage legumes contribute nitrogen from vegetative biomass, which can be comparable to cover crop nitrogen contribution if the forage has time to regrow between the last cutting and termination. Cereal and other non-legume crops in rotation can be useful for building soil organic matter–an important reservoir of nutrients in organic farming (G2283). Root architecture is important to sustainable crop rotation. Deep-rooted crops like alfalfa can scavenge immobile nutrients like phosphorus and leached nutrients like nitrate-nitrogen from deep soil layers, which may be released near the soil surface with decomposition of the crop residue for availability to subsequent crops in rotation.

Table 7. Nitrogen credit to subsequent crop following legume

Conclusions

Nutrient management on organic farms requires long-term planning and a diverse combination of cultural practices and inputs. There are an increasing number of commercially-available organic fertilizers and biofertilizers, but the most profitable organic farms typically source nutrients on or very near the farm by using organic wastes, scavenging residual soil nutrients, and biological fixation of nitrogen. When off-farm or manufactured fertilizer inputs are necessary, it is essential to verify that the product is allowed in organic production by consulting the OMRI list or the certifying agency.