Nutrient Management on Organic Farms

Many organic farmers speak about the importance of a proper balance of nutrients in the soil for plant growth. They use the Basic Cation Saturation Ratio method for estimating crop nutrient requirements. This approach is based on an ideal ratio of exchangeable bases (in particular Ca++, Mg++, and K+) held at cation exchange sites.

Farmers believe that an ideal ratio will optimize plant nutrient utilization and crop yield. These are the ideal ratios:
Ca:Mg 6.5:1
Ca:K 13:1
Mg:K 2:1

If they were ideally balanced, 85 percent of the exchange sites would be occupied by Ca++, 10 percent by Mg++, and 5 percent by K+. If these ratios are not present, then a farmer assumes that a deficiency exists in one or more of these nutrients.

Feed the Soil Approach

The purpose of this approach is twofold. When organic nutrients are added to the soil, microbial activity increases. In this sense, organic farmers are “feeding the microbes.” Increased microbial activity improves soil physical properties. For example, when microbial activity increases, soil tilth improves. In addition, microbial activity speeds nutrient cycling, increasing the availability of nutrients for plant uptake (when mineralization exceeds immobilization by microbes.

Crop Use Efficiency
Regardless of rooting conditions, crop roots will not find all of the applied nutrients. Some crops are much more efficient than others at finding and taking up nutrients. For example, a cucumber crop may take up as little as 20 to 25 percent of applied Nutrients fertilizer. The Nutrients use efficiency of cucumbers, therefore, is 25 percent. The Nutrients use efficiency of corn is only 50 percent. Corn may uptake only 50 pounds from a 100-pounds-per-acre application of fertilizer. Nutrient uptake is influenced by the density of plant roots, which in turn is influenced by the soil’s physical, chemical, and biological properties. Even when soil quality is excellent, plant roots may explore less than 5 percent of the entire soil volume. Many other factors contribute to inefficient use of applied nutrients, such as fertilizer placement, rainfall and irrigation amounts, and soil temperature.

The feed-the-soil approach stops short when nutrient concentrations in soils are already very high. In these cases, feeding the soil can result in nutrient additions that increase the potential for environmental pollution or plant toxicity.

Avoiding Over-applications of Phosphorus and Potassium

In some instances, basing fertilization rates on crop nutrient removal calculations is useful—for example, when farmers use manure to meet crop nutrient requirements. Where soil tests show that P and K values are very high and no additional P or K is recommended, a manure application rate calculation that is based on crop N needs will oversupply P and K. This is because manure contains significant amounts of these nutrients. In these cases, the most sustainable practice may be to apply manure based on the plant removal rates of P or K. Any resulting shortfall in the crop N requirement can be met with an-other N source that doesn’t contain P or K.

Over-application of P is especially problematic when organic amendments are applied to soil surfaces, as when using no-till systems or perennial cover crops. Although N may be lost by many means in a no-till system (leaching, runoff, and denitrification, for example), P is typically lost through erosion, runoff, and subsurface flow. Losses of soil P to streams and rivers through these processes can degrade water quality in lakes, reservoirs, and marine estuaries.

Additions of manures, composts, and other organic byproducts can and do result in a buildup of available P in organic farm fields over time. For this reason, it makes sense to calculate P application rates based on the P removal rates (where soil is sufficient in P). In any case, calculations of crop removal rates of P and K are useful in accounting for additions and removals of nutrients from farm fields over time.

Avoiding Over-applications of Nitrogen

Nitrogen application rates for a particular crop grown on a particular soil in a particular field should be based on a realistic yield expectation (RYE) for that crop grown in that field. A number of soil-related factors can affect the realistic yield expectation, including these:
  • Depth to subsoil, rock, or other limiting horizons
  • Organic matter content
  • Permeability, infiltration, and drainage
  • Landscape position
  • Climate

Fertility management strategies can overcome many of these site-specific properties, but farmers will have to spend more time and money in terms of irrigation, nutrients, labor, and skill to achieve high yields on poorer soils.

Determining a Realistic Yield Expectation

The best method of determining the realistic yield expectation is to use historic production records for each field. To obtain a truly representative value, farmers can average the three highest economic yields (yields that provide the highest net returns) in the last five years that the crop was grown.

Unfortunately, data is sometimes not available on a field-by-field basis, especially where a new crop is being grown.

Calculating a Nitrogen Application Rate

Once a realistic yield expectation for a field has been determined, an appropriate nitrogen rate can be calculated by multiplying the realistic yield expectation by a suggested Nutrient application rate which must be determined from personal experience, a reliable consultant, or local farmers.

Constructing a yield response curve

In the absence of yield response curves for organic operations, organic farmers must construct their own curves to determine N application rates that produce realistic yields of specific crops. Farmers can start with one year’s data on N application rates and the crop yields they produce. The data can be expanded over several years of production that represent a range of growing conditions. The data can be averaged to provide a better estimate of realistic yields and N rates that produce those yields.

All of this will require dedicated record-keeping

Tissue Analysis

It’s a good idea to have plant samples analyzed periodically to determine if crops are receiving adequate levels of nutrients. Public and private laboratories will analyze nutrient concentrations in plant leaf tissue. Results indicate the nutritional status of plants, identify deficiencies or toxicities, and provide a basis for determining whether additional applications are needed, such as a sidedressing or foliar application.

Taking a Sample
The laboratory analysis requires less than 1 gram of tissue. However, a good sample contains enough leaves to represent the total area sampled. For example, 8 to 15 tomato leaves should be adequate. Take separate samples from separate fields or management zones, or from production areas where problems exist.

Crop Rotations

Crop rotation is a system where different plants are grown in a recurring, defined sequence. Crop rotations constitute a substantial mechanism for nutrient supply within organic systems, considering that they modify the physical characteristics of the soil, including the size and activity of the soil microbial biomass. Moreover, rotations are the primary means of controlling weeds, pests and diseases in organic farming.

Organic rotations are divided into phases that increase the level of soil nitrogen and phases that deplete it. The nitrogen building and depleting phases must be in balance, or show a slight surplus, if long-term fertility is to be maintained. This type of rotation provides the basis for forward planning of nitrogen supply, necessary in the absence of soluble nitrogen fertilizer.

Farmers must consider long-term cropping plans or rotations when designing a fertility management plan. If it is agronomically feasible, nutrient application and utilization can be considered for the entire cropping cycle rather than on a crop-by-crop basis. All soil management plans should include a description of the normal cropping sequence, the nutrient needs, and the nutrient removal rates of all crops in the system.

The implementation of a crop rotation plan should include, but not limited to, sod, cover crops, green manure crops and catch crops. These crops must provide the following functions (USDA, 2000):
  • Maintain or improve soil organic matter content.
  • Provide for pest management in annual and perennial crops.
  • Manage deficient or excess plant nutrients.
  • Provide erosion control.

Nutrient Placement

In the absence of chemical or biological inhibitors, roots grow and proliferate in soils with good tilth. However, where root growth is restricted, placement of fertilizer near the developing root is important. Generally a placement that is 2 cm below and 2 cm to the side of the seed or transplant will ensure that nutrients will be available to the crop.

Restricted root growth will occur in compacted soils with high bulk density values or with compacted soil horizons. In these cases, nutrient uptake efficiency may improve if fertilizer placement reduces the distance between fleshy or tap roots and fertilizer material, particularly when fertilizer nutrients are relatively immobile in soil. This is particularly critical where soil test levels are low; in seasons when root growth is slowed due to cold weather; or for plants with restricted root systems due to other physical, chemical, or biological factors, such as nematode damage.