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    Best Management Practices: Soil Testing

    Soil testing is one of the most important management practices for crop production in the new millennium. It is certain to be listed among the best management practices recommended by industry and university agronomists, consultants, and farm managers for the benefit of their farmer clients. Fertilizer dealers also continue to use soil testing as a planning and marketing tool and as a management support service for their customers. Likewise, the environment benefits from improved management of soil resources and fertilizer materials made possible by soil testing.

    Periodic soil testing, combined with a good record-keeping system for each field (including information on prior soil tests, fertilizer and manure applications, and crop yields) serves as a gauge to indicate whether soil fertility is increasing, decreasing, or remaining constant. Fertilizer use and crop yield statistics indicate that soil fertility on many farms may be declining due to deficient nutrient management.

    The consequences of “mining” soil nutrients may not become apparent for several years. If no nutrients are applied in a corn-soybean rotation with high or very high soil tests, the P1-phosphorous (P) soil test will typically drop 5 to 6 pounds per acre per year (lbs/yr), and the potassium (K) soil test will decline approximately 10 to 15 lbs/yr. If this trend continues, substantial loss in crop yield potential will result, and it may take several years of higher fertilizer rates to restore optimum productivity. Allowing soil test levels to decline is destructive to future productivity and profitability.

    Farmer benefits

    • Farmers should use soil testing as a management tool for making scientifically sound management decisions about their soil fertility program. A fertility management program based on soil testing benefits the farmer in many ways:

    • Improved yields and profitability from providing needed nutrients for the crop.

    • Increased uniformity of nutrient availability across a field, optimizing response to other management inputs.

    • More uniform crop growth, which makes individual plants more competitive with weeds and simplifies other management practices such as cultivation, spraying, etc.

    • More uniform plant maturity within a field, simplifying crop harvesting and drying and improving market quality.

    • Allocation of fertilizer dollars to the nutrients that will give the greatest increase in profit. Intensive sampling and variable-rate fertilizer application allow fine-tuning of fertilizer applications within a field.

    Environmental benefits

    The environment benefits from increased soil testing because fertilizer applications based on soil tests and realistic crop yield goals help ensure proper rates are recommended and applied. More efficient use of plant nutrients by growing crops means less potential losses from leaching or surface runoff into waterways. Site-specific fertility management ensures growers identify nutrient needs properly and make the proper corrective fertilizer applications. Withholding needed fertilizer may be more damaging than applying the proper amount of fertilizer to produce a high-yielding, profitable crop, since poorly nourished crops leave less plant residue to hold soil in place. Further, providing optimum levels of all nutrients helps increase yields and may help reduce the need for intensively farming some marginal land.

    What is soil testing?

    Soil testing is a management practice that helps identify the variability of nutrient content within a field and among different fields on a farm. The process involves chemical analysis of representative samples of soil from a given field, along with calibration data derived from research on similar soils, to provide an estimate of the expected yield responses to applications of fertilizer materials.

    The greatest potential for error in the process lies in field sample collecting. Since a 1-pound sample of soil submitted for lab analysis actually represents the soil from 1 to 40 acres (and up to 80 million pounds of soil in the top 6 inches), getting truly representative samples is a critical step. The laboratory will actually use less than a teaspoon of soil from the sample. If a good sample is collected, the results of the test should provide a reliable estimate of the nutrient status of the soil. Increasing the number of samples from a field can help improve the reliability of the recommendations.

    The nutrient level found in a test sample indexes the relative amount of that nutrient in the soil and its availability to plants. The soil test number itself is meaningless without the appropriate set of calibration data with which to compare it. The result of the correlation and calibration data is reflected in the soil test ratings — low, medium, high, and so on.

    Soil testing is agronomically sound

    Soil testing is based upon years of scientific study of sampling procedures, crop yield responses to fertilizer application and interactions of plant nutrients with other production factors. Scientists have proved that soil tests provide the information necessary to make intelligent decisions on the amounts and composition needed to achieve a selected yield goal for a specific soil-climate-management system.

    Soil testing is economically efficient

    To grow a crop efficiently and to maximize return on every dollar spent on fertilizer, it’s important to apply the right combination and rates of fertilizer materials. Soil test information paired with a realistic yield goal is essential to making the best decisions on fertilizer needs. Without soil tests it’s impossible to know which nutrients are in short supply and which ones are adequate for the crop. Total fertilizer costs are sometimes reduced through use of soil tests, but even more important, farmers invest their dollars on the nutrients most needed by the crop.

    Soil testing is environmentally responsible

    The potential for environmental problems can be greatly reduced through a fertilizer management program based on soil testing. Fertilizer application according to soil test results ensures the proper combination of nutrients for the most efficient utilization by the crop. Potassium fertilizer, for example, can increase the efficiency of nitrogen (N) utilization. As K application is increased, the most profitable level of N application is also increased. But without knowing the K soil test level, it’s impossible to determine the most efficient rate of N.

    Balancing N, P, K, sulfur (S) and magnesium (Mg) is essential to efficient use of each of these nutrients, attaining the most profitable yields and protecting the environment. In an Illinois research project, the most profitable combination came when growers balanced N, P and K applications based on soil test information. This was also the combination that produced the highest yields and left the least N in the soil for potential loss to groundwater and surface water. Leaving out P or K caused reduced yields, reduced efficiency, and left a portion of the N fertilizer unused by the crop.

    Frequent soil testing helps farmers decide whether their current management is robbing future productivity and profits. Combined with local calibration data from university research, soil testing serves as the best guide available for determining nutrient needs for growing crops. Soil testing to provide a balanced fertility program is a vital component of sustainable farming programs that are profitable, efficient and environmentally responsible.