Nov 19, 2012
A good test is essential to determine soil fertility

Soil testing is an excellent method for estimating the fertility status of a soil, and it provides valuable information for developing a sound fertility management program.

There are four critical steps to a soil test: sampling, analysis, interpretation and recommendation.

Sampling: A soil test is no better than the care given to taking samples. Soil test interpretations and recommendations are based on a specific sampling procedure. Therefore, it is very important for accurate interpretations and recommendations to follow the sampling instruction from your lab as closely as possible. Arbitrary changes in sampling procedure, such as sampling to a different depth than recommended, may result in incorrect interpretations and recommendations. Whatever lab is used, it is critical that the instructions are followed for the results to be meaningful.

Analysis: It is important to use soil-testing methods that are appropriate for local conditions. Which test will be used in a given area is based on research to determine how well the test works under local conditions. The tests used by Penn State’s Agricultural Analytical Services Lab have been determined, through extensive research, to work best for various conditions.

It is sometimes tempting to send samples to a lab based solely on the lab’s reputation for quality, turn-around time, user-friendly recommendations or perceived slant toward or away from organic agriculture. While these all are legitimate considerations in choosing a lab, the most important consideration is whether the test methods used by the lab are appropriate for the conditions where the test will be used. It is simple to send a soil sample off to a lab in another part of the country based on the lab’s outstanding reputation, but if that lab is not using methods appropriate to your local conditions, even the highest-quality lab might provide incorrect results, interpretations and recommendations.

Information on the methods used is also useful, if you compare analytical results from different labs. Direct comparisons can be made only between labs using exactly the same procedures. Many methods of analysis are in use around the country, each containing both strong and weak points relative to local conditions.

Interpreting the results: The results of the laboratory analysis are meaningless by themselves; they must be interpreted by relating the lab values to known crop responses under local conditions, through calibration research. The relationship between the soil test level and crop yield can be represented as a response curve. As soil test levels increase from very low levels, the yield will increase until it reaches a yield plateau – the point above which yield no longer increases with soil test level. The optimum soil test level lies at that point. Because of natural variations in soil test levels, a range of soil test levels just above the optimum point is usually designated as the optimum range for plant availability of that nutrient in the soil. The goal of soil testing is to manage the soil so that its nutrient levels are in the optimum range for crop production.

Eventually, if soil test levels continue to increase, yield reduction may occur in some instances. Once the response curve has been determined by field research in the area where the soil test will be used, the interpretation levels for the soil test can be established.

Unfortunately, there is no standard terminology for these soil-testing categories. Be sure to determine the exact meaning of the interpretation terms used on your soil test. An important consideration in interpreting soil test results is the balance of nutrients. The “Law of the Minimum” indicates that whatever plant growth factor (including nutrients) is most limiting will control plant growth potential. Thus, applying excess of one nutrient if some other factor is limiting will not overcome the limitation. Also, there can be positive interactions between nutrients such that an adequate supply of one nutrient may enhance the availability or uptake of another nutrient. Therefore, having the appropriate balance of nutrients is critical. The optimums established for a soil test must not only consider the amount of each individual nutrient, but also the balance between the nutrients.

Recommendations: Recommendations are made for a specific crop based on the soil test level and research on how that crop responds to soil nutrient levels and applied nutrients under local conditions. The most common recommendation approach is to manage the soil to maintain the test levels in the optimum range, so that there is an adequate supply of soil nutrients to meet crop needs. In this approach, below optimum soils will have recommendations to apply nutrients to build the soil into the optimum range, and then to maintain the soil in the optimum range by replacing nutrients removed by crops as needed. This approach is in line with organic soil fertility management.

The second approach commonly used is to manage nutrients to achieve the optimum response by the current crop. This approach puts more emphasis on the immediate response to added nutrients than on managing soil nutrient levels.

Records of soil tests can be used to monitor soil fertility levels, fine-tune recommendations to maintain optimum levels and evaluate odd values. A decrease or increase in a soil test level at a relatively constant yield might indicate under-application or over-application of nutrients, respectively. Adjust soil test recommendations according to observed trends. Also, this type of monitoring can help detect potentially erroneous test results. Soil test levels will vary from one test to the next; however, if an obviously odd value is observed, the lab can recheck the results and/or test a new sample for confirmation.

In a crop rotation, the objective is the same: to maintain the soil test levels in the optimum range over time. This is complicated, however, by the different crops in the rotation. On a dairy farm, for example, manure is typically applied to the corn crop and not to the hay crop. This results in a buildup of nutrients during the corn part of the rotation that can be used by the hay crop later. In this type of system, the P and K soil test levels fluctuate during the rotation, but the average trend over time remains in the optimum range. If manure is applied to all crops in the rotation, especially at N-based rates, then there will most likely be a continuous upward trend in soil test P and K levels over time, which can lead to crop production and/or environmental problems.

Liming recommendations for managing pH are always based on a soil management approach. Limestone is recommended based on the measured soil pH, the desired soil pH for the crops grown and the acidity level of the soil as measured by the test. Generally, limestone recommendations are for an application every few years to maintain the soil pH in the optimum range for the crops being grown. The limestone recommendations must be adjusted for the quality of the limestone being used. The most important limestone quality parameters are calcium carbonate equivalent and fineness.

Nitrogen recommendations are not based on the complexity of the nitrogen cycle, which is extremely dynamic and difficult to predict in humid climate regions. Soil tests for N can be run but provide little useful information for predicting N response in a given situation. Nitrogen recommendations are made based on extensive crop response research and are designed to meet the N needs of the current crop. All potential sources of N must be considered when making an N recommendation. This includes residual N from legumes in the crop rotation and from previous manure or compost applications.

There are some in-season tests, such as the pre-side dress soil nitrate test, that can be used to adjust N management during the season. These tests are based on testing the soil just prior to the period of maximum N uptake, and using that to adjust N management.

By Douglas Beegle, Penn State University

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