Monitoring Nutrient Status
Soil Analysis
A soil test provides basic information on the nutrient supplying capacity of the soil and other characteristics such as the acidity or pH level. Soil tests are not perfect, so a soil test value should be considered not a single value, but rather a value within a range. There are multiple reasons why soil tests are not perfect: a soil test represents a measurement at one point in time, while a crop utilizes nutrients through an extended period, and typically under very different soil-water and temperature conditions than at the time of sampling; the information generated typically comes from a sample from the plow layer, but the crop roots extract nutrients below that layer.
Soil Sampling
The objectives of soil sampling are to determine the average nutrient status of a field and to provide some measure of nutrient variability in a field. Knowledge of factors influencing soil nutrient levels including soil type, topography, cropping history, manure application, fertilizer application, and leveling for irrigation will help the producer determine the most effective sampling approach. The spatial variability of available nutrients in a field makes soil sampling the most common and greatest source of error in a soil test. To collect samples that provide a true measurement of the fertility of a field, the producer must determine the sampling method; collect samples at the proper time; collect samples from precisely the same areas of the field that were sampled in the past; collect samples to the proper depth; determine the number of cores per composite soil sample; place samples in approved containers; and maintain complete field records.
Sampling Methods
Soil sampling is the first step in generating field-specific information to make fertilizer decisions. Selecting an appropriate sampling method ensures that the soil in a field is collected in a manner that produces the most accurate and reliable soil test results. Because soils in fields can vary significantly, it is important to use a sampling method that best captures that variation. Proper sampling is particularly important when a site-specific management approach is embraced. The three most commonly used soil sampling methods are whole-filed sampling, grid sampling, and zone sampling. Each method requires different sampling techniques and influences how the results are used and how nutrients are applied
Whole-Field Sampling: Whole-field sampling uses one composite sample to represent a field of no more than 25 acres (or orchard blocks). It is not recommended for large acreage crop fields because field variability. Each composite sample should consist of 10 to 15 cores, depending on the nature of the soil and the size of the area being sampled in the field. A smaller number of cores can introduce variability into the results from different sampling years. When gathering soil cores to make a composite sample, use a W-shaped sampling pattern (Figure 11.1) over the whole area the sample represents.
Grid Sampling: Grid sampling can be used as an improvement over whole-field sampling. First, a field is systematically divided into areas of uniform size and shape (called cells) to form a grid. Cells often consist of 1 to 5 acres, with smaller grids (e.g., 2.5 acres or less) providing the best results. The smaller the grid area chosen, the higher the sampling intensity, thus increasing the costs. For a one-acre grid cell, collect at least five cores. For grid cells between 2.5 and 5 acres, collect between 8 and 10 cores. Thus, each cell has its own sample and analysis result. A global positioning system (GPS) is used to record the geographical coordinates of each sample.
Zone Sampling: In zone-based sampling, the goal is to collect samples that represent the average soil within each zone (Figure 11.3). Zones, often referred to as management zones, are typically developed based on differences in soil types, slope, degree of erosion, drainage, historical variability (previous crop yields, field leveling), or other factors that may influence soil nutrient levels. Consider using other sources of information to delineate management units. Soil surveys, soil electrical conductivity (EC) maps, maps produced from remote sensing data (aerial photos or satellite imagery), yield monitors, and/or grain sampling for protein can be used to identify management units (Section 11.4).
Time of Sampling
The most convenient time to collect soil samples is when there is no standing crop in the field (i.e., in the spring prior to planting or in the fall after harvest). Both sample collection times can be useful for a given management program. It is better to collect samples every year in fields that grow multiple crops and intensive crops such as fruits or vegetables. For less intensive cropping sequences, sampling every two to three years is enough. In either case, fields should always be sampled during the same month of the year because results for some nutrients, pH, and lime requirement will vary seasonally due to weather and other conditions. This will make year-to-year comparisons much easier. In-season sampling is typically done to diagnose problems. Any change in cropping practices should be preceded by thorough soil testing.
Sampling Tools
A stainless-steel soil sampling auger is the most commonly used tool for collecting soil samples under normal conditions (Figure 11.4). The soil probe provides a continuous soil core with minimal disturbance to the soil that can be readily divided into various sampling depths. Vehicle-mounted hydraulic probes are available and are a better choice under adverse soil sampling conditions.
Sampling Depth
Laboratory tests are calibrated to specific depths. It is vital to collect samples from appropriate depths because a core taken deeper or shallower will generate erroneous results. For cereals and other shallow-rooted crops, soil samples from 6 to 8 inches may be suitable. The top 6 to 8 inches of soil has the most root activity and fertilizer applications are generally restricted to this depth.
Sample Handling and Record Keeping
Soil samples should be collected into clean plastic buckets and mixed well. Break up and mix cores well before taking a composite subsample for laboratory analysis. Most soil testing laboratories provide small moisture-resistant boxes, sacks or bags that hold about half a liter of soil.
Soil Tests
All laboratories generate reports for each sample submitted for soil analysis. All reports will contain the same basic information although individual labs may present this information in their own unique format. Labs use various extraction methods in determining the level of a certain nutrient in the soil. Soil testing laboratories use a solution that works well with soils that predominate in the region they serve and the situation. There is an advantage in using a local soil testing laboratory. Since labs use diverse extraction methods, the nutrient analysis of one lab is not directly comparable to another lab unless both are using the same procedures.
Testing for Nitrogen
Plants use nitrogen in the nitrate (NO3¯) and ammonium (NH4⁺) forms. However, ammonium levels are generally very low in soils so soil nitrate tests are used to measure plant available nitrogen. Testing for the nitrogen supplying capacity of soils is particularly challenging. While the nitrate present in the soil at the time of sampling can be measured, a nitrate test is only a snapshot of the nitrate levels in the soil at that exact moment in time. Nitrate levels can fluctuate rapidly as organic matter is mineralized and as nitrogen is lost through leaching or denitrification.
Soil Test Recommendations
Soil tests, and proper interpretation of results, are an important tool for developing a farm nutrient management program (Figure 11.5). However, deciding how much fertilizer to apply—or the total amount of nutrients needed from various sources—is part science, part management philosophy, and part art. For this reason, it is best to think of soil test reports as more indicative than quantitative. Recommendations for nutrient applications take into consideration practical knowledge of the plants to be grown, the characteristics of the soil under study, and other environmental conditions. Management history and field observations can help relate soil test data to fertilizer needs.
Soil Test Ratings
In addition, to reporting nutrient levels in parts per million or some other unit of measure labs often report values classified as deficient, low, medium, high, and excessive (Table 11.2) which are useful for growers in assessing the nutrient status of a soil. Fertilizer recommendations are made accordingly, taking yield targets into consideration. Note that some labs use different word categories such optimum in place of high.
Fertilizer Recommendations
Soil test results typically include fertilizer recommendations. Recommended fertilizer rates may need to be adjusted based on weather and soil properties for the specific location due to the high degree of variability within the region. The recommended nutrient rates shown on a soil test report are for the actual amount of nutrient, not the amount of fertilizer. To determine fertilizer amounts, the fertilizer “grade” (the three numbers on every fertilizer) is needed. Grade equates to the percentage of total nitrogen, available phosphorus (P2O5), and soluble potassium (K2O) present.
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