Soil Sampling May 3, 2011

This publication provides information on techniques of soil sampling and analysis of soil samples for horticulture and agriculture. Soils are sampled to determine physical conditions, fertility (nutrient) status, and chemical properties that affect their suitability as plant growing media. Through a combination of field and laboratory research, analytical methods have been developed which provide quantitative estimates of plantavailable nutrients. Field research determines the optimum
soil test levels for various nutrients for specific soil and crop combinations. Optimum fertilizer practices can be determined by knowing the optimum test level of each nutrient for a specific crop and soil, and by knowing how much fertilizer is required to change soil test values. Soil testing is comprised of four steps:
• Collection of a representative soil sample
• Laboratory analyses of the soil sample
• Interpretation of analytical results
• Management recommendations based on interpreted analytical results
This publication will focus on the first two of these steps. The reader should gain an understanding of the proper methods for collecting soil samples, and of the potentials and limitations of soil testing.

Soil Sampling
Soil testing begins with soil sampling. A soil analysis can only be as good as the sample sent to the aboratory. It is important to realize that only a tiny portion of a field is actually analyzed in the laboratory. For example, a 1 lb soil sample collected from a 5 acre field represents just 1/10,000,000 of the field! Therefore, it is critical that a soil sample be representative of the entire field. The most common and economical method for sampling an area is composite sampling, where subsamples are collected from randomly selected locations in the field, and the subsamples are composited for analysis. The analytical results from composite sampling provide average values for the sampled area. The actual number of subsamples depends on field size and uniformity. Generally, a larger field or a less uniform field should be more intensively sampled than one that is small and uniform. No less than 5 subsamples should be taken, and 15 to 25 are preferred. Alternatively, areas can be grid-sampled in a regular pattern. Each sample is analyzed separately, so that variability in soil properties can be determined. With data provided by grid sampling maps of soil test values can be constructed from the results.

 This information can be entered into a geographical information system (GIS) and combined with additional geospatial data, such as soil texture, crop yields, leaf analyses, etc. and used in precision agriculture systems for variable application of fertilizers and other crop inputs. This is a much more expensive method of soil analysis because of the number of analyses required, although it provides valuable information about geospatial uniformity which can be used in precision agriculture. Ideally, samples should be collected with a soil probe or auger, to the depth of tillage (usually 6 to 8 inches) or to the effective rooting depth of plants. Deeper samples may be collected for evaluation of subsoil properties, such as salt or nitrate accumulation. It is helpful to sample to the same depth each time a soil is sampled, so that year to year samples can be directly compared to monitor changes over time. A small shovel or trowel can be used if a probe is not available. Each subsample should be approximately equal in size. The sub-samples should be placed in a clean plastic bucket and mixed thoroughly. The desired sample amount is then removed from the bucket and the remainder discarded. Check with your testing laboratory to find out how large a sample they require. The area or size of the field sampled is dependent upon management practices. Sample the smallest unit that will be managed separately. For example, if a field has two distinctly different halves, perhaps one half level and the other sloped, then sample the two areas separately, and fertilize each half separately to obtain optimum results. However, if each half of the area will not be fertilized or managed individually, there is no need for separate sampling. A single, representative sample will be less expensive and just as useful. Sample the smallest management unit.

 Soil samples should be air-dried or taken to a test laboratory as soon as possible. To dry a soil sample, spread the soil out in a clean, warm, dry area, and let it dry for two to three days. It is best not to heat or dry soil samples in an oven because soil chemical properties may be altered. Bag the sample and send it
to a laboratory for analysis. Soil samples can be refrigerated for several days if they cannot be dried immediately. When is the best time of year to collect soil samples? Soil test values change slightly during the year, but the primary consideration for timing of soil sample collection is convenience. Collect samples early enough in the cropping cycle to allow results to be used to adjust management practices. How frequently should soil samples be collected? The frequency with which soil samples should be collected depends on the specific soil test, environmental conditions, and value of the crop. Status of some soil nutrients can change quickly, whereas others do not. For example, phosphorus levels in soil are unlikely to change rapidly and annual testing may be unnecessary. Nitrogen levels, on the other hand, change very quickly and frequent tests are required to obtain accurate determinations of plant-available levels. A new soil analysis might be necessary after heavy rains or after a prolonged period of water-logging if one needs an accurate soil nitrogen level estimate.

When making substantial changes to soil fertility levels, it is a good idea to make the change over a period of two to three years, retesting the soil annually. If a crop does not have a high economic value, then occasional soil testing (once every 3 to 4 years) may be considered adequate in the absence of any noticeable nutritional problems. In contrast, commercial production of high value crops may warrant annual testing to ensure maximum yields