How do I collect Soil Samples

Collecting the samples correctly will be necessary in order to obtain an accurate analysis:

1. At least five probes of soil, 5-7" deep, should be taken at random throughout the area to be sampled. Samples of differing depths may lead to very different results and recommendations for the application of nutrients may actually be incorrect for the area that was sampled. Free sampling kit is available.

2. Each soil sample should be from the same soil type, ie; sandy, clay, loam, etc. and the

color must be somewhat the same for all probes taken for a sample. This ensures that the tests will be more accurate than if soil types or soil colors are mixed together to form a single sample.

3. The size of the sampled area should not exceed ten acres per sample. One to five acres would be more practical, depending on the pasture or paddock size.

4. Make sure that areas being sampled are large enough to treat with the appropriate materials. An area that is, perhaps, 10 ft. by 20 ft. within a larger paddock or pasture may be impractical to treat even though it may not be very productive.

5. Try to avoid areas where animals leave a large amount of droppings or areas where they congregate and leave the soil bare.

6. Samples should be air dried, but not heated above 100° F as this may affect laboratory readings.

7. 1-1½ cups of well mixed, dried soil per sample area should be sent to the lab.

8. The soil sample bags are to be folded down and placed tightly in box to be shipped. Box included with free sampling kit.

Fields with non-cultivated crops, i.e. pastures, can be sampled during the dormant season. Adequate lead-time should be allowed for sample analysis, data interpretation, fertilizer recommendation and actual application.

To account for seasonal variations, soil samples should be collected at approximately the same time each year. Soils should be re-tested as often as necessary to determine the influence of cultural practices and crop production on soil chemical properties. Nitrate-nitrogen concentrations should be determined on an annual basis.

Sampling Materials

Common tools used to sample soils for routine analysis include: spade, hand probe, hand auger, bucket auger, Oakfield probe and King tube, or vehicle-mounted hydraulic probe and auger.

Sampling equipment should be clean, free of rust and chrome plated or made of stainless steel, especially for micro-nutrient analysis. Store sampling tools in a location free from contamination for example, away from fertilizer materials. Free sampling kit is included with analysis.

Obtaining a representative soil sample is the key to a successful soil testing program. A representative soil sample gives an average estimate of the whole area sampled. If sampled as one unit, a field with a high degree of variability may not reflect the field's average condition.

Specialized areas, such as dead or back furrows, manure piles, fences, roads, eroded knolls, low areas, salty or wet spots and other variable areas should be sampled separately or avoided. Separate soil samples are suggested for areas within a field that have had different crop rotation and fertilizer treatments, or that vary in slope, texture, organic matter and depth, as shown by soil color, plant growth or yield.

Table 1. Variability in soil tests of 40 individual soil cores from an
80-acre field.

	Soil Test Value
Analysis	Range	Average
pH	4.5 - 6.3	5.6
Nitrate-nitrogen	1-34 lbs/acre	11 lbs/acre
Phosphorus	3-14 ppm	5 ppm
Potassium	74-385 ppm	153 ppm

Individual soil cores from a minimum of six locations should be mixed thoroughly in clean plastic containers. A sub-sample of the soil mixture is removed and placed in a soil sample bag, often lined with plastic. This sample, commonly referred to as the composite sample, consists of a mixture of the individual cores.

The alternative to compositing is physical averaging of the analytical results of individual soil cores (a costly procedure) to obtain an arithmetic average. Compositing decreases the cost of individual analysis for each soil core. The bags must be properly labeled with field identification, sampling depth, management history and other descriptive characteristics. Moist samples must be air-dried before submission to a laboratory to prevent alteration of the nutrient concentrations by soil microorganisms. Sample contamination from dust and foreign materials should be avoided during the air-drying process. Frozen samples are an alternative, but usually are not practical. Oven drying moist soil samples is not recommended because modification of nutrient form and availability make soil test results invalid.

Preliminary results indicate that microwaving soil samples for two to three minutes may prevent nitrogen transformation by microorganisms. The goal is to stop microbial functions, with no increase in soil temperature. This method, which requires further study, may be useful for only nitrate-nitrogen analysis and not for a total fertility analysis program.

Generally, a 100-acre field comprises the largest area to be sampled as one unit. The number of cores to represent an average soil from a field is determined by field variability, not by the number of acres. The number of cores obtained determines the accuracy and precision of the analytical result.

Crop residues should be removed from the surface before sampling. Sampling depth for most soils traditionally has been the tillage depth. The top 6 inches is the area that has the majority of root activity and fertilizer applications generally are restricted to this depth. Deep-rooted crops, such as wheat, barley and grasses, need deeper sampling if nitrogen fertilizer recommendations are desired. Subsoil samples from the 6- to 24-inch depth or to a root (or probe) restricting layer, are used to estimate available nitrogen and, in some cases, sulfur. The 6- to 24-inch sample should be divided into the 6- to 12 and 12- to 24-inch depth intervals if cost is not prohibitive. Nitrate-nitrogen and sulfate-sulfur are mobile in soil and will move below the 6-inch tillage layer. If leaching has not moved these nutrients below the rooting depth, they will be available for plant uptake. Soil samples down to 48, or even 60 inches, can be obtained with a hydraulic probe designed for deep sampling. Deep sampling can improve fertilizer management for certain crops and/or situations.

Uniform fields can be sampled in a simple random, stratified random or systematic

pattern. The result from these sampling plans, the soil test value, provides an estimate of the entire population of possible soil test results. As the number of cores increases, the error, or chance of obtaining an inaccurate estimate of the average soil test value, decreases. Practically speaking, the time required to obtain soil samples
governs the number of cores taken. A good sampling plan helps to ensure the accuracy of the soil test result. Recent research termed "prescription farming" or "farming by soil" has attempted to integrate the inherent variability of soils with differential fertilizer application. This may or may not include sophisticated application equipment. From a soil sampling perspective, a field is divided into management areas that have similar soil types, terrain position (ridge, side slope, bottom) or other unique features.