TAKING SAMPLING SOIL
The Sampling Area
Soil test results can be no better than the sample collected. Thus, each soil sample submitted to the laboratory should be representative of the area for which fertilizer recommendations are to be made. A composite sample should be collected, consisting of small portions of soil taken from approximately 20 locations. For field crops, soil portions should be taken from an area not to exceed 10 acres (Figure 1). For lawns and gardens, soil portions should be collected at random from eight to 10 locations. Areas of contrasting soils, problem spots or portions of fields where crop response is significantly different should be sampled separately, provided the area can be fertilized separately.
Sampling Tools and Depths to Sample
Several types of tools can be used for collecting soil samples. One is the soil tube or probe. A uniform portion of soil is collected rapidly and accurately by pushing the tube into the ground to the desired depth and removing a soil core (Figure 2).
The most common tool used is a shovel or spade. With this tool, a uniform portion of soil is collected by first making a V-shaped cut into the soil to the depth of sampling. Next, a 1-inch thick vertical slice of soil to the same depth is removed from the smoothest side of the cut (Figure 2). From this, a 1-inch strip of soil the length of the slice is removed, as indicated in Figure 3. If other tools are used for sampling (garden trowel, auger, etc.), make sure that a uniform amount of soil is collected to the desired depth from a sufficient number of sites within the sampling area.
Remove organic debris, rocks and trash from the soil surface before collecting the sample. For determination of pH, P, K, micronutrients and organic matter, take soil samples to a depth of 6 inches.
For NO-TILL row crops, collect the sample to a depth of 6 inches for pH and nutrient determinations and to a depth of 2 inches for organic matter determinations. For determination of soluble salts, sample within the rooting zone of the affected crop or the expected rooting zone if sample is taken prior to crop establishment. For the corn pre-sidedress nitrate-nitrogen test, collect samples to a depth of 12 inches. Soil portions for each composite sample should be placed into a clean container and thoroughly mixed. From this, remove enough soil to fill a sample box (Figure 4). When sampling for nitrate-nitrogen, the sample should be thoroughly dried within 36 hours to obtain the best results.
When to Sample
Although soils can be tested any time during the year, fall is a very desirable time. Fields are usually drier and more accessible and the laboratory is less rushed than in the spring. Also, testing in the fall allows recommended rates of lime, phosphate and potash to be applied well in advance of spring planting. By sampling at approximately the same time each year, it is easier to compare soil test results with previous results from the same field. Soils should be dry enough to till when sampling. If wet samples are collected, they should be air-dried before being packaged and mailed. Wet samples are difficult to handle, more subject to being lost during mailing and greatly delay laboratory testing. Wet samples cannot be analyzed for nitrate-nitrogen.
How Often to Test Soils
The following general guidelines may be used to determine how often soils should be tested. However, the frequency can vary depending upon cropping intensities, soil types, fertilization rates, tillage methods, weather conditions and new research findings.
1. Continuous Row Crops (conventional) – every two to three years.
2. Double-Cropping Systems – every two years.
3. Continuous No-Till Soybeans (only) – every three to five years.
4. Continuous No-Till Corn or Cotton – every two years.
5. Hay Systems – every two years.
6. High-Value Cash Crops (Tobacco, Vegetables) – annually.
7. Lawns, Gardens and Pasture Crops – every three to five years.
8. Any time a nutrient problem is suspected.
9. At the beginning of a different cropping rotation.
