Importance of taking good soil samples
A soil test is the only practical way of telling whether lime and fertilizer are needed. However, if a soil sample does not represent the general soil conditions of the field, the recommendations based on this sample will be useless, or worse, misleading. An acre of soil to a 6-inch depth weighs about 1,000 tons, yet less than 1 ounce of soil is used for each test in the laboratory. Therefore, it is very important that the soil sample is characteristic of the entire field. The following directions will help you collect good soil samples.
Where to take soil samples
If the field is generally uniform, fewer composite samples may be required than for fields with more variation. A composite sample consists of a core or boring taken from at least 10 different places in the area to be sampled.
Avoid sampling areas such as:
In general, do not sample any area of a field that varies widely from the rest of the field in color, fertility, slope, texture (sandy, clayey, etc.), drainage, or productivity. If the distinctive area is large enough to receive lime or fertilizer treatments different from the rest of the field, sample it separately. If manure or crop residues are on the surface, push aside these organic materials to keep from including them in the soil sample.
On contour strip fields, sample each strip separately if it is approximately 5 acres or more in size, following the sampling intensity guidelines listed in this publication. Cores from two or three small strips that have identical cropping and management histories may be combined following these same recommended sampling intensity guidelines.
Sampling fields for a single recommendation
With conventional sampling, you will receive a single set of results based on sample averages. The sampling guidelines in table 1 are based on when the field was last tested (more or less than 4 years) and whether the fields were responsive or non-responsive the last time they were tested (if within 4 years). The responsive range is considered to be where either soil test P or K levels are in the high (H) category or lower. A non-responsive field is one where both soil test P and K levels are in the very high (VH) or excessively high (EH) categories.
To assure accurate representation of the nutrient needs of the field, each sample should be made up of a minimum of 10 cores. Research has shown that taking 10-20 cores provides a more representative sample of the area than when samples are made up of fewer cores. Use a W-shaped sampling pattern (as shown in figure 1) when gathering composite samples. Be sure to thoroughly mix the cores before placing approximately 2 cups in the sample bag.
It is an advantage to submit multiple samples for all fields. When at least three samples are provided, the Wisconsin soil test recommendation program will remove samples that are significantly higher than the field average. This ensures that no part of the field is under-fertilized. Where only one or two samples are submitted for a field, no sample can be discarded, whereas one sample can be discarded if three of four samples are submitted, and up to two samples may be discarded from fields having five or more samples.
Recommended sample intensity for "uniform" fields.
Figure 1. Recommended W-shaped sampling pattern for a 15-acre field. Each sample should be composed of at least 10 cores.
How often to sample For field crops, sampling the soil once every 3-4 years or once in a rotation is sufficient. Fields that are more susceptible to changes in nutrient levels, such as those with sandy soils or those used to raise high-value crops such as potatoes should be sampled more frequently.
Tillage system considerations when sampling
Moldboard plowing. Sample to the depth of tillage.
Chisel plowing and offset disking. Take soil samples to ¾ of the tillage depth. When possible, take soil samples before spring or fall tillage. Sampling before tillage lets you determine the sampling depth more accurately and you can avoid fertilizer bands applied for the previous crop.
Till-plant and ridge tillage. Sample ridges to the 6-inch depth and furrows (between rows) to a depth of 4 inches. Combine equal numbers of soil cores from ridges and furrows to make up the composite sample.
No-till. Fields that have not been tilled for 5 years or more may develop an acid layer on the surface from the use of nitrogen fertilizer. This acid layer could reduce the effectiveness of triazine herbicides. Unincorporated phosphorus (P) and potassium (K) are also likely to build up in the surface soil. If an acid layer is suspected, take a separate sample to the depth of only 2 inches. When sending the soil to the lab, indicate that the sampling depth was only 2 inches. This sample will be tested for pH only, unless P and K are specifically requested. For fertilizer recommendations, take a separate sample to a depth of 6-7 inches. Fertilizer recommendations require this sampling depth because fertilizer calibration studies are based on plow depth sampling. Sample between rows to avoid fertilizer bands.
This information is adapted from a publication distributed by the UW-Extension. Copies may be obtained by contacting your Wisconsin county Extension officer or from Cooperative Extension Publishing, 45 N. Charter Street, Madison, WI 53715, 608-262-3346. Outside Madison, call toll-free: 877-WIS-PUBS (947-7827). Before publicizing, please check on this publication's availability. To see more Extension publications, visit their website at www1.uwex.edu/ces/pubs/.