Soil testing and fertility analysis has been around a long time in Wisconsin and is one of the most highly recommended practices. It is the only way a farmer or land owner can know for sure what the fertility level is in their fields.
With the increase in nutrient management regulations, more farms are soil testing and using the analysis to determine fertilizer and manure applications. Still, many farmers do not soil test or have not tested on a routine basis. How come? A couple of reasons why can be dispelled quickly.
Soil testing is expensive. Wrong!
University of Wisconsin soil testing recommendations are to collect one composite sample per five acres and to soil test at least once every four years. Using a standard rate of $8 per analysis, this averages out to 40 cents per acre per year. Pretty cheap considering all other costs needed to grow an acre of a crop in Wisconsin. Keep in mind this only considers the cost of the laboratory analysis and not the cost of time and labor to collect the samples.
Maintenance applications of P and K are appropriate in all situations. Wrong again!
Soils testing in the “very low” to “low” range for P and K require additional inputs beyond removal rates to optimize yield. Fact: Soils testing in the “high” to “excessive” range require less than removal rates to optimize yield. In either case, money is lost from either reduced yields or over application of P and K. When soils tests indicate the soil is in the very low to low category, this suggests that there is a very high likelihood that yields will increase due to application of fertilizer. However, it also indicates that the subsequent crops would benefit from building the “fertility” of the soil through additional P and K inputs over time.
Fall and after harvest is a great time of year to have soil samples collected and analyzed for a number of reasons. First, crops will have matured and completed nutrient uptake, which provides a more accurate soil analysis of the nutrients remaining in the soil. It is also easier to take samples when the crop is harvested and out of the way making field access easier. Following harvest, there may be a little more time on the farm to get the samples collected. Finally, fields tend to be firmer in the fall which allows for easier access within fields. Of course, waiting too long and wrestling with a frozen field to give up a sample can be an issue. So take sample this fall before the ground freezes.
Once soil test results are obtained, winter is a great time to develop or update a nutrient management plan. Once legume and manure credits are taken, fertilizer needs will be known and any pre-pay or pricing discounts can be taken.
For most fields, one soil sample for every five acres is needed to ensure accuracy of individual samples and the overall field-test average. Each sample should consist of 10 to 20 cores which should be taken from a W-shaped pattern in the field. These cores should be mixed thoroughly in a bucket and two cups of soil extracted for the sample. For site specific or precision management, a grid can be formed on one to five acre grids to obtain the samples which can be taken using GPS or a guidance system. In this case, 10 cores are taken from a small area around a geo-referenced point.
Individual cores should be taken to a depth of six inches or the depth of tillage. For fields under a no-till system for more than five years, a separate two-inch sample for pH is recommended as an acid layer could form from the use of nitrogen fertilizers. Regarding contour strips, which may be smaller than five acres in size, two or three strips with similar cropping systems and management may be combined to reach the five-acre threshold.
To stay current on soil tests, fields should be tested every four years. High-yielding alfalfa and corn silage fields could be tested every two years to adequately manage nutrient levels in these fields. Samples should be sent to Wisconsin, Department of Agriculture, Trade and Consumer Protection Certified Laboratories which are required to provide fertilizer recommendations based on UW guidelines along with the soil test values. For more information on sampling and testing soils, check out UW-Extension Publication A2100, Sampling soils for testing.
- Clark is the Chippewa County Extension crops and soils agent.