Now is the time to collect soil samples for determining the fertilization and liming needs of your fields. Whether fertilizer and lime are applied in the fall or in spring, an understanding of the soil nutrient levels in each field will help you with better crop planning and fertilizer budgeting.
That advice comes from Iowa State University Extension soil fertilizer specialists John Sawyer and Antonio Mallarino. Sawyer is an associate professor and Mallarino a professor in ISU’s Agronomy Department at Ames.
“The best time to collect soil samples is after crops are harvested in the fall or before you apply fertilizer in spring, “ says Mallarino. “If you fertilize in the fall, be sure to collect your soil samples before applying any fertilizer, manure or limestone.”
Why is fall the best time to sample? After all, some farmers, fertilizer dealers and crop consultants do indeed sample soil at other times of the year. “Sampling soil in winter or in a growing crop can provide inaccurate pH, phosphorus or potassium test results,” says Mallarino.
This is just one of the recommendations the ISU agronomists offer in their recently updated ISU Extension publication, “Take a Good Soil Sample to Help Make Good Decisions”.
There’s a lot riding on a soil sample. Agronomists and dealers make fertilizer and lime recommendations and farmers decide how much fertilizer or lime to apply, based on the results of a soil test. Sampling the soil is the weakest link in the entire process. You need samples that truly represent the field.
“The opportunity for error is greatest when soil samples are being collected in the field, not in the lab analyses,” says Mallarino. “Collection of a soil sample is the foundation you’ll use to make a number of decisions. Making sure your sampling is done correctly will help you make good decisions.
ZONE OR GRID SAMPLING?
Mallarino and Sawyer advise that you use either zone or grid pattern sampling strategies to select where to collect soil samples within fields.
Zone sampling can use several sources of field information to decide where soil samples will be collected. Examples include soil maps, yield maps, aerial images, field management and nutrient application histories.
Both methods have strengths and weaknesses, and both are acceptable for determining variation in soil tests across fields. These methods can also provide needed soil test information for developing the Natural Resource Conservation Service’s phosphorus “P-index” ratings for fields.
Once you’ve determined where you’ll collect soil samples in a field, make sure you correctly collect them. For example, at what depth should a soil sample come from?
The fertilizer and lime recommendations you can use are contained in another helpful ISU publication, “A General Guide to Crop Nutrient and Limestone Recommendations”. It provides recommendations based on samples pulled from the top 6 inches of soil.
“It’s important that you collect cores at that depth,” says Sawyer. “Also, the greater the number of cores you collect per sample the better each sample will reflect the average test level of the area represented by the sample.”
ISU agronomists recommend you collect a minimum of 10 to 12 cores per sample. If possible, put all cores in the sample bag and send it to the lab. Collect more cores if fertilizer or manure has been previously applied in a band in the field.
Sawyer says soil testing is even more important if you apply manure to your fields. Soil test levels – especially for phosphorus – can change rapidly when manure application rates are based on the crop available nitrogen supply from manure. Applying too much phosphorus can cause environmental problems.
There’s also another good reason to have your soil tested when using manure. “Some nutrients, like potassium, may not be supplied by manure at adequate rates to meet crop needs,” notes Sawyer.
Collecting soil samples on a regular schedule helps you build a soil test database of information, useful for monitoring long-term changes in soil test levels of your fields. The ISU agronomists suggest you sample each field every two to four years, or sample once in the crop rotation.
“Only by sampling routinely can you determine these soil fertility trends,” emphasizes Sawyer.
Reprinted with permission of Farm Progress Companies