Soil Tests Essential for Planning '09 Fertilizer Expenses

October 27, 2008

Des Moines, IA – Farmers are encouraged to pull soil samples to begin their nutrient planning for next season’s crops. Following harvest, fall is an ideal time to collect soil samples. 

State Agronomist Barb Stewart, with the U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS), says discovering the nutrients still available in the soil can prevent over- or under-applying fertilizer, saving producers money. “Soil testing is the first step of a comprehensive nutrient plan. It is used to establish phosphorus, potassium and lime recommendations,” she says. Stewart says soil sampling usually costs about $15, although more extensive test results can be requested.

Soil sampling is important in nutrient management because the cost to fertilize a 200-bushel-acre-field of corn could be $160-$225 per acre. “The soil sample is a relatively small expense to find the nutrient levels in a particular field,” says Stewart.

When soil sample results are determined, farmers can calculate how much fertilizer, if any, is needed. “In a tight year, this may be an opportunity to reduce expenses,” says Stewart. It can ensure next year’s crop receives the nutrients it needs for optimal yield and prevent an over-application of costly fertilizer, which could potentially be lost to the environment.

If a producer intends to apply manure yet this fall, Stewart says it makes sense to pull the soil samples before spreading the manure. “Producers should take into account all sources of nutrients to figure out their fertilizer recommendations, including the legume credits, manure credits, soil test levels, and match those up with crop needs,” she says.

Crop consultants and farmer co-ops can sample soils for producers, but Stewart says producers can do it on their own. She says the first step is to take samples at the correct depth. For nutrients such as phosphorus, potassium and pH, take a zero to six-inch sample. (Take a late spring N sample at 12-inch depth.) Once acquired, soil samples should be dried, bagged and labeled, so the lab can extract the appropriate nutrients.

Randomly pull 15 to 20 cores in a 10-acre sample area, avoiding hills and draws to get a whole sample area composite. About one pint from each composite sample should be used. Soil samples should be kept cool or frozen until sent to the lab for analysis. Farmers with precision farming technology, such as variable rate manure spreaders, may want to take more extensive samples based on soils, topography, and other factors which provide a basis for a better prescription for those fields.

For more information about why and when to test soil, how to get started, and how to apply results, visit your local NRCS or ISU Extension office or go online to www.extension.iastate.edu/Publications/NMEP1.pdf.

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