As soon as autumn harvest is complete, farmers are encouraged to pull soil samples to begin their nutrient planning for the next season’s crops. Conservation Agronomist Eric Barsness with the Natural Resources Conservation Service (NRCS), Mitchell , SD , says discovering the nutrients still available in the soil can prevent over or under applying fertilizer, which can save producers money.
“Just a regular series soil sample, which includes nitrogen, phosphorus, potassium, and a few other nutrients, is usually about an $11 test,” says Barsness. More extensive tests results can be requested. Sampling soil is important in nutrient management because the cost to fertilize a 150-bushel-acre-field of corn could be up to $180 an acre. So, Barsness says, “the soil sample is a relatively small expense in order to find out the nutrient levels in a particular field.”
With a realistic yield goal in mind, farmers can then calculate how much additional fertilizer will be needed. If a producer intends to apply manure to the field yet this fall, Barsness says it makes sense to pull the soil samples before spreading the manure. “That brings up a good point; 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.”
Many crop consultants and farmer co-ops can take soil samples for producers but Barsness says it’s not difficult if farmers want to do it on their own. He says the first step is to sample soil at the correct depth – for immobile nutrients such as phosphorus, potassium, and zinc, take a zero to six-inch sample. Take a 6-to 24-inch depth sample for mobile nutrients, such as nitrate-nitrogen, sulfur, and chloride. Barsness says once acquired, those samples should be dried, bagged, and labeled, so the lab can extract the appropriate nutrients. Although they are separate samples from two depths, this would equal one sample with one lab fee.
About 15 to 20 cores should be pulled randomly in an 80-acre field avoiding hills and draws to get a whole field composite. About one pint from each depth should be used as the composite sample. Soil samples should be kept cool or frozen until sent to the lab for analysis. Since a pound of soil is literally alive with billions of microorganisms, keeping the sample cold slows decomposition and further break down of organic matter which could influence the nitrogen test result. 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 will provide a basis for a better prescription for those fields.
Barsness says taking a soil test is the first step in a comprehensive nutrient plan. It can ensure next year’s crop receives the nutrients it needs for optimal yield and it can prevent an over-application of costly fertilizer which could potentially be lost to the environment.
Brochures with guidelines for correctly taking soil samples are available through local Cooperative Extension Service, NRCS, or conservation district offices or online at the comprehensive nutrient management planning page of the South Dakota NRCS Web site .