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Using Post-Harvest Soil Sampling as Part of your Soybean Nutrient Management Program

Every farmer knows that fertilizer and nutrient inputs are some of the biggest crop expenses on the farm and are essential to the vitality of crops. Yet, each year, farmers spend thousands of dollars adding inputs to their fields without sampling their soil first and knowing what those fields need when it comes to fertility.

Without regular soil testing, farmers may be throwing away money on unnecessary inputs or neglecting the soil by not replacing essential nutrients that have been extracted by a crop.

“Soil sampling should be considered an integral part of every management plans since it impacts the way farmers will implement every other action on their crops,” says Bobby Golden, Ph.D., assistant professor of plant and soil sciences at Mississippi State University.

Soil samples should be taken at least every three years, following the same crop in a rotation system, and should be taken at the same time each year, preferably after harvest for spring-planted crops. Sampling at this interval provides a consistent basis for comparing fields and detecting trends over time. Samples should also be taken when the soil has moisture levels suitable for tillage. The samples should be used to test for phosphorus, potassium and pH levels, plus minor nutrients, organic matter and cation exchange capacity.

“Soil sampling doesn’t always mean that farmers should expect to have to build up a lot of nutrients in the soil,” says Golden. “Sometimes it can just mean a small addition or no addition at all to maintain natural levels.”

Soil-test results can help farmers monitor the nutrient levels in their fields and create a nutrient-management plan for the next growing season. This plan can also minimize their environmental impacts by preventing excess nutrients from being applied to fields. Their plan should document all available nutrient sources they plan to use, along with the production and management practices that affect their soil fertility. Through better planning, farmers can have better nutrient availability and crop productivity, and become better stewards of the environment.

Proper nutrient application can also help farmers to increase yields and improve overall crop health. Without soil testing and nutrient management, farmers run the risk of having insufficient or excessive nutrient levels in the soil, which could limit or stop plant growth and development, decrease yields, lower profitability and increase environmental risks.

Mississippi State University Extension and other laboratories analyze soil samples for nutrient content, but it is important for farmers to be able to read and understand their soil test results. When used with yield data, test results can help farmers make the best management decisions for their fields.

Farmers should remember these recommendations to collect the best sample and get proper test results:

  • Use only stainless steel or other non-reactive metal sampling equipment and clean plastic buckets to gather samples.
  • Soil probes or augers are the best tool for sampling because they ensure a consistent depth and that the correct amount of soil is gathered.
  • Gather core samples at random in a zigzag pattern in the area being tested, composite and mix well.
  • In tilled fields, gather soil samples from a depth of six inches. In minimum- or no-till fields, take samples from a four-inch depth. And in pasture and hay fields, take samples from a two-inch depth.
  • Consistently sample during the same season from year to year.
  • Maintain accurate records with field maps, sampling points and timing, crop and fertilizer history and other management activities.

For more resources on soil sampling, visit the Mississippi Soybean Promotion Board website: and the Mississippi State Extension website,