Reducing Fertilizer Use with a More Accurate Soil Test

By Dennis O'Brien
July 10, 2014

Soil tests that determine fertilizer needs measure nitrate in the soil, but they don't sufficiently account for soil microbes, which mineralize organic nitrogen and make more of it available to a crop. As a result, farmers often apply more fertilizer than they need.

Richard Haney, a U.S. Department of Agriculture (USDA) soil scientist in Temple, Texas, has developed a soil test that replicates some of the natural processes that occur in a field and accounts for that microbial activity, along with measuring nitrate, ammonium (NH4), and organic nitrogen.

Haney is with the USDA Agricultural Research Service (ARS) Grassland, Soil and Water Research Laboratory in Temple. ARS is USDA's principal intramural scientific research agency.

The new soil test is known as the Soil Health Tool. It involves drying and rewetting soil to mimic the effects of precipitation. It also uses the same organic acids that plant roots use to acquire nutrients from the soil. The tool measures organic carbon and other nutrients, accounts for the effects of using cover crops and no-till practices, and will work for any crop produced with nitrogen or other types of nutrient fertilizer.

Haney has made it available to commercial and university soil testing laboratories and has worked with farmers to promote it. Growers who use it receive a spreadsheet that shows the amounts of nitrogen, phosphorus, and potassium available to crops. On average, they reduce fertilizer costs by about $10 to $15 per acre. With less fertilizer applied, there is less of it running off into surface water.

Haney and Daren Harmel, an ARS agricultural engineer at the Temple lab, evaluated the tool in fields where they raised wheat, corn, oats, and grain sorghum at nine Texas sites over four years. They applied traditional fertilizer rates; no fertilizer; and the amounts dictated by the Haney soil tests. They planted and harvested on the same dates at each site, and kept track of fertilizer costs and application dates, crop prices, and overall profits.

They found that the tool reduced fertilizer use by 30 to 50 percent and reduced fertilizer costs by up to 39 percent. The enhanced testing methods had little effect on corn production profits, but increased profits by 7 to 18 percent in wheat, oat, and sorghum fields. The results were published in the Open Journal of Soil Science.

Read more about this research in the July 2014 issue of Agricultural Research magazine.