Knowing Residual Soil Nitrogen Can Lower Fertilizer Bill

With high fertilizer costs, soil sampling is vital for next season. Compiled by staff 

Published on: Nov 22, 2005

Reduce those high fertilizer bills by taking into account residual soil nitrogen.

UNL research has shown that many areas in Nebraska have residual nitrogen in the soil profile leftover from past fertilizer applications, says soils specialist Richard Ferguson. Soil testing can detect this leftover nitrogen and save farmers money in reduced commercial nitrogen costs.

"Higher fertilizer costs combined with low corn prices make soil testing even more important," Ferguson says.”

For example, if you found 80 pounds of residual nitrogen per acre, you could save about $16 per acre when commercial nitrogen costs 20 cents per pound and $24 per acre when it costs 30 cents per pound, according to Matt Stockton, UNL ag economist in North Platte.

Ferguson recommends taking deep soil samples--at least 24 inches--to test for nitrogen yearly. He also recommends testing soil organic matter about every 5 years. Soil organic matter does not change as frequently as nitrogen. Testing soil organic matter requires a shallower test--about 8 inches.

When taking soil tests, a 160-acre field should be divided into four areas and 12 to 15 samples should be taken in each quadrant. The samples should be mixed well, then about a pint of soil should be sent to the lab for analysis. Fewer deep soil samples, about eight to 10 per quadrant, are necessary for residual soil nitrogen analysis.

Soil sampling and analysis costs about $1 per acre. With nitrogen at 20 cents per pound, as little as 5 pounds of residual nitrogen covers soil sampling costs and analysis.

Residual soil nitrogen also is often greater in fields where manure has been applied. David Tarkalson, soil fertility and nutrient management specialist in North Platte, says, “Manure can be a good source of nitrogen, but be sure to have the manure analyzed at a lab for ammonium and total nitrogen so you can match the application rate with your crop needs."

For more information about correct manure application, visit the UNL Soil Fertility Web site at soilfertility.unl.edu/.

Fall and spring both are good times to sample soils. Fall is especially a good time because it allows you to get soil analyzed and back before spring nitrogen applications.

Ferguson recommends staying within UNL's nitrogen fertilizer application rates for corn even with the higher nitrogen prices.