Strengthening the case for soil testing

Farm Forum

Fertilizer has been a critical component in agriculture since the Native Americans taught the Pilgrims how to place one or more fish in each hill when they were planting corn. When settlers ventured to more fertile lands than the thin, rocky soils of Plymouth Rock, it likely didn’t take long for them to realize they could raise good crops without “fertilizer”; at least for a while. It’s probably anyone’s guess as to whether they knew they were “mining” the soils’ native organic matter, but history books state that crops were commonly raised in fields until yields fell off and then new sites were planted because the soil was “worn out”.

An interesting piece of agricultural history involves the “Morrow Plots”, a tract of land established as experimental plots on what is now the University of Illinois’ Urbana campus in 1876. The Morrow Plots are the oldest agronomic experiment fields in the United States, and include the longest-term continuous corn plot in the world. At the time, farm wage rates in Illinois were $15/month, corn sold for $0.30/Bu, and the state’s average corn yield, 30 Bu/Acre.

In the first 29 years, nothing was added to the soil, the plots were merely tilled and planted, but one set of plots were planted to corn each year, while others were in various crop rotations. By that time, the researchers learned that the fertile prairie soil could be depleted with cropping, and the depletion could be postponed with crop rotations. Beginning in 1904, manure, lime and phosphorus (MLP) were applied to half of each plot, which was also managed in crop rotations that included legume hay. Fifty years later, the unfertilized, continuous corn averaged 25 Bu/Acre while the fertilized corn in a 3 year rotation averaged over 100 Bu/Acre.

In 1955, limestone, synthetic nitrogen (which became widely available following WWII), phosphorus and potassium (LNPK) was applied to a plot that had previously received no treatments and one that had been getting MLP for 50 years. The unfertilized, continuous corn yielded 29 Bu/Acre (this was hybrid corn, introduced in 1937), while the previously untreated, continuous corn plot that received the LNPK produced 113 Bu/Acre. The LNPK treatments also increased corn yields on the plots that had earlier received MLP.

More recently, researchers investigated why corn growth and yields had been about 20 percent lower during the past 50 years for the continuous corn plots than for the corn-oats-hay plots, despite considerably greater inputs of fertilizer nitrogen and residues. They analyzed soil samples for organic carbon and learned that all of the residue carbon had disappeared, and there had been a net decrease in soil organic carbon. Regardless of the crop rotation (or lack of), the decline was greater with higher nitrogen rates. Organic carbon is, of course, organic matter, the most important characteristic of a soil due to its contribution to water holding capacity, soil structure, microbial activity, mineralization to nitrogen, etc.

The researchers don’t question the importance of nitrogen fertilizers for crop production, but they say excessive application rates cut profits, and are bad for soils and the environment. Many people assume that plants get more nitrogen from fertilizer than the soil, but in most cases the opposite is true. This supports the need for soil testing in determining optimum nitrogen fertilizer rates, as well as diverse crop rotations, and cover crops to maintain or enhance organic matter content. To learn more, visit “The Morrow Plots: A Century of Learning”: