Fertilizers provide mixed benefits to soil in 50-year study
When he tested soils from the experimental plots, he saw soil organic carbon concentrations rise gradually with increases in nitrogen fertilization at soil depths from 0 to 6 inches, although not at deeper ones. Similarly, phosphorus fertilization increased soil organic carbon at depths of 0 to 3 inches and 6 to 12 inches.
But Blanco observed a different trend in soil aggregate stability, especially when nitrogen and phosphorus were applied together at high rates. At a depth of three to 12 inches, for example, adding more than 80 pounds of nitrogen per acre reduced the number of stable soil aggregates by 1.5 times when no phosphorus was applied, by 2.1 times at 18 pounds of phosphorus/acre, and by 2.5 times at 36 pounds of phosphorus/acre.
Blanco can’t say for certain why this occurred, but he has some hypotheses. Some studies suggest that adding fertilizers rich in ammonium ions may cause soil particles to disperse rather than aggregate, thereby offsetting any positive effects of increased soil organic carbon content. Because tillage periodically disturbs the soil, it may also negate any benefits of fertilization.
Blanco is now testing these hypotheses in three additional long-term experiments in Nebraska that encompass a wider range of tillage practices and cropping systems. The effects of nitrogen and phosphorus fertilizers on crop yields are well-researched, of course. Likewise, reduced tillage, cover crops, intensified cropping systems, and other conservation practices are known to build the soil long-term. Blanco now wants to see the two come together.
“It’s clear that we need inorganic fertilizers to meet the increasing demands for food production, so it’s important to look at how the extensive use of inorganic fertilizers affects soil properties in the long term,” he says. “The hypothesis is that inorganic fertilization combined with conservation tillage—strip till, no-till, and others—may improve soil structural properties relative to conventional tillage systems.”