Aug. 27, 2012 -- Variable-rate application of nitrogen (N) and phosphorus (P) fertilizers has become one of the most widely-used precision agriculture practices in the U.S. Corn Belt, with potential to enhance farmers’ profitability and reduce environmental impacts associated with both excessive and deficient fertilization.
While the technology to implement precision agriculture is highly advanced, the agronomic understanding needed to support site-specific management has lagged behind. In particular, relatively little quantitative information exists on how within-field variability in soil cycling of N and P may influence optimum fertilization rates. University of Minnesota and University of Alaska-Fairbanks scientists analyzed the results of a six-year field experiment to determine how soil N release and P buffering vary over space and time.
Substantial quantities of a crop’s N requirement can be met by N released through microbial mineralization (decomposition) of soil organic matter. The amount of potentially mineralizable soil N is known to vary across fields; however, farmers have not known how this variation might change over time. Moreover, relating potential N mineralization to actual N availability has been challenging.
In this experiment, the researchers found that patterns of potentially mineralizable N were stable over six years. On a uniformly well-drained field, patterns of potentially mineralizable nitrogen values related consistently to soil nitrate levels; at a poorly-drained site, they did not.
What these results indicate is that site-specific N fertilizer applications based on predictions of soil mineralization will most likely succeed when soils are uniformly drained, allowing mineralization to occur under uniform conditions across a field. But when field moisture conditions are not uniform, predictive approaches are likely to fail and farmers should rely instead on in-season sensing of crop N status.
In contrast to excess N, which is readily leached from fields or lost by conversion to N gases, fertilizer P not used by a crop can be banked in the soil from year to year. Past research has given farmers and agronomists excellent information on the soil test P level necessary to meet crop needs. Currently, site-specific P fertilizer applications are made assuming that the effect of fertilizer P on soil test P is uniform across fields.
However, there are sound reasons to expect that the quantity of nutrient addition required for a unit change in soil test level may differ among soils. These reasons include differences in soils’ ability to retain nutrients in forms unavailable to plants, differences in mineralization capacity, and nutrient loss or accumulation due to runoff and erosion.
After accounting for P removal in grain, researchers in the current study found substantial differences in the quantity of fertilizer P necessary to raise soil test P levels. These differences were primarily related to soil pH, which affects the chemical forms that P takes when applied to a field.
By taking these differences into consideration, site-specific P fertilizer rate recommendations could be substantially improved. In addition, knowledge of the quantity of fertilizer P necessary to attain a critical soil test P level will allow producers to calculate economic-optimum P rates.
While these results provide some new guidance for site-specific N and P management, considerable work remains. Given the magnitude of differences in P buffering capacity and N mineralization, the researchers foresee potential benefit in developing and including buffering capacity and mineralization assessments alongside traditional soil tests of nutrient levels.
These assessments would provide agronomists with important components of P and N cycling that are currently underutilized for site-specific management decisions. This information would also help farmers minimize environmental impacts of misapplication of fertilizer nutrients, help optimize use of scarce fertilizer resources, and allow farmers to more efficiently attain the most profitable nitrogen and phosphorus levels for crop production.
Peter Anthony, Gary Malzer, Mingchu Zhang and Stephen Sparrow. 2012. Soil Nitrogen and Phosphorus Behavior in a Long-Term Fertilization Experiment. Agron. J. 104(5): 1223-1237. View abstract