IN THE FIELD
PHOSPHORUS CONTROL PRACTICES OFFER VARYING TRADE-OFFS The management of phosphorus loss must account for the different forms and seasonal characteristics of the nutrient. by Ryan Heiderman and Eric Cooley
M
itigating phosphorus loss from cropland is often a balancing act, with numerous environmental and management factors interacting simultaneously. There are trade-offs when determining how to tackle this challenge. Different forms of phosphorus exist within an agricultural system, each with characteristics that must be matched to an appropriate conservation practice or management strategy to effectively reduce losses. Phosphorus (P) can be in the organic or inorganic form, and it can either be dissolved in water or attached to soil particles in runoff. The behaviors of these various forms should be considered when applying strategies aimed at reducing P loss to the environment. Scientists have long documented the links between harmful algal blooms and other water quality problems to phosphorus, but aquatic ecosystems are particularly sensitive to dissolved P.
Seasonal effects It is well known that surface runoff will happen; the amount and timing of precipitation are uncontrollable, but what is contained in that runoff is controllable. Over the past 20 years, Discovery Farms Wisconsin has been actively monitoring runoff events on farms across the state. Analysis of data from over 127 site years and 2,184 runoff events has shown March to be the month when the most runoff occurs in Wisconsin, with more than double the
the soil has thawed and been worked, particulate phosphorus attached to the soil particles is carried with the runoff.
Weighing the options
For two decades, Discovery Farms has been monitoring runoff events around the state. average runoff amount of the next highest month. March is not, however, the month with the greatest soil loss. June, closely followed by May, loses far more soil than any other time of the year. As such, March and June are the months with the highest P loss, but the type of P lost in these months is different. Similar proportions of the total annual P loss happen in January, February, and March at 39%, compared with May, June, and July at 45% (Figure 1). However, in the early months of the year, 71% of the P is in the dissolved form (moves with water), while in the middle of the year, 70% is of the particulate form (moves with soil). In late winter and spring, runoff is influenced by snowmelt and frozen soils, where the majority of the P loss in this runoff is in the dissolved form. Soils are often frozen and fieldwork is minimal, leaving soil particles in place, and thus, less particulate phosphorus is found during these runoff events. Come summer, when rainfall is often highest and
6 | Journal of Nutrient Management | February 2022
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The implementation of conservation practices should account for different forms and seasonal characteristics of P loss. While a practice may reduce one pathway of loss, it may potentially increase another. For example, conservation tillage and cover crops are an effective means of reducing soil runoff. However, over time, these practices have been shown to cause more stratification and accumulation of P in the upper soil layer, leading to greater water-soluble P, which leads to more dissolved P in runoff.
Finding a balance The Discovery Farms edge-of-field monitoring program has found similar results as other P studies, which demonstrate that the implementation of soil health or conservation practices has some potentially unintended consequences regarding P loss. Keeping the soil in place with minimal disturbance has many erosion control and soil health benefits, but these practices should be combined with P nutrient management strategies. A balance must be made between placing nutrients below the soil surface but not with so much disturbance that soil loss becomes an issue. Injecting or banding fertilizers in combination with conservation tillage practices is one such jofnm.com
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