Contributors to increased phosphorus loss in drainage water and possible solutions
Farmers are challenged by increased phosphorus loss due to greater phosphorus availability in the soil and increased drainage. Research is finding a suite of best management practices to help.
Excess phosphorus can cause harmful algal blooms and eutrophication, as seen in recent years in Western Lake Erie. Much of this excess phosphorus comes in the form of dissolved reactive phosphorus (DRP), and is an unintended consequence of current farming practices necessary to meet the growing food demand. Although there is a range of possible factors, two have been identified as key contributors to increased DRP load in the Western Lake Erie Basin. These two factors are increased water transport efficiency and higher phosphorus availability in the soil.
One of these factors is due to subsurface (tile) drainage, which is vital to crop production as it removes excess water from the field creating a suitable environment for crop growth as well as improving trafficability. This exit pathway for excess water also allows phosphorus to bypass the natural filter of the soil, and be transported more efficiently to surface water. In fact, Jarvie et al. (2017) noted that 65 percent of the increased DRP loads in Western Lake Erie arose from the increased DRP delivery. The increased DRP delivery was attributed to two factors, one of which was increased water transport efficiency and the other was greater phosphorus availability in the soil. Increased transport efficiency has occurred due to more land being subsurface drained and systems designed with closer drain-pipe spacing.
The other key contributor is the expanded prevalence of no-till and reduced-tillage (strip-till and mulch-till) practices that aim to reduce erosion and increase soil organic matter in the Western Lake Erie Basin. These practices increase phosphorus availability in the soil, which is one of the two factors contributing to the increased DRP delivery as found by Jarvie et al. (2017). While these practices are beneficial to crop production, Baker et al. (2017) reported that these practices increase the risk of DRP transport in surface runoff due to the buildup of phosphorus in the upper layer of the soil, called P stratification, which often goes undetected in traditional soil tests. The researchers found that the mean soil-test P (STP) in the top one inch of soil was 55 percent higher than that of samples used for fertilizer recommendations, which could lead to unnecessary fertilizer application, only increasing the buildup of phosphorus near the soil surface. Furthermore, no-till increases the development of macropores in the soil, which would allow some of the stratified phosphorus to move from near the soil surface to the drain pipe very quickly without having to go through the soil to reach the drain, streamlining its course to subsurface drainage and surface water. Williams et al. (2016) recognizes the benefits of no-till, but found that tillage disrupted macropores and temporarily reduced dissolved phosphorus load after rainfall events, and noted that annual P loads could be substantially decreased by tilling after fertilizer application.
Given this information, what can be done to improve or protect surface water quality, while still improving soil health with implementing these conservation practices? For fields with high amounts of P stratification, Baker et al. (2017) recommends an occasional soil inversion with mixing. This will help disperse the buildup of STP, and reduce DRP load in surface runoff and macropore transmission to subsurface drains.
One conservation practice cannot be the answer to surface water quality issues, but adoption of a suite of best management practices (in-field and edge-of-field) could help improve surface water quality as well as crop yields in the future. Examples of edge-of-field practices are controlled drainage and saturated buffers that can reduce nutrient transport in agricultural drainage water.
Research regarding controlled drainage and saturated buffers is being done by Michigan State University to quantify the effectiveness of these edge-of-field drainage conservation practices. More information about the research can be found in the previous Michigan State University Extension article entitled “Drainage Monitoring Workshop to be held July 19, 2018.” This research can then be used to recommend best management practices, specifically related to decreasing DRP delivery through subsurface drainage.