Decreasing Dissolved Phosphorus Loss and Building Soil Health with Multi-Purpose Conservation Systems
Phosphorus (P) is an essential nutrient for all life, and P inputs to agriculture are critical for maintaining and increasing the food supply for a growing world population. However, excess P inputs to surface water are a major factor contributing to harmful algal blooms (HABs), which are increasing in number. Non-point runoff from agriculture is a primary source of P inputs to fresh water and a contributor to the prevalence of HABs, especially for agriculturally dominated watersheds in the Great Plains and Central Corn Belt. Decreasing both the total P and dissolved P losses from agricultural land must be a priority for the long-term control and reduction of HABs in the United States.
It is critical that we design agricultural conservation systems that maintain the productivity and economic sustainability of our farms, protect and build soil health, and decrease nutrient loss. We are nearing the end of a 5-year study where we have been measuring the effects of cover crops and fertilizer placement on water quality at the Kansas Agricultural Watershed (KAW) Field Lab. We found that cover crop implementation in a no-till, corn-soybean system slows down runoff, decreases peak runoff rates, and decreases erosion by over 60%. However, cover crops increased the dissolved P loss by 60% with no change or even an increase in annual total P loss. In the same study we found that fertilizer management was the most effective method to reduce P loss, with a 30% reduction in dissolved P loss when P fertilizer is placed below the soil surface. These results were obtained for soils with soil test P above the critical limit (> 20 ppm Mehlich III soil test) and where P application rates were equal regardless of cover crop treatment. There are indications that cover crops may reduce the need to maintain high soil test P. The question remains, can cover crops reduce P fertilizer requirements and thereby decrease soil test P and P loss while maintaining crop yield? However, there are not any data available to answer this question.
We hypothesize that the full benefits from cover crop use will only be obtained when soil test P is maintained below the critical concentration. When soil test P is below the critical concentration, increased biological activity and diversity from cover crops will increase P cycling and availability and reduce the P fertilizer requirement, thereby reducing P application rates and P loss while maintaining crop yield. Our project will collect the necessary data to validate or refute these claims.
This research will be conducted at the KAW Field Lab (http://ksu.edu/kaw), a unique state-of-the-art facility with 18 small watersheds that are fully equipped to measure edge-of-field water, sediment, and nutrient losses. We will have three fertilizer treatments in a no-till, corn-soybean cropping system, including i) no P application, ii) low P application (P applied only if there will be a crop response), and iii) high P application (P applied to maintain Mehlich III soil test P between 20 and 30 ppm). Each fertilizer treatment will be present with and without a cover crop. All six treatments will be replicated 3 times. We will collect water samples from each runoff event and analyze them for total suspended solids, total P, dissolved P, total N, nitrate, and ammonium. We will also measure crop yield, nutrient uptake, nutrient removal, and annual changes in soil test P, total C, total N, and total P.
Results from this study will help producers make the appropriate adjustments in P fertilizer management to economically decrease P loss from cropping systems with cover crops, thereby reducing the chance for HABs while realizing the soil conservation and soil health benefits of cover crops.
Project timeline
October 2019 to September 2024
Funding
USDA Natural Resource Conservation Service
