Phosphorus plays a crucial role in sustainable crop production. Made from finite natural resources, phosphorus fertilizers support high and increasing crop yields, but their use can also elevate the risk for reduced water quality. Increasing the adoption of 4R phosphorus application practices—applying the right source at the right rate, right time, and right place—has great potential to improve both crop yields and water quality. This paper reviews a science-based effort to describe such practices for five major commodity crops produced in North America.
By Tom Bruulsema, Phosphorus Program Director, IPNI
Phosphorus for Sustainable Crop Nutrition
Phosphorus (P) fertilizers contribute considerably to the yields attained in North American cropping systems. For example, long-term research on irrigated corn in Kansas showed a yield reduction of more than 40% where P was not applied for over 30 years (Schlegel and Havlin, 2017). On the other hand, P losses from agricultural soils have contributed to eutrophication issues (Jarvie et al., 2017) and approximately 40% of lakes in the continental USA are considered “most disturbed” owing to high P levels (USEPA, 2016), with nonpoint source runoff from agricultural cropland a substantial contributor.
The 4R Nutrient Stewardship concept emphasizes applying the right source of each plant nutrient at the right rate, right time, and in the right place. “Right” is defined in terms of making the cropping system more sustainable. Thus, 4R P application practices are those that support improvement of both crop yield and water quality. All four attributes—source, rate, time, and place—of P application can have a large influence on these two impacts, and need to be considered together (IPNI, 2012). The right combination, however, is specific not only to each crop and regional cropping system, but also to the factors unique to each farm and field. These factors include soils, local climate, weather, farmer management ability, markets, logistics of field operations, and vulnerability of local ecosystems. The site-specific nature of 4R practices limits the degree of detail with which they can be described across large regional cropping systems.
There is nevertheless a need to describe 4R P practices, particularly in the context of agricultural sustainability initiatives, in which it is important that a wide range of stakeholders, with different levels of technical knowledge of crop production, have an appreciation for the practices employed by producers, and for the opportunities and costs associated with changing and optimizing such practices.