Leopold Center for Sustainable Agriculture

Completed Competitive Grant

Agronomic and environmental soil testing for phosphorus and threshold levels in soils

Project ID: 2001-11

Abstract

Greater knowledge of soil phosphorus (P) is needed to develop application recommendations for Iowa farmers. This project provides more data on the topic by addressing both agronomic and water quality issues.

Key Question: How can farmers apply phosphorus to greatest advantage for their operations?

Findings: Variable-rate and deep P placement are two technologies that have great promise for better nutrient management and are becoming more accessible to farmers. The results of this project showed that the main justification for their adoption is to manage P better and not necessarily to increase yield, at least in the short term. Therefore, the producer should carefully consider costs and environmental benefits before adopting these technologies. Another useful result is that farmers should trust the P index and in particular watch for factors that increase risk of P loss with surface runoff. Major efforts should be directed at reducing soil erosion and surface water flow, placing P deep into the soil, and avoiding overly high soil-test P values. High soil-test levels and practices that produce little P loss through tile drainage can have major impacts on P loss and water quality when the risk of soil erosion and surface runoff is high.

Lead investigator: Antonio Mallarino, ISU Agronomy, et al

Year of grant completion: 2004

This competitive grant project was part of the Leopold Center's Initiative.

Topics: Nutrient management