Diagnostic of Soil Phosphorus Availability for Crops and Concepts of Fertilizer Recommendations

Profitable crop production requires appropriate soil phosphorus (P) levels. Careful P management planning is required because recently grain and fertilizer prices have fluctuated significantly and excessive or badly applied P impairs water quality. Soil-test P interpretations and estimates of P removal are key pieces of information that must be used together with fertilizer/crop price ratios when deciding P application rates. In contrast to N management, properties of most soils of the Corn Belt that result in P retention by soils in crop-available forms allow for managing soil-test P and fertilizer application over time. For example, P can be applied for one crop or only once before the first crop of a two-year rotation, and an excess of P for a first crop does not necessarily result in a long-term economic loss or higher risk of water quality impairment. The possibility for long-term management together with consideration of the probabilities of soil sampling and testing errors and of crop response result in several alternatives for soil-test interpretations and fertilizer application. Other considerations that seldom are clearly included in recommendations further expand the realm of viable interpretations and management alternatives. These include producers’ attitude concerning risk when investing on fertilizers, land tenure, business management philosophy, and potential impacts of practices on water quality among others. Some of these issues cannot be easily included in recommendations but must be considered by producers and consultants making fertilization decisions because they may result in different “best management practices” depending on the specific situations. Many producers and crop consultants prefer simple answers and “one fits all” recommendations, but this may not be always possible and over-simplification of recommendations may not result in better nutrient management, more profitable crop production, or better water quality. This presentation will address these issues based on the Iowa experience in a way that may be useful to producers and consultants in other states where production conditions may be similar or different.

Back to Index

Dr. Antonio Mallarino Professor of Agronomy
Iowa State University
apmallar@iastate.edu


Antonio P. Mallarino is Professor of Agronomy, Soil Fertility Research and Extension Specialist, at Iowa State University. He began his career at the University of Uruguay in 1976 and joined the Iowa State faculty in 1993. He develops research and extension programs related to agronomic and environmental issues of nutrient management with emphasis on phosphorus, potassium, and liming. Issues addressed include soil and plant-tissue sampling and testing, fertilizer placement methods, use of variable-rate technology, and phosphorus management impacts on water quality. He has published more than 50 scientific articles and 200 technical/extension articles. Mallarino oversees the Iowa State University Soil and Plant Analysis Laboratory, represents Iowa at the regional USDA-CSREES committees Soil Testing and Plant Analysis for the North Central Region (NCERA-13) and Minimizing P Losses From Agriculture (SERA-17/IEG), represents the NCERA-13 at the oversight committee for the North American Proficiency Testing Program (S890), has served as Associate Editor of Agronomy Journal and Soil Science Society of America Journal, and is Fellow of the American Society of Agronomy and the Soil Science Society of America.