Variable-Rate Application for Phosphorus and Potassium: Impacts on Yield and Nutrient Management

Soil fertility management can be improved by use of precision agriculture technologies. Global positioning systems (GPS), yield monitors, various forms of remote sensing, geographical information system (GIS) software, and variable-rate technology (VRT) are available for use by producers. Dense soil sampling, crop scouting, and other practices complete the new technological package. Soil testing is a diagnostic tool especially adapted for site-specific management. At the same time, GPS and GIS can greatly improve soil testing when these technologies are used to better describe nutrient levels across a field. The spatial variation of plant nutrients over a field makes soil sampling the most important and common source of error in soil testing. Therefore, Georeferenced soil sampling, soil test mapping, and fertilizer application with VRT can improve the efficacy of fertilization compared with the conventional practice of collecting a composite soil sample from large areas and applying a single fertilizer rate over a field

On-Farm Implementation of the Phosphorus Index Observed Risk Ratings and Impacts on Fertilizer and Manure Phosphorus Management

The Iowa phosphorus (P) index is an assessment tool that was developed to assess the risk of P loss from fields to water resources. It provides a risk rating that also can be used to prioritize fields or field zones for manure or fertilizer P application and for implementing improved soil conservation practices. Scientists have been proposing such a tool since the early 1990s, and it has evolved from a simple subjective tool to a more complete objective tool without becoming a complex model of P flow. The need for a P index has its origin in two main issues. One is that P accumulation in many soils in excess of amounts needed by crops has increased P losses from fields and has resulted in poor water quality in many streams and lakes. Water quality is impaired through a process known as eutrophication, which occurs when nutrient levels in water (mainly P) are high and stimulate excessive algae growth. Excessive algae growth reduces water oxygen levels and creates ecological imbalances that result in reduced populations of desirable fish species as well as reduced drinking and recreational value of lakes and streams. The second issue is that P loss from fields cannot be appropriately assessed or predicted only from knowledge of soil-test P, manure or fertilizer P applied, and method of P application used. Although these P source factors are important, how P can move off fields through various transport mechanisms is as important. Therefore, a new tool that integrates P source and transport factors was needed to estimate risk of P loss from fields. The P index also provides information useful to decide among several soil conservation and manure or fertilizer P management practices that can maintain a low risk of P loss or can reduce it

Potassium Fertilization Guidelines in Iowa: Are They Working and Should Applications Be Adjusted with High Fertilizer Prices?

Iowa State University (ISU) researchers have conducted much research over time with help from Iowa farmers, crop consultants, and agribusiness to study potassium (K) fertilization of crops and use of soil-test K as a tool to determine crop K needs. Results of this research are reflected on current fertilizer recommendations, which are explained in the ISU Extension publications Pm-1688 and Pm-1310. The research continues to keep pace with changes of production practices, new hybrids and varieties, and new questions. This year Iowa farmers are looking very carefully at fertilization practices because recent sharp increases in fertilizer and fuel prices have not been matched by increases in grain prices. The ISU K recommendations were last updated for the 2003 crop year. The most significant changes were (1) to recommend maintenance of higher soil-test K levels for optimum crop production and (2) to suggest deep K fertilizer placement for grain crops managed with ridge-till and no-till systems. In this presentation we review the K recommendations and share highlights of recent on-farm research. Decisions about fertilization for any economic situation should be made based on knowledge of the probability and size of crop response to fertilization for different soil-test levels, fertilization rates, and prices