Variability in Soil Test Potassium and Crop Yield

The goal of this research is to improve K fertilizer management for corn and soybean in Iowa and to generate results useful for other regions. Several sub-projects are based on short-term or long-term interrelated trials. This summary highlights results of two sub-projects. Research that included 33 corn site-years and 14 soybean site-years investigated K uptake and recycling to the soil.

The primary objective of this project is to improve Iowa State University K fertilizer recommendations for maize and soybean. This research project is actually a compilation of several interrelated experiments being conducted to meet this objective.

Work was completed on the four year study on "Potassium Uptake, Recycling, and Soil-Test Temporal Variability in Soybean and Corn". The initially planned project was improved upon by also evaluating P recycling. Main findings were that K recycling to soil from standing soybean plants at physiological maturity in the fall and from residue until early the following spring was much more complete and faster than for corn.

We advanced at summarizing, for publication, a project to study the impact of genetic rootworm resistance on corn response to K. Work also continued on five long-term trials with corn-soybean rotations managed with and without tillage to understand soil-test K (STK) temporal variability and relationships among K rates, placement, STK, and grain yield. A summary was completed of 16 years for the no-till treatment, which included measuring removed K.

A project studying impacts of rootworm incidence and genetic rootworm resistance on corn response to K fertilization was completed. Treatments were two hybrid isolines with or without rootworm resistance and five K fertilizer rates. Data from 27 site-years showed that root injury by rootworms was much less for the resistant hybrid. The grain K concentration seldom differed between hybrids, but grain yield and K removal were greater for the resistant hybrid half the time.

This project studied early growth, early K uptake, and corn grain yield response to in-furrow fluid starter K and broadcast K across the landscape using eight replicated strip trials and precision agriculture technologies. A low-salt liquid starter fertilizer (0-0-30) was applied at 15 to 22 lb K2O/A alone and after broadcasting 120 lb K2O/A (a common removal-based rate used by farmers for corn of corn-soybean rotations).

Four conventional plot trials with corn were established at four research farms in 2006, five additional trials were established in 2007, and evaluations continued in 2008 for a total of 22 site-years. Treatments replicated four times were two hybrids and five K fertilizer rates (0 to 180 lb K2O/A). All sites had histories of rootworm infestation and soil test K ranged from the upper range of the Very Low class to a value between Optimum and High. No root insecticide was applied.

In 2007, this project evaluated the impact of rootworm resistance genetics on grain yield and nutrient uptake by corn. Nine conventional plot trials with corn were established at five Iowa State University (ISU) research farms. The treatments included two corn hybrids (i. e. , rootworm resistant and susceptible) and five K fertilizer treatments (0 to 180 lb K2O/A). All sites had corn planted the two previous years, which showed evidence of rootworm infestation.

How great is the variability in potassium (K) soil test levels across fields and years? How does this variability impact crop responses to applied K? How well do currently-used soil test K methods predict the likelihood that crops will respond to K additions? These questions are being actively researched in Iowa, both at research stations and in farmer fields.

How great is the variability in potassium (K) soil test levels across fields and years? How does this variability impact crop responses to applied K? How well do currently-used soil test K methods predict the likelihood that crops will respond to K additions? Research in Iowa is showing that the current practice of using dried samples in soil test K procedures may sometimes provide inaccurate indications of whether or not crops will respond to additional K.

How great is the variability in potassium (K) soil test levels across fields and years? How does this variability impact crop responses to applied K? How well do currently-used soil test K methods predict the likelihood that crops will respond to K additions? Research in Iowa is showing that the current practice of using dried samples in soil test K procedures may sometimes provide inaccurate indications of whether or not crops will respond to additional K.

How great is the variability in potassium (K) soil test levels across fields and years? How does this variability impact crop responses to applied K? How well do currently-used soil test K methods predict the likelihood that crops will respond to K additions? Research in Iowa is showing that the current practice of using dried samples in soil test K procedures may sometimes provide inaccurate indications of whether or not crops will respond to additional K.

A need to update Iowa soil-test potassium (K) interpretations was first suggested during the mid 1990s by an increasing frequency of K deficiency symptoms in corn. These symptoms occurred mainly when spring rainfall was below normal, but were observed in other conditions—even in some soils that tested optimum according to current interpretations.

The primary objectives of this research are (1) to study the variability in soil test potassium (K) and corn response to K fertilization and (2) to evaluate soil tests for K currently used in the Midwest as well as a new soil test for K with potential to improve the predictability of response. This research is based on evaluation of long-term trials at three research farms and several on-farm replicated strip trials.

The main objectives of this research are 1) to study the variability in soil-test K and corn response to K fertilization and 2) to evaluate soil tests for K currently used in the Midwest and a new soil test for K with potential to improve the predictability of crop response to K fertilization. In the 2000 season, the second year of the project, soil samples and grain yields were collected from three long-term, conventional trials at three research farms and from three on-farm strip trials.

The objectives of this research are to: (1) study the within-field variability in soil-test K and corn response to K fertilization, (2) evaluate a modified K soil test that has the potential to improve the predictability of crop response to K fertilization, and (3) study the relationships among yield response, K removal in grain, and long-term changes in yields and soil-test K.