Variability in Soil Test Potassium and Crop Yield

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). Soils were sampled using a dense grid sampling approach and yield was harvested with yield monitors. Two trials were managed with no-till and the others used chisel-plow/disk tillage.

Mean field soil test K ranged from 102 (low) to 223 ppm (very high), but each field had values ranging from very low or low to high or very high. Averages from the entire length of the strips showed that K applied as either source: (1) did not affect early plant dry weight or K uptake consistently; (2) often increased early plant K concentration; and (3) increased grain yield at three sites. At two sites, broadcast K increased yield more than starter K and at one site both fertilizers increased yield similarly. Analyses of data for within-field areas with different soil series or with soil test K in different interpretation classes showed no consistent differences in response to starter or broadcast K. An interesting result was that a yield response to K sometimes was observed for soil test K levels higher than those for which K application is recommended in Iowa and neighboring states. The few instances with a early plant growth or K uptake response did not show a yield response. Starter K applied in addition to broadcast K never increased yield further.

Comparisons of these results with others using N-P or N-P-K starters indicate that application of in-furrow K has little or no true starter effect, which confirms previous Iowa research conducted with granulated fertilizer applied beside and below the seed row. However,. the low liquid starter K rate applied often increased corn yield as much as the higher broadcast rate, which shows that farmers have management options concerning use of these K fertilizers sources for production of corn. IA-09F


Justification

There are two types of variability that strongly influence how well producers are able to evaluate their K fertilization strategies for a particular field: spatial and temporal. Spatial variability refers to changes in soil test K across a field. Temporal variability refers to changes with time. The larger the variability in either type, the less clear management strategies become. To help producers manage K, it is necessary to help them unravel the causes of the variability observed.

There are several factors that can contribute to both types of variability. Spatial variability in soil test K can arise from differences in soil composition, drainage, past nutrient management, and nutrient removal by crops. The amount of K removed by harvested crop portions depends upon both K content in plant tissue and yield, both of which may vary within a field. Temporal variability of soil test K can originate from changing weather conditions, crop removal, and sampling techniques. The soil test procedures conducted by laboratories also contribute to variability. For instance, in the late 1980s, the Iowa State University soil testing laboratory made a switch in its procedures from using field-moist soil samples to using dried samples. Previously, Iowa was the only state using field-moist samples. The switch was prompted by customer demand for quicker analyses and faster turn-around times. Analysis of soil test K results before and after this procedural change indicated that dried samples resulted in greater variability.

To help producers optimize their management of K, this study investigates factors believed to account for a major portion of temporal and spatial variability in soil test K levels. Among the measurements taken for this study are crop yield, K content in grain, and soil test K as measured by several soil testing procedures. These data are being collected on several K fertility studies in the state of Iowa. Analysis will target the contribution of each factor upon observed variation in soil test K.



Objectives

The overall objective of this research is to investigate the causes of variability in soil test potassium (K) to assist farmers in their management of K. Specific objectives are as follows:

To study the within-field variability in soil test K and the response of corn to K fertilization

To evaluate a recently proposed soil test for potassium with potential to improve the predictability of crop response to potassium fertilization on the basis of soil tests

To study the relationships among crop yield response, potassium removal in corn and soybean grain, and long-term changes in yields and soil-test potassium


Methodology

The general approach in this research is to measure variables believed most important in describing spatial and temporal variability in soil test potassium (K) and corn grain yield where K nutrition is the controlled variable. The measurements taken are:

• corn grain yield (bu/acre)
• soil test K level (ppm)
• K content in corn grain (%)

Since soil test K procedures are another significant source of variability, four procedures are being compared on specific soil samples:

• Ammonium acetate K extraction on dried samples
• Ammonium acetate K extraction on field-moist samples
• Mehlich-3 extraction on dried samples
• Sodium tetraphenyl boron extraction on dried samples

Data are being collected on K nutrition experiments across the state of Iowa. These experiments fall into two categories: small plot replicated trials, and on-farm replicated strip trials.
The goal of the analyses of these data is to model relationships among crop removal of K, changes in soil test K, and corn grain yield for each soil test procedure.