Evaluation of Cotton Yield, Quality, and Plant Growth Response to Soil-applied Potassium

Studies were initiated at two field sites with a previous history of K+ deficiencies, including Williamson county in the Blacklands region and Wharton county in the Upper Gulf Coast region. In mid April, cotton cv. Phytogen 499WRF was planted into a Lake Charles clay loam at the Wharton site and cv. DP 0935B2RF into a Burleson clay at the Williamson county site. Based on soil test results, 35 and 60 pounds K₂O per acre were recommended for the Wharton and Williamson sites, respectively. Treatments were 0, 25, 50, 75 and 100 lbs of K₂O/a applied shortly after planting using liquid 2-6-12 at the Wharton site, and 0, 40, 80 and 120 pounds of K₂O per acre applied both as liquid 2-6-12 and as granular 0-0-62 before planting at the Williamson site. Phosphorous and nitrogen were equalized for all the K+ treatments. Monthly rainfall during the season was below normal at both study sites. Lint yield responded to rates of applied K equal to and greater than that recommended at both sites. Liquid K applied in a subsurface band had a greater positive effect on lint yield compared to granular K surface applied and incorporated. Applied K improved length, strength and uniformity at the Williamson site. These studies will be repeated to better assess the influence of seasonal differences in rainfall, crop rotation and soil properties on the results. Below are some result summary graphs and tables demonstrating the impact of the potassium application on cotton lint yields, quality, and net returns.


Justification

The frequency and severity of potassium (K+) deficiency symptoms on the highly productive clay soils in the Central Blacklands and Gulf Coast regions of Texas have increased in recent years. While continuous dry conditions have undoubtedly contributed to this consistent occurrence of deficient K+ symptoms, the frequency and widespread geographic nature of the K+ deficiencies in multiple row crops, specifically cotton, is a major concern to producers and scientists.

For the past decade, Texas has continued to become a larger percentage of the total U.S. cotton production. For the past several years, Texas has accounted for nearly 50% of the total U.S. cotton acres. Much of the cotton production in Texas occurs on clay soils on the Blacklands of Texas and Gulf Coast production regions, and K+ deficiencies have been reported in these regions in various years over the past 20 years. However, the frequency of the K+ deficiency symptoms seems to be on the rise, and the geographic occurrence seems to be increasing also as more K+ is mined from the soils.
Additionally, under K+ deficient levels, cotton plants are more prone to multiple foliar diseases that can further reduce the yield potential.

Potassium is required in large amounts by cotton for normal growth and fiber development. Potassium plays a major role in photosynthesis, activation of protein enzymes, increases disease and drought resistance, and positively affects cotton fiber yield and quality. Previous research has shown a two bale cotton crop will remove 30 lb/acre K+ annually. However, increased yield potential in new varieties and better pest management have pushed cotton yields to 3-4 bales and can exceed 5 bales on irrigated land. These high yields put a substantial demand on the roots ability to uptake sufficient K+ and other nutrients to meet the physiological demand of the plant, seed, and lint. As K+ demand continues to increase, deep profile soil samples indicate a reduced level of plant available K+ in some production areas. According to the Nutrient Use Geographic Information System (NuGIS) webpage, K2O balance is negative (-11 to -50lbs/a) for the majority of the cotton production regions in the Blacklands, and Coast Bend, and some of the Upper Gulf Coast (IPNI, 2012).

It is well documented that cotton has been shown to be more sensitive to low K+ availability than most other major field crops, and often shows symptoms of K+ deficiency on soils not considered deficient (Cassman et al, 1989). However, in this case, cotton maybe the first crop to reach the tipping point for low K+ levels and may serve as a “miner’s canary” for K+ deficiencies to come in other row crops.


Objectives

The first objective is to quantify the K+ soil levels, surface and with depth, from several major cotton production regions in Texas experiencing K+ deficiencies. The second objective is to evaluate the impact of applications methods on K+ and rates on cotton yield, quality, and return on investment. Based on these finding, soil K+ recommendations will be re-evaluated and modified as appropriate to optimize yields.


Methodology

Three small plot replicated trials will be initiated on three different soil types in the Blacklands and Upper Gulf Coast regions. Soil samples will be collected to a four foot depth and macronutrients will be quantified for the 0-6”, 6-12”, 12-24”, 24-36”, and 26-48” depth. The three sites will be selected to try to obtain a low, medium, and high level of plant available K+ as reported by the soil test results from Texas A&M AgriLife Extension Service. The K+ application rates will encompass the recommended rate of K+ to accomplish a respectable yield for each region. The application methods will include both a broadcast incorporated and a deep banded application prior to planting cotton. The application rates are included in the Table 1 below. In-season plant measurements will include stand counts, early season vigor, physiological measurements (first fruiting branch, boll distribution, crop maturity), yield, and fiber quality.

Table 1. K+ application method and rates for 2013 trials

Application method	Rate (lb/a)
Broadcast	0, 40, 80, 120, and 160
Deep banded	20, 40, 80, 120, and 160