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Rising Input Prices and K Fertility

According to recent articles (e.g. Farm profits threatened by volatile fertilizer, pesticide costs” and Farmers optimistic despite rising costs), there is little doubt that rising input prices will be a major consideration for soybean producers for the 2022 growing season. This may cause some producers to cut back on certain fertilizers–e.g. potassium (K)–that they normally might apply ahead of next year’s plantings. This reaction is certainly understandable, and may in fact be an acceptable option if producers have been diligent with their past fertility additions so that the soil reservoir of K is deemed sufficient for an optimum or near-optimum yield.

One of the negatives of the decision to cut back on K fertilizer addition is the real possibility that the crop will suffer from a K shortage during the growing season. This of course can and likely will result in a yield reduction if K in the amount needed for maximum yield is limiting. Thus, knowledge during the growing season about K sufficiency will be valuable to producers.

In an article titled “Dynamic critical potassium concentrations in soybean leaves and petioles for monitoring potassium nutrition”, authors Nathan Slaton et al. describe research results that provide such knowledge. Details about this research and its findings follow.

•    Potassium is a macronutrient that is often present at below-optimum levels in the soil, thus requiring K fertilization to ensure its sufficiency for optimum soybean yield.

•    Tissue analysis is a commonly-used tool for assessing plant levels of required nutrients, and to develop or change fertility programs.

•    Tissue analysis can be a very effective tool as long as the proper plant tissues are sampled at the appropriate developmental stages of the soybean plant.

•    The use of a single critical tissue-K concentration to assess K nutrition across soybean growth stages can lead to inaccurate assessments of plant K sufficiency.

•    The objective of the research reported in the above article was to determine dynamic critical-K concentrations in recently matured/fully developed trifoliolate leaves and their petioles from one of the top four nodes of soybean plants during the reproductive period.

•    Ten K fertilization trials providing a range of soil-K availability and fertilizer-K rates were used to address the objectives. All trials were conducted in Arkansas on silt loam soils that were cropped to a flood irrigated rice-soybean rotation.

•    Soybean cultivars in the 4.4-5.5 relative maturity group (MG) range were planted in April or May each year. Seed yields were determined in all trials, and relative yield in relation to the treatment with the greatest seed yield was calculated.

•    The 10 research trials provided >1400 samples of both leaves and petioles for determining K concentration during the R1-R6 stages. The determined relationships in the article are specific for leaf and petioles as separate tissues.

•    Soybean in all 10 trials responded positively to K fertilization, with yield increases ranging from 11.4 to 29.0 bu/acre. Soybean receiving no fertilizer K yielded 59-82% of the maximum yield.

•    Sufficient K concentrations in trifoliolate leaves were greatest at R1 and declined with time. Trifoliolate K concentrations were 1.98% , 1.85%, 1.57%, and 1.13% at 1 (1 DAR1), 20 (20 DAR1), 40 (40 DAR1), and 60 (60 DAR1) days after R1. These times approximate R1, late R2, R3-4, and R5-6 stages, respectively.

•    Sufficient K concentrations in petioles were also greatest at R1 and declined with time. Petiole K concentrations were 4.97%, 4.0%, 2.84%, and 1.55% at 1 DAR1, 20 DAR1, 40 DAR1, and 60 DAR1. Thus, in the early stages of reproductive development, critical K concentrations in petioles were 2.5 times greater than those in leaves, but at the last sampling time, critical leaf- and petiole-K concentrations were numerically similar.

•    Tissue samples collected during the R1-R2 period and during late reproductive development were less accurate for predicting critical K concentrations than those collected during mid-reproductive development.

•    The authors caution that the reproductive stages and the time between stages may vary widely based on planting date and soybean MG, as well as environmental conditions affecting the growing crop.

It is interesting to note that the critical leaf-K values at only the 1 DAR1 and 20 DAR1 sampling times are above the general critical level of 1.7% that is shown in the table in “Sufficient Soil and Tissue Nutrient Levels for Soybeans” that is posted on this website. Thus, this general critical value for K sufficiency in soybean is not as accurate as the specific values for specific reproductive stages that were determined in this research.

The results from this research conclusively show that 1) the time that tissue samples are collected for K analysis will provide differing results based on the reproductive stage of the soybean plant, 2) it is important that the time of tissue sampling in relation to soybean reproductive stage to determine K sufficiency in leaves and petioles be documented to ensure that the data are interpreted accurately, and 3) if K fertilization is reduced or curtailed based on price and availability of K fertilizers, then tissue sampling during the mid portion of soybean reproductive development can be used to most accurately determine K sufficiency in soybean plants.

A detailed discussion of “Soybean Tissue/Seed Analysis to Determine Plant Nutrient Status” appears on this website.

Composed by Larry G. Heatherly, Nov. 2021,