Behind nitrogen, potassium (K) is the nutrient absorbed by soybean plants in the greatest amount.2 Much of the absorbed K is removed with the seed at harvest each year. As soybean yields increase, it is important to consider overall amounts of crop nutrients required.
Absorption of K
Peak absorption of potassium (K) occurs from flowering through early pod development. High-yielding soybeans (101 bushel/acre) can have a K2O uptake rate of 9.6 lb/acre/day during the early pod stage.1 A shortage of K during this period can result in yield loss without obvious foliar symptoms.
Causes of Potassium Deficiency and Symptoms
Often the cause of deficiency is due to low K levels on soil tests.4 Soil tests estimate the ability of a soil to supply the nutrient to a growing crop.1 Potassium moves to the root surface over short distances. One to two percent of potassium in the soil is readily available, and 10 percent is thought to be trapped between layers of clay mineral and is slowly available to plants.2 Soil conditions that affect this movement with lower availability include soils with compaction, dry conditions, and low clay or organic matter.
Alternatively, the rate of K diffusion can increase with a rise in temperature. Almost all K in the soil is involved in the structural component of soil minerals, and dry conditions can cause K to become fixed and unavailable. Stress from insects and disease can also induce deficiency symptoms.
Symptoms typically begin at the leaf tip and extend down the margins toward the leaf base. Older leaves are affected and appear with yellowing or browning on the leaf margins.
Figure 1. Potassium deficiency in soybeans begins as yellowing on the leaf margins of older leaves. Daren Mueller, Iowa State University. Bugwood.org
Sampling and Management
While soil testing is the standard for fertilizer recommendations, plant tissue analysis can provide additional useful information, especially when deficiency symptoms are spotty throughout the field. Plant tissue analysis steps for sampling include:
1) Timing - at first flowering (R1 growth stage), to allow for maximum tissue and root development before nutrients move from leaves to seeds.3
2) Leaf tissue - sample the top fully developed leaf (three leaflets plus stem) and take a soil sample from the same area at the same time. Tissue should not be damaged by insects; chlorotic or dead; or contaminated with soil.
3) Amount of samples – samples of 20 plants throughout a selected area with each sample being at least 100 grams of fresh tissue.
4) Storage and shipping – store samples in paper bags and deliver to the lab immediately or dry them to prevent spoilage.
Critical concentration for K in soybean tissue has recently been raised from 1.2 to 2.0 percent.3 Potassium concentrations at or above the critical concentration indicate that plants have the necessary amount of K for maximum crop growth and yield potential.
By the time deficiency symptoms develop in the growing season, it is usually too late for an economical and effective, corrective treatment for the current crop. For future crops, a soil-applied fertilizer can be broadcast and worked into soil during fall or spring field work. Fertilizer placed with a planter should use an attachment to keep the product 5 cm (2 inches) to the side and 5 cm (2 inches) below soybean seed. If plant tissue analysis indicates K concentrations of less than 2 percent and soil tests report adequate K levels, there may be root restrictions from soil structure or moisture issues in the field.
1 Leikam, D. et. al. 2010. Are current soil test-based phosphorus and potassium fertilizer recommendations adequate? Crops& Soils. American Society of Agronomy.;
3 Bohner, H. 2007. Potash deficiency in soybeans. OMAFRA. http://www.omafra.gov.on.ca/. (verified 2/20/2012);
3 OMAFRA. 2011. Interpretation of plant analysis for soybeans. http://www.omafra.gov.on.ca/ (verified 2/20/2012).;
4 Mallarino, A.P. 2005. Potassium deficiency symptoms in corn and soybean: what can we do about them? Integrated Crop Management. Iowa State University. IC-494(15).
Rehm, G. and M. Schmitt. 1997. Potassium for crop rotation. University of Minnesota. WW-06794.