Potash Deficiency in Soybeans

Many soybean fields showed characteristic potash (K) deficiency this year. Potash is one of the most prevalent soybean nutrient deficiencies evident in the province. After nitrogen, K is the nutrient absorbed by soybean plants in the largest amount. A large portion of this K is stored in the seed, and therefore removed from the field each year at harvest. K deficiencies will severely cut soybean yield potential. K influences nodule formation, as well as the plant's ability to resist diseases.

Fall is an excellent time to apply K for next year's soybean fields. But not all fields that showed deficiency symptoms this year were truly soil deficient. What was going on in these fields? Regular soil testing is always recommended, but if K deficiency symptoms were evident a soil test is crucial to determine K soil levels.

Figure 1. Potassium (K) deficiency appears as yellowing or browning of leaf margins, generally on the older leaves.

Factors Affecting K Availability

Although there is considerable K in the soil, only 1 to 2% of the soil's potassium is readily available for the plant. About 10% is “slowly” available. A number of factors influence the uptake of K. These include:

1. Soil temperature - As temperatures increase the rate of diffusion and root growth also increases, making the potassium more available.
2. Soil compaction - Under poor aeration caused by soil compaction the low oxygen level in the soil decreases the uptake of the K. Roots cannot access as large a volume in compacted soils.
3. Dry conditions - When conditions are dry, the movement of the K in the soil to the root is slower. Low moisture also results in more of the K becoming fixed in unavailable forms.
4. Clay and Organic Matter - Soils that are low in clay or organic matter retain less K. Sandy soils may need more frequent applications of K than other soils. 

Stress Induced Potash Deficiency

Factors that limit root growth, such as dry conditions, sidewall compaction, insects, and diseases, will reduce K uptake. Even if soil K levels are adequate, symptoms may be evident. This describes what happened in many fields in 2007. Because of the dry conditions, the roots were unable to take up K from the soil, even if soil K levels were sufficient. A soil test is the only reliable way to know if a field is truly low in K, or only showing stress-induced potash deficiencies.

Fertilizer Recommendations

Soil applied fertilizer at the recommended rate is the best way to ensure enough K will be available for next year's crop.

Leaf Samples

Fertilizer recommendations for soybeans are normally based on soil test results, but plant tissue analysis can provide useful additional information. There has been a recent update to the Critical Concentration for K in soybean tissue. Due to newer research the Critical Concentration of K has been revised upward from 1.2% to 2.0%, and the Maximum Normal Concentration from 2.5% to 3.0%. For more information on leaf samples, please refer to OMAFRA Factsheet "Interpretation of Plant Analysis for Soybeans".:

Related Links

• OMAFRA factsheet "Interpretation of Plant Analysis for Soybeans"

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