Three tools are used to evaluate crop nutrient availability: soil analysis, plant analysis, and visual diagnosis of nutrient deficiencies. Soil analysis is used to predict the fertilizer requirement for a certain crop while the plant analysis is used to check the plant’s nutritional status. Based on what is seen with an understanding of specific plant nutrient deficiency symptomology, the visual diagnosis can be an important part of diagnosis and provide insights as to whether plant nutrition is actually involved. When these three tools are integrated, hopefully soil analysis and the application of appropriate lime and fertilizer rates will reduce nutrient limitations, while plant analysis based on either a routine sampling schedule or a visual problem diagnosis is used to document specific additional nutrient limitations.
Nutrient deficiency occurs when one of the 17 plant essential nutrients is not available in sufficient quantity to meet the requirements of a growing plant. Producers, agricultural Extension agents, and crop consultants should be able to recognize the main symptoms of nutrient deficiencies. It is important to interpret the scenario correctly to determine if timely intervention can minimize current crop yield loss, or whether it is more effective to wait and resolve the nutritional problem for future crops.
Each nutrient plays a different role in the plant and lack of a particular nutrient will first impair specific physiological processes that lead to a identifiable visual abnormality. Additionally, some nutrients are mobile in the plant while others have low mobility. When the uptake of a mobile nutrient decreases, it can be translocated from the older to the younger parts of the plant. As a consequence, for mobile nutrients (nitrogen, phosphorus, potassium, magnesium, molybdenum, and chlorine) deficiency symptoms appear first in older leaves, while for immobile nutrients (calcium, sulfur, iron, manganese, copper, zinc, and boron), symptoms appear first in the young leaves or rapidly forming reproductive tissues (flowers, fruits, seeds).
Although the visual symptoms indicate that something is wrong with the crop, it may be difficult to correctly identify a specific nutritional cause. Limitations to visual symptoms include:
- Deficiency symptoms of different nutrients can appear similar;
- More than one nutrient can be deficient at the same time, making the identification difficult;
- Different species (or varieties) can express symptoms differently;
- Other factors can produce symptoms similar to some plant deficiencies, such as diseases, droughts, insects, herbicide side-effects, and excessive rainfall;
- Crops can present “hidden hunger,” where the nutrient is below the optimum level but there are not visual symptoms;
- Deficiency symptoms evolve with time, and the symptoms may appear different than “expected” symptoms;
- Deficiency of a certain nutrient can negatively affect the uptake of others. Therefore, the visual symptoms can be misleading, pointing to the wrong factor.
For these reasons, when plants with visual symptoms of nutrient deficiency are found in a field, it is very important to perform soil and tissue testing to correctly identify the problem. Nematode assay may be important too. In this factsheet, we present the main symptoms of potassium deficiency in soybeans and the recommended management for North Carolina soils.
Management recommendations are based on experiments conducted on research stations and commercial farms in North Carolina and other regions. Producers wishing to conduct their own on-farm trials can utilize resources available through Cooperative Extension agents and NCDA&CS Regional Agronomists as well as precision technology to apply products and monitor crop yields. Such studies are most likely to provide useful information when they are planned following a few simple guidelines.
- Fertilizer treatments should be compared to a “control,” usually an untreated area.
- Treatments and controls should be replicated, which avoids the problem of basing conclusions on a single field area which may not be representative.
- There should be random assignment of control and fertilizer treatments to field areas.
Plant tissue sampling following foliar fertilizer application should allow adequate time for rain to wash the product off the leaf surfaces. Otherwise, laboratory results may suggest greater nutrient uptake even though the nutrients may not have moved into the plant.
Soybean has a high potassium (K) requirement, leading to high amounts of K exported with grain. Potassium can be leached from coarse-textured soils due to excessive rainfall. For these reasons, potassium availability in North Carolina soils can vary much more than phosphorus, and a regular soil testing program is essential.
Potassium deficiency is commonly seen as yellowing, reddening and dying leaf margins on lower (older) leaves during vegetative growth and on upper (younger) leaves during grain fill and/or stunted plants and slow growth are common. Since potassium uptake is diffusion-driven, deficiency may be more often seen under drought conditions.
The visual deficiency diagnosis needs to be confirmed by diagnostic soil and plant analysis. Procedures for diagnostic samples can be found at the Diagnostic Samples webpage. The most effective way to avoid K deficiency is to maintain a regular fertilization program based on frequent soil testing, at least every other year. When a K deficiency is detected in a soybean field during the early vegetative stages, broadcast granular fertilizer at agronomic rates recommended by the soil test are likely to result in yield responses than are the very low K rates that plants can tolerate as a foliar spray. If needed, consult your Cooperative Extension agent, NCDA&CS regional agronomist, crop consultant, or CCA for help in rate and fertilizer selection.
Publication date: Dec. 4, 2020
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