Three tools are used to evaluate crop nutrient availability: (1) soil analysis, (2) plant analysis, and (3) visual diagnosis of nutrient deficiencies. Soil analysis is used to predict the fertilizer requirement for a certain crop, while plant analysis is used to check the plant’s nutritional status. With an understanding of specific plant nutrient deficiency symptomology, the producer may make important visual diagnoses and gain insight on whether plant nutrition is actually involved. When the three evaluation tools are integrated, it is hoped that 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 1 of the 17 essential plant nutrients is not available in sufficient quantity to meet the requirements of a growing plant. Producers, 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. Lack of a particular nutrient will first impair specific physiological processes that lead to an identifiable visual abnormality. In addition, 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, deficiency symptoms appear first in older leaves for mobile nutrients such as nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), molybdenum (Mo), and chlorine (Cl). For immobile nutrients, symptoms appear first in the young leaves or rapidly forming reproductive tissues (flowers, fruits, and seeds). These immobile nutrients include calcium (Ca), sulfur (S), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), and boron (B).

Although the visual symptoms indicate that something is wrong with the crop, it may be difficult to correctly identify a specific nutritional cause. Diagnosing via visual symptoms includes the following limitations:

• 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,” in which the nutrient is below the optimum level but there are no 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 other nutrients. Therefore, the visual symptoms can be misleading, pointing to the wrong factor.

Due to these limitations, 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 P deficiency in soybeans and the recommended management for North Carolina soils.

Management recommendations are based on experiments conducted at research stations and commercial farms in North Carolina and other regions. Producers wishing to conduct their own on-farm trials can use resources available through Cooperative Extension agents and the North Carolina Department of Agriculture & Consumer Services (NCDA&CS) Regional Agronomists. They can also rely on precision technology to apply products and monitor crop yields. Such studies are most likely to provide useful information when they follow 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 that may not be representative.
• Control and fertilizer treatments should be randomly assigned to field areas.
• Allow adequate time for rain to wash any foliar fertilizer off the leaf surfaces before taking any plant tissue samples. Otherwise, laboratory results may suggest greater nutrient uptake, even though the nutrients may not have moved into the plant.

In general, native North Carolina soils are low in P, but after decades of fertilizer applications, it is rare to find agricultural soils with low soil test levels of P here. Phosphorus deficiency is likely to occur in soils with low pH in which P is less available for plants. Young plants in cold and wet soils can present temporary P deficiency, even when the soil test results present adequate P availability. Phosphorus deficiency can also occur in acid and compacted soils with compromised root systems.

Stunted plants and leaves with dark green, purple, or bluish tint appear first on lower (older) leaves when P is deficient. Root development may not be extensive (Figure 1).

Figure 1. Normal plants (top) and P-deficient plants (bottom).

Carl Crozier

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Figure 1. Normal plants (top) and P-deficient plants (bottom).

Carl Crozier

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The visual deficiency diagnosis needs to be confirmed by diagnostic soil and plant analysis. Procedures for diagnostic sampling can be found at NC State Extension’s soil fertility webpage. The best way to avoid P deficiency is to maintain a regular fertilization and liming program based on soil testing, keeping the soil pH within the recommended range based on the crop and soil class. When a P deficiency is detected in a soybean field during early vegetative growth stages, there is some potential for foliar or granular P fertilizer applications to help the status of the current crop; but the most effective correction of this problem will be with future crops. If needed, consult your Cooperative Extension agent, NCDA&CS regional agronomist, crop consultant, or certified crop advisor (CCA) for help in rate and fertilizer selection.