Ask the Experts
Q. What causes my peanuts to yellow early in the season?
A. Robert Lemon, Associate Professor and Extension Peanut Agronomist, Texas A & M University
Yellow, unhealthy looking peanuts may have many causes.
In 2000, we experienced a lot of rainfall in June, but not much after that. The waterlogged soil conditions hindered the Rhizobium bacteria from fixing nitrogen for the plant, and it also resulted in reduced root-growth and nutrient uptake. In this situation, the yellowing is mainly due to reduced levels of nitrogen and deficient levels of certain micronutrients, most likely iron, manganese, zinc or copper or some combination. Generally, after the stress of too much water is relieved and good growing conditions return, the Rhizobium bacteria resume nitrogen fixation, nutrients are absorbed in adequate amounts and the plants will become green and healthy.
However, another situation exists in high pH soils and that is high pH induced micronutrient deficiency. As pH increases, these micro-nutrients become less available, thus giving rise to deficiency symptoms. Because these elements are not mobile in the plant, they will not be reallocated from other tissue. Consequently, the chlorosis or yellowing first appears in the youngest leaves, as compared to a nitrogen deficiency, a mobile nutrient, which first appears on the oldest leaves.
Soils that have a lot of caliche on the surface are extremely prone to this problem. In fact, there are many fields in West Texas that should not be planted to peanuts simply because of micronutrient-deficiency problems. These fields remain yellow and stunted throughout the season and seldom produce acceptable yields. Applying foliar sprays to these fields rarely corrects the problem and is certainly not cost effective.
On the other hand, many productive fields will show signs of micronutrient deficiency at some time during the season. In most cases, foliar spray applications will solve the problem. The question is, "Does it improve yield and quality?" If it is an early- to mid-season problem, then foliar feeding is justified. If problems occur late-season, it is a questionable call whether it adds to yield potential.
Determining which element is deficient can also be a challenge. Although these elements exhibit somewhat different symptomology, they can also look very similar in the field. Visual inspection and plant tissue analysis does not always reveal which element is causing the problem. Unless you are very confident in your diagnosis, it is best to apply a balanced foliar feed that contains a combination of iron, manganese, zinc and copper. Best results are usually obtained from multiple applications, applied about two weeks apart.
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