A combination of events occurring during the 2005 growing season resulted in potassium deficiencies in numerous cotton fields in Virginia and North Carolina. Early symptoms appeared as interveinal chlorosis and mottling on leaves in the upper regions of the plant canopy. More advanced cases become necrotic on the outer margins of leaves and have increased leaf spot disease incidence, finally resulting in premature defoliation and boll opening.
There are numerous factors that can contribute to potassium deficiencies in cotton including moisture stress, poor root growth, potassium leaching, reduced application rates due to high prices, low soil residual potassium levels, and heavy boll loads.
Early season conditions in many portions of the Cotton Belt including North Carolina and Virginia were extremely wet and cool. From mid-to-late season, rainfall has been limited to isolated showers and many areas have sustained long periods of drought and record-breaking heat. This combination of conditions has resulted in poor root growth and thus reduced potassium mobilization. Boll retention was very high in most fields in 2005 and the fruit load is compact as it was put on over a short period of time (most varieties displaying symptoms are short season), contributing to an increased demand for potassium.
Not surprisingly, the worst potassium deficiencies symptoms are appearing in sandy spots in fields. Wes Alexander, Extension agent in Southampton County, Va., measured greater than four feet of sand in areas where potassium deficiency was severe, versus one foot of sand followed by a clay layer in areas of the same field where deficiency symptoms were moderate. This clay layer improves moisture retention and restricts potassium movement, ultimately enhancing root growth and increasing potassium uptake. Poor root growth was observed in the sandier areas of the field, likely due to midseason moisture stress.
Many of the fields displaying symptoms have heavy fruit loads with high demands for potassium. Most cotton fields examined followed various row crops in 2004, including corn, cotton, peanut, and soybean. There was likely little residual soil potassium present as a result of last year’s high crop yields. Keith Edmisten, Extension cotton specialist with North Carolina State University, also noted that deficiencies were severe in fields following coastal bermudagrass, a plant that will scavenge potassium in excess of necessity.
Severe potassium deficiency can result in significant yield loss and deterioration of lint quality. This year’s occurrence of potassium deficiencies will probably not warrant changes in potassium application recommendations as poor root growth due to environmental conditions is suspected to be the largest contributor to the problem. Observations this year do appear to support the need for split applications of potassium in order to ensure availability during boll development.
Joel Faircloth is Extension cotton specialist with Virginia Tech. Contact him at firstname.lastname@example.org.