Late Season Potash Deficiency in Cotton

The evolution of cotton cultivars has led to higher yield potentials and shorter growing seasons compared to the historical full season cultivars favored in the Mid-South.  These newer, faster fruiting cultivars may require more nutrients than older outdated cultivars.  Recently, across the Mid-South, many consultants, state extension and research specialists have observed numerous cases of potash deficiency many of which have occur later in the season.  Abaye, 2009 summarized that K deficiency across the cotton belt was primarily related to i) cultivation of cotton on soils testing low in potassium, ii) a general inefficiency of cotton to absorb K and iii) the new superior faster fruiting cultivars being grown.  In the past 25 years, the time period between planting and first bloom has shortened by a week to 10 days.

In Mississippi, like most cotton belt states, supplemental potassium applications are required to produce optimal fiber yield on soils testing low in potassium. Averaged over the last 15 yr., approximately 64% of Mississippi’s cotton acreage has received some form of potash fertilization (NASS). Over the last several years, trials have been established on known sites were late season potash deficiency has been noted.  To date we have observed very limited responses potash application on sites were soil test K is marginal to adequate, and potash deficiency shows up late in the season near cutout.

Late season symptoms of potash deficient cotton

Late season potash deficiency being observed throughout the Mississippi, may be due to multiple origins.  The first being those sites where a true potash deficiency exist (i.e. those that have a low soil test K value), and secondly those fields that have marginal or borderline soil test K that have trouble assimilating soil K.  The latter is what I feel like may be happening currently.   With heavy boll loads, a heavy demand for K is present in the cotton plant, due to physiological, environmental, soil issues or some combination of all, today’s short season fast loading varieties may have trouble assimilating enough potassium from the soil.  In general any stress that influences root growth and development will influence the plants ability to obtain potash from the soil.  For instance in a year like 2014, with numerous amounts of rainfall, we have a smaller, shallower root system that may not be extensive enough to scavenge for deeper soil K once we encounter drier soil conditions.  In those instances translocation of potassium from leaf tissues to the sink (i.e. bolls) is what we feel is occurring, thus causing the visual deficiency symptoms to present.