Potassium (K) is the second most critical nutrient after nitrogen (N) for cotton production. Potassium deficiency can cause significant lint yield reduction with reduced fiber quality. Cotton experiences K deficiency for several reasons.
One of the main reasons is cotton grown in soils with low soil-test K level and received no potassium fertilizer. Also, any biotic or abiotic stresses during the growing season such as insect damage, diseases, nematode, compaction, drought etc. that suppress cotton root growth result in reduced K uptake and increased deficiency.
Although we had plenty of rain this year, the last couple of weeks we had to turn our irrigation on to minimize water stress especially during blooming period, the most critical stage to set maximum yield potential and any stress during this period will cause significant yield reduction. Figure 1 shows cotton response to K deficiency.
A bale of cotton requires about 52 lb potash (K2O) per acre with the peak accumulation occurs couple of weeks after first blooming, when cotton uptakes around 2.2 to 3.5 lb K2O per acre per day. Any shortage of K supply during this peak accumulation period will result K deficiency and yield reduction.
Not all the plant parts of cotton are similarly sensitive to K deficiency. The order of sensitive to K deficiency is stems > roots > bolls > leaves. Stem is the most sensitive and leaf is the least sensitive to K deficiency. So, any K deficiency symptom in cotton leaves signify all the other plant parts are already affected.
Potassium deficiency symptom first appears as yellowish-white mottling between the veins, then the center of these yellowish spots dies followed by numerous brown specks between veins, around the margins, and at the tips of leaves and eventually the whole leaf becomes reddish-brown and rusty colored.
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Potassium deficiency symptoms usually appear on the lower older leaves early in the growing season, but most frequently appear on the upper younger leaves of heavily fruiting cotton plants later in the growing season.
Cotton plants can experience K deficiency without producing any deficiency symptoms. This is called hidden K deficiency or hidden hunger. The best way to diagnose K deficiency with or without deficiency symptoms in cotton is to take tissue samples during early blooming period.
For tissue sampling, at least 20 uppermost recently mature leaves without petioles from the 3rd to 5th node from the terminal (a quarter-sized main-stem leaf at the top of the plant should be counted as the 1st leaf) should be collected during the early blooming period and sent immediately to the diagnostic lab (e.g., LSU AgCenter Soil Testing and Plant Analysis Laboratory, Baton Rouge, LA) for K concentration.
The critical leaf-K concentration ranges from 1.5 to 3.0% at the early bloom stage and 0.75 to 2.5% at the late bloom stage. Therefore, cotton fields with leaf K concentration lower than 1.5% K at the early blooming period should be fertilized with potassium fertilizer to rescue some yield losses and cotton, if deficient in K, is very responsive to K fertilization during blooming stage.
Top-dress of 100 lb dry potash (Muriate of Potash; 0-0-60) per acre should be enough for in-season management of K deficiency. Irrigation or at least 0.25” rain followed by in-season application would result rapid K uptake and better response.