August Is the Right Time for Foliar Potassium Spraying on Cotton

Because the content of available potassium (including water-soluble potassium and readily available potassium) in the soil decreases year by year, available potassium has become the number one demand in cotton nutritional physiology. When 30-40 jin of high-quality potassium fertilizer is applied in the base fertilizer, due to the consumption by cotton growth and development and leaching after heavy rain, by August the available potassium content in the soil will again drop to its original level.

Entering August, as soil aeration deteriorates and the ground temperature drops after the cotton field closes its rows, the soil's potassium supply capacity weakens day by day. If potassium fertilizer cannot be supplemented in time, obvious potassium deficiency will appear. Potassium deficiency in cotton first shows on the lower old leaves; generally the veins remain green while the mesophyll gradually loses its green color. Because potassium is highly mobile within the plant and concentrates in the young, metabolically active parts, when the cotton plant lacks potassium, the potassium is first transferred from the old leaves to the young tissues. If in early to mid-September a potassium-deficient cotton field suddenly encounters low temperature and rain, the supply of available potassium in the soil is almost completely interrupted. When even the basic potassium nutrition of the cotton leaves cannot be maintained, the cotton plant will suddenly show physiological collapse; within two or three days the cotton leaves rapidly turn yellow or red, then dry up and fall off, the stem withers and dies, and the cotton plant dies. This is also called red leaf stem blight, and at this point no measure taken will be in time.

Therefore, early to mid-August is the critical period for potassium supplementation in cotton, and foliar potassium spraying is the most economical and effective method of potassium supplementation. Effective potassium must be supplemented before the cotton plants become severely potassium-deficient. If in August the cotton plants are in a state of severe potassium deficiency and cannot be supplemented timely and effectively, it will cause reduced or no boll setting on the upper part, small and late bolls, poor boll opening, reduced lint percentage, leaf shedding, and even the serious consequence of whole-plant death. There have already been many painful lessons in past years.

What potassium fertilizer variety is best for foliar spraying? There are 4 principles for choosing fertilizer: (1) the high-potassium principle; (2) the high-solubility principle; (3) the principle of easy absorption and utilization and safety and harmlessness; (4) the low-input principle. According to the above 4 principles, potassium sulfate is the best variety for foliar spraying: (1) high-quality potassium sulfate contains more than 50% potassium oxide, far higher than any other compound or blended potassium fertilizer variety; if 1 jin of potassium sulfate is sprayed per mu each time, it can provide the cotton with the potassium needed to produce 10 jin of seed cotton; (2) it is easily soluble in water; at 30°C, 13 grams of potassium sulfate can dissolve in every 100 grams of water, making it convenient to spray under any conditions; (3) it is easily absorbed and utilized by cotton and is safe and reliable within a certain spraying concentration; (4) calculated per unit of effective ingredient, its price is lower than any other potassium fertilizer variety.

Spraying method: the concentration should preferably be 2%, with 60 jin of liquid sprayed per mu, spraying only the upper half of the cotton plants, wetting both the front and back sides of the leaves. The spraying time should avoid the period of greatest evaporation from after 10 a.m. to before 3 p.m. The finer the spray droplets, the safer for the cotton. Be careful not to spray the cotton plants at the ends and edges of the field repeatedly. Spray once every seven or eight days, for 3 consecutive sprays. If mixed with pesticides, reduce the fertilizer amount and increase the number of sprayings. According to the practical experience of cotton farmers, powdered potassium sulfate produced in Russia has the best spraying effect, with a potassium oxide content as high as 52%; it dissolves quickly and has almost no sediment. Large-granule potassium sulfate that does not dissolve well is not suitable for foliar spraying. Grade II and Grade III potassium sulfate have low potassium content and much filler, and are not suitable for spraying. Potassium chloride has a potassium oxide content as high as 60% and also dissolves well, but cotton leaves are relatively sensitive to chloride ions; if used for foliar spraying, a 1% concentration should be used to ensure safety.

Potassium dihydrogen phosphate is an outdated variety of cotton foliar fertilizer; it has low potassium content but high phosphorus content, requires a low spraying concentration, and involves high input. It is suitable for cotton fields with not much potassium deficiency in the 1970s-1980s. In severely potassium-deficient cotton fields, spraying potassium dihydrogen phosphate is far from meeting the cotton plant's craving for potassium. In prematurely aging cotton fields, early-maturing cotton fields, and cotton fields with vigorous reproductive growth but weak vegetative growth, phosphorus should no longer be sprayed. Potassium dihydrogen phosphate is suitable for cotton fields with excessively vigorous vegetative growth and a tendency toward late maturity.

Potassium nitrate is a raw material for gunpowder and industry, is expensive, and involves high input. It is suitable for cotton fields lacking both potassium and nitrogen in the later stage.

"Potassium sulfate type compound fertilizer" belongs to compound fertilizers, not potassium fertilizer; its potassium content is low, and it cannot be sprayed as potassium fertilizer.

For general cotton fields, spraying high-quality potassium sulfate is the simplest and easiest to carry out, and is also economical and affordable. Practice has proved that it can achieve a relatively good potassium supplementation effect.