Potassium Fertilizer for Rice: Role, Application Timing and Dosage Explained

Rice is a grain crop with a high potassium requirement. In recent years, super hybrid rice is being promoted and applied, and hybrid rice has a greater potassium demand than conventional rice, sometimes as much as 30%-40% higher. So, what are the roles of potassium fertilizer for rice? When is the best time to apply potassium fertilizer to rice, and how much should be used?

—Potassium depletion in general paddy soils is severe, especially in the acidic soil regions of southern China where potassium-supplying potential is not high. In some high-yield rice production systems, potassium may be in a deficit state, which is very unfavorable for achieving high yields. Therefore, when developing high-yield and even super-high-yield rice production today, it is necessary to supplement potassium fertilizer in appropriate amounts and at the right time. One point to note here: in order to improve the recycling of potassium in farmland, for paddy fields where straw is returned to the field, the input of chemical potassium fertilizer can be appropriately reduced, because 70% of the potassium absorbed by rice in the current season is present in the straw. However, in paddy fields where straw is not returned, effective potassium application is very important.

The physiological role of potassium in rice: the potassium content in stems and leaves of rice at different growth stages is about 1.5%-3.5%, while the content in the panicle is lower, generally below 0.5%-1%. In the plant body, potassium exists almost entirely in ionic form, with part in an adsorbed state in the protoplasm. Unlike nitrogen and phosphorus, potassium is not a constituent component of protoplasm, fats, cellulose, etc. However, in some important physiological metabolisms such as the decomposition and transfer of carbohydrates, potassium has a catalytic effect and can promote these processes smoothly. Potassium also aids nitrogen metabolism and protein synthesis, so the more nitrogen applied, the correspondingly greater the potassium requirement. Potassium acts as an activator for many important enzymes in the plant body. An appropriate amount of potassium can improve photosynthesis and increase the carbohydrate content of the rice plant, and can thicken the cell walls, thereby enhancing the plant's ability to resist disease and lodging.

Symptoms of potassium deficiency in rice: root development stagnates, and root rot is likely to occur; the degree of dark green leaf color is similar to that when too much nitrogen is applied, but the leaves are relatively short. In severe potassium deficiency, yellow-brown spots first appear at the leaf tips and gradually spread to the entire leaf, the stems become soft, and stem height elongation is inhibited. Potassium has great mobility within the plant body and can transfer from old leaves to new leaves, so potassium deficiency symptoms appear first on the lower leaves. When potassium is insufficient, starch, cellulose, and carbohydrates decrease. When rice is under conditions of luxuriant growth and shading or insufficient light, growth can mostly be improved after increasing potassium fertilizer application.

In fertilization for super-high-yield rice, the application rate of nitrogen fertilizer is generally high. From the perspective of nutrient balance, more attention should be paid to the corresponding supplementation of potassium fertilizer. This is because adequate potassium supply can make rice stalks thick and strong, increase their strength, and improve mechanical properties, which both resists lodging and can effectively block the invasion of pathogens and pests. Therefore, under the condition of large amounts of nitrogen fertilizer application, if the potassium fertilizer rate cannot keep up and the nitrogen-potassium ratio becomes imbalanced, it will cause the problem of too much nitrogen and too little potassium or obvious potassium deficiency. Symptoms of potassium deficiency in paddy fields: for example, potassium deficiency at the seedling stage may cause stiff seedlings; insufficient potassium nutrition at the jointing stage shortens internodes, and the lower old leaves become speckled with rust-colored spots; severe potassium deficiency causes leaf margin scorch, and the rice plant becomes soft in structure, with soluble nitrogenous compounds accumulating in the body, which reduces the stress resistance of rice. The plant stalks become weak and prone to lodging, leaves lose water, and damage from pests and diseases is more severe, so the safety of achieving high yields is greatly reduced.

When is the best time to apply potassium fertilizer to rice? How much should be used? It is reported that relevant agricultural technology departments have carried out research and demonstrations on the optimal application rate and application methods of agricultural potassium fertilizer for rice in different seasons. Preliminary results show that timely topdressing of potassium fertilizer after field drying has a great effect on increasing rice yield, with an average yield increase of about 6%. Analyzing the reasons for the yield increase: potassium application can increase the panicle-bearing rate of super hybrid rice, reduce tiller degeneration to varying degrees, promote tillers to bear panicles, and increase the number of effective panicles. It also improves the seed-setting rate to a certain extent, and the 1000-grain weight also tends to increase. Therefore, the experiment participants believe that topdressing potassium fertilizer can reduce tiller degeneration, thereby improving the tillering efficiency of rice. Potassium application can also enhance the grain-filling intensity of rice, improve the seed-setting rate and grain weight, and thus lead to increased rice yield.

In addition, the results of experimental demonstrations on how to apply agricultural potassium fertilizer in mechanical direct-seeded rice cultivation show that the optimal potassium fertilizer rate is 10 kg K2O per mu, with all fertilizer applied as a basal application before transplanting, and after application, raked into the soil layer below 5 cm with an iron-toothed harrow. For mechanical direct-seeded rice, applying potassium fertilizer increased yield by 11.6% compared with no potassium application. The stalk thickness of the first, second, and third internodes at the base of the rice increased, especially the thickness of the basal internodes increased noticeably, indicating that potassium application enhanced the lodging resistance of rice stalks. The effective panicles, filled grains per panicle, and seed-setting rate were all relatively high.

In general, potassium fertilizer can be applied entirely as a basal fertilizer, but in sandy soil or super-high-yield cultivation, potassium fertilizer should be applied in two applications: basal fertilizer and topdressing. For hybrid rice, the tillering to booting stage is the peak period of nutrient uptake, so topdressing is best carried out before heading, so that the plant maintains a relatively high level of nitrogen and potassium nutrition during the peak growth period, which can improve the seed-setting rate and 1000-grain weight.

Although the amount of nitrogen chemical fertilizer used has increased year by year, while the amount of organic fertilizer used has relatively decreased, including the fact that straw return is not widespread enough, the trend of possible potassium deficiency in rice is increasing year by year in some areas. Potassium deficiency in paddy fields often causes problems such as low seed-setting rate and lodging in rice, which has become one of the obstacles to achieving the goals of high and stable yields. Therefore, the role of potassium fertilizer for rice is very significant, and the application timing and dosage of potassium fertilizer for rice also require great attention.