Potassium is an essential element for plant growth and production. The Vertisols are generally rich in available potassium reserves for supplying potassium to plant. In present agriculture scenario the negative balance of potassium goes on increasing because crops remove more K than N and P. Under such condition there is need to focus on K fertility status of soil. On the other hand, there is an anomaly regarding crop response to applied potassium in some Vertisols of India. This review paper provides information important to the understanding of soil potassium status and its behavior in Vertisols.

Keywords: vertisols, potassium, potassium status of India, fertility

In India, shrink-swell (Vertisols and Vertic Intergrades) soils are found mostly in the Peninsular region extending from 8o45’ to 26o0’N latitudes and 68o0’ to 83o45’E longitudes. These soils are developed in alluvium derived from weathering of Deccan basalt.¹ The Vertisols and associated soils occupy about 116 Mha in India.² Vertisols dominated by smectites clay minerals, are generally rich in exchangeable or available potassium and give an impression of adequate K reserves for supplying potassium to plants. Potassium is known as a yield plus quality nutrient. It is involved in the working of a large number of enzymes, in the production and movement of photosynthates from leaves to storage organs, water economy and providing resistance against pests, diseases and stresses. Potassium is the seventh most abundant element in the earth’s crust. Potassium, one of the 17 chemical elements required for plant growth and reproduction, is often referred to as the “the regulator” since it is involved with over 60 different enzyme systems in plants. Potassium aids plants in the production of starches, controls root growth, and regulates the opening and closing of pores in plant cells (called stomata), which is important for efficient water use. All plants require potassium, especially crops high in carbohydrates, such as potatoes.

In the present agriculture scenario, the net negative NPK balance is 19% N, 12% P, and 69% K. The large proportion of K is partly because crops remove an average 1.5 times more K than N and K application through fertilizer is much lower than of N or P. However, total fertilizer nutrient consumption is increased especially N and P but the consumption of K fertilizers declined, which lead to wide gap in NPK ratio leading to mining of soil K and imbalance nutrition.³ In most of the intensive cropping system in India potassium balance is negative since the additions of K seldom match the K removals resulting in larger dependence on soil K supply. Under such conditions, there is greater pressure on non-exchangeable K for meeting the K requirements of the crops. Long-term intensive cropping, in the absence of K inputs, adversely affected the K supply to crop plants and consequently crop yields.⁴ Therefore, it is necessary to continuously emphasize the role and importance of K in crop production as balanced fertilizer use for the long term and sustainable use of agricultural lands. The removal of K needs to be balanced by adequate K inputs if a decline in soil fertility is to be avoided. In India, so far there was a general understanding that Vertisols are rich in K and there was no need for its application. However, with time it is likely that in some soils deficiency of K could occur due to continuous cropping, leaching loss, and soil erosion so on. This paper critically reviews the work done on soil K status and crop response to K fertilization.

Potassium status in Indian soil

Potassium is one of the three main pillars of balanced fertilizer use, along with N and P. while India is the third largest user of NPK fertilizers in the world, with current annual consumption at about 18 million tons (Mt) of N+P₂O₅+K₂O, K constitutes only one seventh of the total. In the present agriculture scenario, the net negative NPK balance is 19% N, 12% P and 69% K. The large proportion for K is partly because crops remove an average 1.5 times more K than N, and K application through fertilizer is much lower than that of N or P. The available K status generated in 1976 5 forms the benchmark information. Most of Indian soils are rated as medium to high in available K status. The nutrient index values based on more than 11 million soil test data⁵ shows that out of the 371 districts 76 (21%) are low in potassium, 190 (51%) are medium in potassium and 105 (28%) are high in potassium. this data is based on 1N ammonium extraction method, soils containing 130 kg K²O/ha were categorized as low, 130 to 335 kg K²O/ha as medium and above 335 kg K²O/ha as high.⁶

Need for intensive research on potassium in Vertisols

There are anomalies in crop response to applied potassium in different K fertility status of soil. Usually soils which contain less than 120 kg ha^-1 K (144 kg K²O) are rated as low in available K, between 120 and 280 kg ha^-1 K (144-336 kg K²O) as medium and above 280 kg ha^-1 K (336 kg K²O) as high in available K.⁶ Unfortunately, these ratings limits are irrespective of crops or soils. For example,⁷ studied the response of two wheat varieties to potassium on farmer’s field in shrink-swell soils. Though these soils were adequate in ammonium acetate extractable K, crop responded to 30 kg ha^-1 K²O. They have established a critical limit of 14.4 kg ha^-1K water-soluble K but failed to establish a critical limit based on ammonium acetate K. This indicates that ammonium acetate does not give proper indication of viability of K in shrink-swell soil.⁸ studied available K status and crop response at Jabalpur and indicated that Soybean started showing response to applied K when available K status reached to 316 kg ha^-1. Whereas, wheat showed response to K even before. Similarly at Akola, sorghum started giving response to applied K at 324.4 kg ha^-1 whereas, a larger response in both the crops noted at available K status of 307.6 kg ha^-1. It means middle value of K status is 312 kg ha^-1 (average of 307.8 and 316.0) could be considered as critical value for Vertisols. This is greater than the threshold value (280 kg ha^-1) is being used for rating the soil as high in K status. Thus, for Vertisols there is need to increase the threshold value of K in soil to get the actual recommendation of K in soil to sustain the productivity and maximize benefit from applied nutrient. Under this situation, we have to revise of the ammonium acetate method is warranted which is used as soil test crop response all over the country.^9,10

The available K status in Indian soils showed gradual decline from medium to low even in black soils. These trends were due to low K application by the farmers, imbalanced use of NPK fertilizers, misapplication of K recommendation leading to K deficiency in soils and crops. The present fertilizer recommendation is four decades old and still being used, warrants revision and revalidation. Also, we have to revise ammonium acetate method which is used as STCR all over the country.

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The authors declare that they have no conflict of interest.