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Air-drying of soils for testing can cause fixation/release of potassium (K), showing a K status different from that of the field. We compared different extractants to find one with the least deviation in K-extractability between field-moist and air-dried soils. We chose two treatments (No-K and 150% of recommended K fertilization) of a field experiment (maize-wheat) for the study. Field-moist soil (0–15 cm) was collected at 3, 4, 5, 6, 7, 8, 23, 24, 25, and 26 days after the irrigation (DAI) from each replicated plot at the flowering stage of the 2018–19 wheat crop. Soil K, before and after air-drying, was extracted with 1 M ammonium acetate (NH4OAc), 0.1 M nitric acid (HNO3), 0.01 M calcium chloride (CaCl2), and 0.01 M organic acid mixture (OAM, mixture of 0.0025 M each of citric acid, malic acid, tartaric acid, and oxalic acid). Air-drying significantly (P <.05) affected the soil K for all methods except HNO3 in plots without K fertilization. However, air-drying's effect on K release was more pronounced in No-K soils with high initial moisture content (>10%). Averaged over K-fertilization and initial soil moisture, the K-extractability using HNO3 and OAM showed the least deviation (9.6% for both) by air-drying. It was significantly (P <.05) lower than NH4OAc or CaCl2, which showed as high as 21.1–23.0% deviation due to air-drying. Hence, one should use 0.1 M HNO3 or 0.01 M OAM instead of 1 M NH4OAc to get an available K status closer to that in the field. [ABSTRACT FROM AUTHOR]

Potassium (K) is essential for plant growth and vital for soil health. However, despite high crop demand, K fertilization continues to be ignored or severely inadequate in Indian agriculture. Over time, this could lead to severe depletion of soil K reserve, irreversibly alter K-bearing minerals, and adversely affect soil fertility and crop productivity. Hence, we should comprehensively assess the alarming situation of soil K mining in India and come up with appropriate solutions. Keeping the above in mind, here we review the soil K pools concerning plant availability, their contents in major soil orders of India, reasons and status of soil K mining under major crops and cropping systems in India, and the impact of long-term K mining on soil K pools and clay minerals. We also address various aspects of sustainable K management in agriculture and suggest future action courses focusing on India. Our main findings are: (i) farmers in India mainly apply nitrogen and phosphorus, but little or no K; consequently, (ii) net K balance continues to be negative for most crops and cropping systems across India (e.g., - 3.29 million tonnes [Mt] in 2000–01, - 7.2 Mt in 2015–16); (iii) long-term K mining primarily depletes the exchangeable and non-exchangeable K pools in soil, and alters clay minerals to various extents; and (iv) the existing K fertilizer recommendations need an upward revision. We further enlist indigenous non-conventional alternatives of K fertilizers to meet the agricultural K demand. This is the first comprehensive review to simultaneously address the ongoing soil K mining in India, its impact, and its potential mitigation strategies. The points raised here would help reduce soil K mining, plan research work, and make policy decisions on K fertilization and residue management with the ultimate goal to prevent soil health deterioration and ensure sustainable crop production. [ABSTRACT FROM AUTHOR]

Abstract Fertilizer use in Indian agriculture is skewed towards nitrogen (N) and phosphorus (P), and farmers generally neglect potassium (K) input ignoring K requirement of crops. We, therefore, studied the long-term effect of imbalanced fertilization (i.e. without K) on K supplying capacity of a kaolinitic red soil (Typic Haplustalf) after 42 years of intensive cultivation. Soil samples (0–15 and 15–30 cm) were collected after the completion of 42 cropping cycles from an on-going long-term fertilizer experiment located at Birsa Agricultural University, Ranchi, India, from six treatments, viz. control (unfertilized), fertilizer N, NP, NPK, 150% NPK (1.5 times of NPK) and NPK + FYM (NPK + farmyard manure), along with adjacent uncultivated land. Soil K pools, clay minerals, quantity-intensity relationship and K release kinetics along with grain yield and K uptake of soybean and wheat were studied. Among the treatments, NP treated soils showed the lowest amounts of labile (K L) and non-exchangeable K (K nex). Equilibrium activity ratio (AR e K) and non-specifically held K (−ΔK 0) were also lowest under NP treatment. The gap between equilibrium exchangeable K (EK 0) and minimum exchangeable K (EK min) was minimum under NP. Cropping and fertilization-induced transformation in clay minerals was noticed wherein continuous neglect of K fertilizer led to a decrease in illite content with a concomitant increase in interstratified minerals. On the other hand, noticeably lower alteration in clay minerals and comparatively less depletion in overall K-supplying capacity of soil were recorded under K addition as compared to imbalanced fertilization (i.e. without K). Soybean showed a significant positive response to K application. The study thus underlined the necessity of adequate K input for the sustenance of soil health, inherent K-supplying capacity and crop productivity. Highlights • We studied long-term effect of cropping with or without K input on soil K-fertility. • Imbalanced fertilization with respect to K caused depletion of soil K pools. • Imbalanced fertilization adversely affected quantity, intensity and K release rate. • Continuous neglect of K addition led to distortion in clay minerals. • Cropping with K addition maintained much better K-supplying capacity of soil. [ABSTRACT FROM AUTHOR]

Purpose: Potassium (K)-fractions, thresholds of K release and fixation, quantity-intensity (Q/I) parameters of K, K-release kinetics, and K-fixation capacity were compared for their effectiveness in differentiating the effect of various nutrient management practices on K supplying capacity of an Aeric Endoaquept soil after 45 years of puddled rice cultivation.Materials and methods: Soil samples (0-15 cm) were collected after the completion of 45 rice-rice cycles from an on-going long-term fertilizer experiment located in ICAR-National Rice Research Institute, Cuttack, India. The treatments involved control (unfertilized), N (nitrogen fertilizer), NP (N+ phosphorus fertilizer), NK (N+ potassium fertilizer), NPK (N + P + K fertilizer), FYM (farmyard manure), N + FYM, NP + FYM, NK + FYM, and NPK + FYM.Results and discussion: Rice cultivation without K fertilizer application resulted in lower values of soil K parameters than the K-fertilized treatments. Treatment effects were most prominent on release threshold concentration (RTC), followed by cumulative K release, K-release rate constants, and K-fixation capacity. Parameters of K-release kinetics and Q/I relationships showed better correlation with rice grain yields than soil-K fractions. Soil K thresholds were closely related with exchangeable (Kex) and non-exchangeable K (Knx), but not clay minerals.Conclusions: Among the soil K parameters, RTC, cumulative K release (Kf) with 0.01 M CaCl2, release rate constants (bR and bS) of parabolic diffusion equation, and K-fixation capacity were most effective in revealing the nutrient management induced variations in soil K fertility. In the studied soil, K-thresholds were significantly related to Kex and Knx. [ABSTRACT FROM AUTHOR]

• Effect of long-term cropping on K supplying capacity of Inceptisol and Vertisol was studied. • K omission depleted K ex in both soils and NEK reserve in Inceptisol. • Cultivation without K adversely affected quantity, intensity and K release rate. • Cultivation with K input maintained relatively better K-supplying capacity of soils. • Existing K fertilizer recommendations appeared inadequate in both soils. Despite removal of potassium (K) by crops in large quantities, often exceeding that of nitrogen (N), K fertilization has long been neglected compared with N and phosphorus (P) in Indian agriculture leading to mining of soil K. We, therefore, studied the effect of long-term fertilization (with or without K) on K supplying ability of two major soil orders of India, viz. Inceptisol and Vertisol after more than four decades of cultivation. Soil samples (0–15 and 15–30 cm) were collected after the completion of 43 (Inceptisol) or 42 (Vertisol) cropping cycles from on-going long-term experiments located at New Delhi (Inceptisol) and Jabalpur (Vertisol), India, from six treatments, viz. no-fertilization (control), recommended N (N), N and P (NP), N, P and K (NPK), 150% NPK and NPK + farmyard manure (NPK + FYM), along with adjacent uncultivated land. Ammonium acetate extractable K (K ex), non-exchangeable K (NEK), quantity-intensity relationship and K release kinetics along with grain yields and K uptake by crops were studied. The K ex and K-saturation of exchange complex in both soils and NEK in the Inceptisol were the lowest under NP followed by N. Cropping without K input resulted in lower quantity (non-specifically and specifically held K i.e. –ΔK 0 and K s) and intensity (equilibrium activity ratio i.e. AR e K) of K in both soils. More negative values of standard free energy of exchange (ΔG0) under cultivation without K in both soils indicated increased affinity of soil solids for K+ owing to K omission. The gap between equilibrium exchangeable K (EK 0) and minimum exchangeable K (EK min) was significantly lower under cropping without K than with K in the Vertisol. Imbalanced fertilization (N or NP) also showed noticeably smaller cumulative K releases and slower rates of replenishment of solution K by solid phase K in both soils. Interestingly, even under cropping with K fertilization at either experimental site, several K supply parameters showed decline compared with adjacent uncultivated soil. Grain yields of wheat and total K uptake by all crops (maize and wheat in Inceptisol; soybean and wheat in Vertisol) showed significant and positive response to K application. The study showed variable effect of long-term intensive cropping with or without K input on the K supplying ability of an Inceptisol and a Vertisol. The findings underlined inadequacy of the existing K fertilizer recommendations, and the need for their refinement to minimize native K mining and sustain K supplying ability of soils. [ABSTRACT FROM AUTHOR]