Abstract
There is a great concern about high intensification of rice, rapid expansion of maize, increased crop yield of both rice and maize and heavy nutrient withdrawal in combination with imbalance fertilization leading to potassium (K) depletion of the soil and to its deficiency in rice and maize crops. The present study was designed to estimate, through a plant-based assay, the magnitude and variation in K supplying capacity of a range of soils for intensive rice and maize growing areas in Bangladesh. A laboratory and a pot experiments were carried out on 18 diverse soils collected from 11 districts (central, north, north-west, south and east) under 11 Agro-ecological zones (AEZs) of Bangladesh. There were two treatments viz. without K and 100 mg K kg⁻¹ soil for rice and maize in pot experiments. On the other hand, field validation trials with five levels of K (0, 30, 60, 90 and 120 kg K ha⁻¹ for rice; and 0, 40, 90, 120 and 160 kg K ha⁻¹ for maize) and two fertilizer recommendations (BRRI and "Rice Crop Manager" fertilizer recommendations for rice, and "Nutrient Expert" and "Maize Crop Manager" fertilizer recommendations for maize) were conducted in 24 farmers' fields in Rangpur, Rajshahi and Comilla to validate the results of the pot experiments. In the laboratory study, exchangeable bases, as determined by 1.0 N neutral NH₄OAc extraction, varied from 0.145 to 0.317 cmol kg⁻¹ soil, with the highest level for the soil of Modhukhali (AEZ 12) in Faridpur and the lowest for the soils of Barura (AEZ 16) and Birganj (AEZ 1). The results indicated that the most of tested soils were K-deficient. The relative yield (RY) of crop reveals that the soil dry matter yielding ability without K in comparison to that with K. The mean Rys over seven successive crops across 18 soils varied from 47 to 75% and from 37 to 85% for rice and maize, respectively, whereas the highest and the lowest Rys for both rice and maize crops were observed in Modhukhali and Durgapur soils, respectively. K uptake by both crops varied significantly in both control and K-added pots, and K uptake in K-added pots was higher than control pots. In control pots, cumulative K uptake over seven successive crops varied from 1.20 to 5.71 and 1.70 to 5.29 g pot⁻¹ in rice and maize, respectively. In general, total K uptake by plant from the pot that did not receive K fertilizer, considered as potential K supplying capacity of the soils, the order in rice was as follows: Mithapukur >Modhukhali >Rangpur Sadar >Gangachara >Dinajpur Sadar >Jhinaidah Sadar >Binerpota >Modhupur >Tarash >Gopalpur >Daudkandi >Paba >Nawabganj Sadar >Birganj >Shibganj >Godagari >Daudkandi >Barura >Durgapur. In contrast, in maize, this order was as follows: Modhukhali >Mithapukur >Rangpur Sadar >Dinajpur Sadar >Jhinaidah Sadar >Gangachara >Binerpota >Tarash >Gopalpur >Daudkandi > Paba >Modhupur >Nawabganj Sadar >Shibganj >Birganj >Godagari >Barura >Durgapur. The cumulative K uptake by rice and maize at the seventh harvest showed significant relationship with the exchangeable K (R²=0.87) but such relationship with non-exchangeable K was poor (R²=0.48). Higher slope of regression lines for exchangeable K pools indicated that crop depended mostly on exchangeable pools for K uptake and thus it might have depleted K greatly, and to meet the K demand it might have explored K from the non-exchangeable pool to a considerable extent. In field validation trials, yield and yield attributes of both crops responded to K fertilizer significantly in all locations. K fertilizer increased grain yield from 11 to 31% and from 18 to 79% over control in rice and maize, respectively. Further, maize crop utilized the applied K more efficiently than the rice crop did. The research established that 59-62 and 111-122 kg K ha⁻¹, respectively for rice and maize, was economically most viable to produce optimum yield from these crops. Further, the study showed that site-specific K management was economically profitable compare to region based fertilizer recommendation. Total K uptake by plant from the plot that did not receive K fertilizer, considered as K supplying capacity of the soils, was in the order: Rangpur >Rajshahi >Comilla for the both crops, which was similar to the results of pot experiments.