Introduction

The general recommended application rate of potash fertilizer for modern rice varieties in Bangladesh is about 35 kg K/ha, whereas an average crop of rice yielding 4.0 t/ha removes at least 70 kg K/ha from the soil (note elemental basis is used here). The present K fertilizer management practice may thus not be adequate to sustain soil K fertility status in the long term. There is therefore a tremendous potential for K fertilizer application both to maintain soil fertility and to increase cereal crop production in Bangladesh. Application of organic matter as crop residues is very limited in Bangladesh because these are largely used as fuel for cooking purposes and also as fodder for livestock.

Materials and methods

A research trial and farmers' field demonstrations with several combinations of potash fertilizer rates were conducted under a wheat-rice (transplanted Aman) cropping pattern in NW regions of Bangladesh during 2003-2006. The research trial was carried out at the Hajee Danesh Science and Technology University (HDSTU) experimental farm, Dinajpur (Old Himalayan Piedmont Plain, medium highland) and the farmers' field demonstrations at Dinajpur, Thakurgaon and Panchagar. The soils of the experimental fields are sandy loam in texture, and strongly to very strongly acidic in nature. They are very low in total nitrogen, and in exchangeable K and S, as well as being very low to optimum in P and low to optimum in Zn content (Table 1).

In the research trial, six K treatments viz. zero K (K₀), recycling of crop residues but no K fertilizer (K₀ + CR), 33 kg K/ha (K₃₃), 50 kg K/ha (K₅₀), 66 kg K/ha (K₆₆) and farmers' practice for K (K_FP) were tested for both rice and wheat using a randomized block design (RBD) with four replications. In the farmers' field demonstrations, three K treatments were compared for both crops, K control (K₀), farmer's practice for K (K_FP) and a soil test based K application (K_STB). The soil test based recommendations for NPS and Zn were applied to all the plots.

Rice and Wheat response to K

Application of K fertilizer significantly increased the grain yield of rice and wheat over the control treatment K₀ (Table 2). It is evident that farmers' practice for K (25 and 30 kg K/ha in rice and wheat, respectively; K_FP) was not enough to produce high yields in either of the cereals. Of the treated plots, where K fertilizer was applied, an application of 66 kg K/ha produced the highest grain yield of both rice and wheat in any season. Potash fertilizer application was more efficient in wheat in the dry season as compared with the wet monsoon rice season (Fig. 1). For instance, the highest rate of 66 kg K/ha increased grain yield of rice on the average by 30 per cent but the comparative figure for wheat was 53 per cent (Table 2). In general, the additional grain yield obtained by potash application was between 0.69 to 0.9 t/ha for rice and 0.65 to 1.23 t/ha for wheat (Fig. 1). As compared with the K₀ treatment, farmers achieved an additional 0.46-0.60 t/ha when using their regular practice (K_FP) or through the application of crop residues (K₀+CR; Fig. 1). Recycling of crop residues significantly increased the grain yield over K₀, with grain yields statistically similar to those of K₃₃ and K_FP for both rice and wheat (Table 2). Crop residue incorporation increased grain yield of the two crops on average by 20-21 per cent, as compared with the K₀ treatment. Thus, potash fertilizer application on light textured soils gave higher crop productivity, as compared with crop residue incorporation alone without potash fertilizer. As farmers in Bangladesh generally remove straw from their fields, recommended rates of potash fertilizer need to be applied to optimize crop K nutrition and to preserve the soil from K mining. Soil test-based K fertilization (K_STB) significantly increased the grain yield of both crops over K₀ and K_FP treatments in farmers' field demonstrations (Table 3). Again, the contribution of K fertilizer to grain yield production was found to be more prominent in wheat than that in rice. On average, the use of K fertilizer in recommended rates in farmers' fields produced 25 per cent higher rice grain yield and 86 per cent higher wheat grain yield as compared with the K₀ treatment.

Economic analysis

Economic analysis was carried out on the mean data of three crop seasons (Table 4 & Table 5). The maximum additional incomes in both rice and wheat were obtained from the same treatment in which K fertilizer was applied at 66 kg K/ha. Applied potash was of greater benefit than that of crop residue incorporation in both rice and wheat production. The additional income earned resulting from K-fertilization was much higher in wheat than in rice. In the case of farmers' demonstrations, K applied on the basis of a soil test always contributed to considerably higher additional benefit than that from farmers' fertilization practice, especially in the wheat crop. In all the treatments, the value cost ratio (VCR) was found to be more than the acceptable limit (VCR=2), but the treatment with crop residue incorporation showed the lowest VCR. The comparably higher VCR for potash fertilizer application was found in the wheat crop.

Edited by E. A. Kirkby.