Introduction

In Bangladesh, despite rising fertilizer input levels, farmers' yields remain stagnant. During the last 5 years (2000-2001 – 2005-2006), application rates of N, P and K have grown by 4, 5, and 23 per cent respectively. On average, in 2005-2006, for N, P₂O₅ and K₂O the corresponding levels applied in the field were 103, 26, and 15 kg/ha. During the same year, the N:P₂O₅:K₂O ratio was 1:0.25:0.15 in Bangladesh thus indicating a considerable imbalance of applied nutrients, especially in relation to potassium. There is thus a major need for a more efficient use of mineral fertilizers based, first of all on a balanced application of plant nutrients.

This paper is a continuation of a publication of research findings from IPI collaborative projects conducted in Bangladesh. In a previous issue (e-ifc No. 12, June 2007), we published results obtained for a Wheat-Rice cropping system in NW regions of the country. In this report we discuss research findings for a double rice cropping system in Central Bangladesh, which is another common cropping pattern of the country.

Materials and methods

A research trial was conducted on a Rice-Rice cropping system, i.e. Boro- Fallow-T. Aman (Transplanted Aman), (Boro is also mostly a transplanted crop) at Bangladesh Rice Research Institute (BRRI) farm (Modhupur tract, medium highland) during 2003-2007. Boro is the dry season (Nov-May) irrigated rice and Aman is the main monsoon season rice (Jul-Dec). The farmers' field demonstrations were carried out in the Gazipur District at sites of 5-7 farmers per season. The soils of the experimental fields are clay loam in texture, low in fertility status and very strongly acidic to slightly acidic in reaction (Table 1). The content of total N, available P and S ranged from very low to low, exchangeable K - from low to medium and available Zn - from low to very high.

In the research trial, six K treatments (note elemental basis is used here): zero K (K₀), recycling of crop residues with no added K fertilizer (K_{0 + CR}), 33 kg K/ ha (K₃₃), 50 kg K/ha (K₅₀), 66 kg K/ha (K₆₆) and the farmers' practice for K (K_FP, 18 and 37 kg K/ha, in T. Aman and Boro, respectively) were tested using a randomized block design (RDB) with four replications. In the farmers' field demonstrations, three K treatments were compared: 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 at the farmers' fields) were applied to all the plots in both Aman and Boro.

Rice response to K

Application of K fertilizer considerably increased the grain yield of rice over the control treatment K₀ in both seasons at the range of 7-17 per cent (Table 2). The non-significant, but positive, effect from K fertilizer use was observed during the first season of T. Aman and the last season Boro. Of the treated plots, where K fertilizer was applied, an application of 66 kg K/ha in the T. Aman season produced the highest grain yield of 3.93 t/ha (the four seasons' average). In the Boro season, a rate of 50 kg K/ha achieved the highest yield of 5.70 t/ha (the four seasons' average) Potash fertilizer application had practically the same efficiency in both the T. Aman and Boro seasons: the optimal rates of K (50-66 kg K/ha in T. Aman and 50 kg K/ ha in Boro) increased grain yield of rice on average by 16-17 per cent. The additional grain yield obtained with optimal potash rates was 0.56-0.58 t/ha for T. Aman and 0.79 t/ha for Boro (the four seasons' average).

It is evident that farmers' practice for K (18 and 37 kg K/ha in T. Aman and Boro, respectively; K_FP) was not enough to produce high yields of rice (Table 2).

As compared with the K₀ treatment, farmers achieved an additional 0.32-0.41 t/ha or 7-12 per cent yield increase (the four seasons' average) when using their regular practice (K_FP) in both seasons.

Recycling of crop residues significantly increased the grain yield of rice over K₀ (except the first T. Aman and the last Boro seasons) with grain yields similar to those of K₃₃ and K₁₈ (K_FP) for T. Aman and K₆₆ for Boro (Table 2). Crop residue incorporation increased grain yield of the two crops on average by 13-15 per cent, compared with the K₀ treatment. In general, potash fertilizer application gave higher crop productivity, compared with crop residue incorporation alone, without potash fertilizer. In the experiment conducted on a light textured soil at the NW region of Bangladesh (see e-ifc No.12, June 2007), it was concluded that crop residue incorporation can not substitute recommended rates of potash fertilizer, which need to be applied to optimize crop K nutrition and to preserve the soil from K mining. So the same conclusion is true for clay loam soil at Gazipur. With this impressive effect of straw incorporation on yield, it is important to mention that farmers in Bangladesh generally remove straw from their fields thus potash fertilizer is a major source of potassium for crop nutrition and sustains soil fertility. Incorporation of straw along with a reduced amount of applied potassium fertilizer may well prove to be the best approach for K fertilization.

The efficiency of K was found to be more prominent in the dry season rice than that in the wet season rice. Better vegetative growth, coupled with increased grain yield production in the dry season than in the wet season from the K fertilized plot, might explain the results.

The use of soil test based K fertilization (K_STB) was examined in farmers' fields (Table 3). In addition to this treatment, doses of 18 and 37 kg K/ha were compared. The K_STB approach which recommends higher K levels of (41 and 67 kg K/ha in T. Aman and Boro seasons, respectively) clearly shows that lower K levels applied are not sufficient to produce high yields. On average, the use of K-fertilizer in recommended rates, according to soil tests in farmers' fields produced 19-25 per cent higher rice grain yield as compared with the K₀ treatment.

Economic analysis

Economic analysis was carried out on the mean data of first three crop seasons of T. Aman rice and of that of Boro rice (Table 4). This analysis shows that on average, the net additional income gained by application of potash is between 22 and 123 USD/ha per season. The additional income earned resulting from K-fertilization was much higher in the irrigated Boro season than in T. Aman because the efficiency of K was found to be higher in dry season rice than wet season rice. In a research experiment, the maximum additional income in T. Aman season was obtained from the treatments in which K-fertilizer was applied at 50-66 kg K/ha. The K-fertilizer rate of 50 kg K/ha generated the highest additional income in Boro season. The treatment with crop residue incorporation showed the lowest value cost ratio (VCR) and the recycling of crop residues was profitable only in the irrigated Boro season but still less than compared with applied potash at 50 kg K/ha. In the case of farmers' demonstrations, K applied on the basis of a soil test in both seasons always contributed to considerably higher additional benefit than that from farmers' fertilization practice.

Conclusions

T. Aman and Boro rice yields were significantly increased at rates of 7-17 per cent in response to medium application rates of potassium. This increase proved to be economical in both T. Aman and Boro seasons, although much more in the Boro season, mainly due to better vegetative growth coupled with increased grain yield production in the dry season than in the wet season in K fertilized plots. Performing soil tests for K at farmers' fields is highly recommended as it provides the farmers a significant yield increase of 19-25 per cent in both rice seasons.

References

• Mazid Miah, M.A., Saha, P.K., Islam, A., Nazmul Hasan, M. and Nosov, V.V., 2006. Efficiency of potash fertilizer application in a rice-rice and a rice-wheat cropping system in Bangladesh. Paper presented at the IPI-PAU International Symposium on "Balanced Fertilization for Sustaining Crop Productivity". Ludhiana, India, 22-25 November 2006.
• IPI-BRRI Collaborative Project Preliminary Report 2007.

Edited by E.A. Kirkby.