Experimental Sites Three experimental sites were established at the Bangladesh Agricultural Research Institute (BARI) experimental farms at Joydebpur, and Ishwordi. The Joydebpur site is within Agro-ecological Zone (AEZ) 9 (Old Brahmaputra Floodplain), on flood-free, flat, medium highland with fine-textured (silty clay loam) and low-permeability soil. The Nashipur site is in AEZ 1 (Old Himalayan Piedmont Plains), on floodfree highland with coarse-textured (sandy loam) and high-permeability soil. The Ishwordi site is in AEZ 11 (High Ganges River Floodplain), on flood-free highland with fine-textured (silty clay loam) and medium-permeability soil (BARC, 1997). Experimental Design, Treatments, and Crop Management Experiments at each site compared RW systems with maize or mungbean as pre-rice crops under soil test-based (STB) and farmers’ practice (FP) fertilizer management. The experiments commenced with wheat sown in late November/early December in 1995 at Joydebpur and Nashipur, and in 1997 at Ishwordi. A nutrient-extractive maize crop was sown at each site before the start of the experiments to identify and minimize site heterogeneity. A randomized complete block design with four replications was used at each site, with two main treatments: pre-rice crop management (three levels) and fertilizer management (two levels). Pre-rice treatments were mungbean, with residues either retained or removed and maize, with residues, removed. For mungbean, residues were either incorporated into the soil, or removed completely, after pod harvest. In rice, wheat, and maize, the above-ground residue was removed but roots were incorporated into the soil. Fertilizer treatments comprised the recommended complete fertilizer dose to achieve high yield (STB) and the typical fertilizer regime used by farmers at each location (FP). Fertilizer N, P, and K for rice across sites ranged, respectively, from 87–120, 18–26, and 0–50 kg ha−1 under STB and from 60–80, 9–26, and 17 kg ha−1 under FP. In wheat, the ranges were 120–140, 15–26, and 33–66 kg ha−1 under STB and 60, 11–18, and 25 kg ha−1 under FP. Maize received 80–120, 18–30, and 0–33 kg ha−1 under STB and 60–80, 9–26, 17–33 kg ha−1, respectively, under FP, while mungbean received 15–20 kg ha−1 N, P, and K under STB but no fertilizer under FP. Each fertilizer treatment was divided into two N rate sub-plots in a split-plot design. In the +N sub-plot, N was applied at the STB or FP rate while N was omitted in the corresponding −N sub-plot. The result was 12 treatment combinations. All plots and subplots were separated by earthen banks lined with plastic to 30 cm depth. Tissue K concentration was measured and analyzed from only seven treatments: 6 +N treatments (T1–T6) and one −N (control, or T7). The control plot was grown to maize without N but with other nutrients as applied by farmers (FP). Details of experimental design, treatments, fertilizer doses, planting dates, crop management, sampling procedures for yields of individual crops, and weather description for each site are provided elsewhere (Quayyum et al., 2001, 2002a, 2002b). Chemical Analysis of Soil, Water, and Plant Samples “Initial” soil samples were collected from each site (0–15 cm) at the establishment and “final” samples were collected after the rice harvest in 2000 from six random locations in each subplot (0–15 cm). Plant samples (straw and grain) from 10 m2 of each plot were oven-dried at 700C for 48 h and finely ground. Rice grain was unhusked. Straw and grain samples, and also the initial and final soil samples, were analyzed for total K by di-acid digestion (Yoshida et al., 1976). Crop K uptake was estimated from K concentrations and the straw and grain yields reported elsewhere (Quayyum et al., 2001, 2002a, 2002b). The nutrient concentrations in irrigation and rainwater were determined occasionally and means were used to calculate the amounts of nutrients added by irrigation and rain. Potassium Balance Calculations The mean annual apparent K balance was calculated using the following simple equation: Ka = (Kf + Kr + Ki + Kc) − Kupt
T1, T3, and T5 denote mungbean residue removed, mungbean residue retained, and maize residue removed (STB fertilizer); T2, T4, and T6 denote mungbean residue removed, mungbean residue retained, and maize residue removed (FP fertilizer), and T7 denotes maize residue removed (with FP fertilizer and no N). 2Least significant difference at α = 0.05. where Ka = apparent K balance (kg ha−1) Kf = k added through fertilizer (kg ha−1) Kr = k added through rainfall (kg ha−1) Ki = k added through irrigation water (kg ha−1)
Kc = k added through crop residue (kg ha−1) Kupt = k uptake by crop (kg ha−1) Potassium and System Productivity Total system productivity at each of the three sites was correlated with (1) the total external K supply (Kex) through fertilizer, irrigation, and rainwater and (2) internal use of K by the crops, i.e., the K uptake (Kupt).