Experimental site: The experiments of this study were conducted with wheat during November through March of 2009 - 2010 at the farm of the Bangladesh Agricultural University, Mymensingh (24.75oN latitude and 90.50oE longitude). The soil was silt loam with 0.48% organic matter, 6.8 pH, 38.2% (v/v) field capacity, 18.37% (v/v) permanent wilting point, 1.33 g cm−3 bulk density and 0.62 dS m−1 electrical conductivity (EC). The climate of the region is sub-tropical humid with an average annual rainfall of 242 cm that is concentrated over May to September. The summer is hot and humid, and the winter (November – February) is mode rate with occasional light rainfall. There was however no rainfall during the period of our field experiments, thus providing a controlled water management.
Treatments and design: Three experiments were designed with different dose combinations of three major nutrients: N, P and K. The treatments of each experiment comprised four doses of one of the major nutrients; the other two major nutrients and, as micro nutrients, sulphur (S), zinc (Zn) and boron (B) were kept at their recommended doses. The variable doses of the nutrients included the smaller as well as the larger quantities than the recommended doses. The doses under test were 0, 80, 100, 120 and 140 kg ha−1 for N; 0, 10, 20, 30 and 40 kg ha−1 for P and 0, 40, 60, 80 and 100 kg ha−1 for K. The recommended doses of N, P, K, S, Zn and B for wheat cultivation in Bangladesh were 120, 30, 60, 15, 5 and 1 kg ha−1, respectively. These nutrients were applied in the form of triple super phosphate, muriate of potash, gypsum, zinc sulphate and borax, respectively. The experiment was laid out in a randomized complete block design with three replications; the size of each plot 2 m x 2 m.
Cultural operations: The entire doses of the fertilizers in the treatments of the experiments, except the urea, were applied to the prescribed plots during the final land preparation and incorporated into the soil. The urea was applied in two splits: two-thirds during the land preparation and the rest as top dress before the first irrigation at 20 days after sowing (DAS). The wheat seeds, @ 120 kg ha−1, were sown in 20-cm apart rows on 4 December 2009. There were weed infestation that was controlled effectively by uprooting. An incidence of cut worms was controlled by spraying the insecticide, Darsbarn. The wastewater of Mymensingh municipality was collected in plastic barrels from the drainage canal of the sewerage system. The barrels were carried to the experimental field in a truck and the water was poured into a pit lined with polyethylene sheet. The water was mixed thoroughly to achieve a homogeneous mixture. Samples of wastewater were collected from the pit and analyzed by a DR/890 Colorimeter (Hach Co., USA) for their chemical properties. The (average) concentrations of B, Fe, K, NO3-N, PO4-P, Na, Pb, Cu, Zn and Cd in the wastewater were below their threshold values set by FAO (1992) for safe use in agriculture; only the Mn exceeded the limit. The concentration of N, P and K in the wastewater was 17.5, 3.7 and 10.3 mg L-1, respectively. The details of the wastewater quality parameters of the Mymensingh sewage system were reported in Mojid et al. (2010). The irrigation was applied on the critical phenological growth stages of wheat: CRI (20 DAS), booting (50 - 55 DAS) and flowering (75 - 80 DAS). The soil water content of the plots was measured before irrigation by a portable moisture meter, Trime FM (Eijelkamp, The Netherlands). The quantity of irrigation water was calculated by the difference between the soil-water content at field capacity and that prior to irrigation. The irrigation requirement was quantified for the effective root zone depth of 60 cm. The soil in the field was characteristically homogeneous and so the soil-water contents in the plots were very consistent. Consequently, an average equal amount of water was applied to each plot in a particular irrigation. This provided an additional control in the treatments. The irrigation was applied manually in check basin. The quantity of irrigation water was 3.0, 4.5 and 5.5 cm in the first, second and third irrigation, respectively.
Data collection: The leaf area and above-ground dry matter were collected four and five times, respectively during the growing season by clipping ten plants, selected randomly, at ground level from each plot. The leaf blades were separated from the sheath at the collar and their area was measured with a LI- 3100 (USA) leaf-area meter. The leaf area index (LAI) was calculated by the ratio of the measured leaf area of the ten plants to the ground area covered by these plants. The mature wheat was harvested manually on 22 March 2010 from an area of one square meter at the middle of each plot that remained unaffected by periodical crop sampling. The total number of the fertile spikes was counted in the sampled crop for each plot. The plant height, spike length and number of spikelets per spike were recorded from randomly selected ten plants from each sample. The harvested crop of each plot including that of the ten sample plants was then threshed after sun drying and cleaned to separate the grains and straw. The grains were dried and weighed at 12% moisture content. One thousand clean and dry grains were counted from the seed stock of each plot and weighed. The biological yield, articulated by the sum of the grain and straw yields, was determined. The harvest index was calculated from the ratio of the grain yield to the biological yield. The analysis of variance of the growth and yield attributes, grain and biomass yields, and harvest index of wheat was done for the Randomized Complete Block Design. The R-package Agricolae (de Mendiburu, 2009) was used for the analysis. The significant level for comparison of the different treatments for the growth and yield attributes was set at p = 0.05.
The nutrient use efficiency (NUE) was calculated by using the difference in the nutrient of interest between the fertilized and control plots. The NUE was expressed as the physiological efficiency (PE), agronomic efficiency (AE) and crop recovery efficiency (RE). The PE, defined by the grain yield per unit of nutrient uptake, entails the ability of a plant to transform a given amount of an acquired nutrient into the grain yield. The AE refers to the increase in crop yield per unit of an applied nutrient. The RE, on the other hand, refers to the increase in nutrient uptake by the plants per unit of an applied nutrient. These nutrient use efficiencies for different N, P and K treatments were calculated following Daradjat et al. (1991).
In the above expressions, the uptake of a nutrient means its uptake in the above-ground biomass and was expressed in kilogram per hectare; the applied dose of a nutrient means its rate of application and was also expressed in kilogram per hectare; and N0, P0 and K0 imply the omission of the respective nutrients.