| The objectives of this study were; (i) identifying the distribution of soil characteristics that affect accumulation of phosphorus and nitrogen in sugarcane fields with Immokalee fine sand and Margate fine sand; (ii) characterizing sorption of phosphorus using different electrolytes; (iii) studying movement of water determined by bromide tracer, phosphorus, and nitrogen in relation to fluctuating water table depth and drip irrigation, and (iv) modeling water (bromide), nitrogen, and phosphorus using linearized sorption coefficients determined using different electrolytes. Characterization of soil characteristics in sugarcane fields was conducted using 80 uniformly distributed (using a 38 m by 38 m grid) and 20 random sample positions. The two sugarcane fields (one with Margate soil and another with Immokalee soil) were 30 acres each. Soil samples sampled from 0-30cm, 30-60 cm, and 60-90 cm depths were analyzed for total carbon, total phosphorus, pH, oxalate iron, oxalate aluminum, oxalate phosphorus, and exchangeable calcium. The A horizon depths were fully explored and measured for the two sugarcane fields. The values of total phosphorus, total carbon, and pH were arranged in ascending order and clustered in to five clusters. One sample from each cluster was randomly selected for conducting phosphorus and ammonium sorption experiments. The A horizon from Immokalee and Margate soil, Bw horizon from Margate soil and Bh horizon from Immokalee soil were used for sorption experiments. A saturated flow experiment where A horizon material was packed in a column (15 cm long and 7.5 cm in diameter) and fertilizer mixture pumped through at a rate of 10mL per minute was conducted. Column leaching experiments were conducted to identify changes in movement of phosphorus and nitrogen (ammonium and nitrate) with fluctuating water table (30 cm to 50 cm). In a lysimeter study, phosphorus, nitrogen, and potassium were applied to a lysimeter where sugarcane was planted and subjected to drip irrigation at water application rate of 2.3 Lh-1. The drip emitters’ spacing was 30.5 cm. The 15 cm depth increment (0-15 cm, 15-30cm, and 30-45 cm) was used for soil sampling. Bulk densities of soil horizons, saturated hydraulic conductivity (Ksat values), moisture release constants, and phosphorus sorption coefficients determined using different electrolytes and sorption kinetics parameters were used to calibrate Hydrus 1D. After calibration, column leaching results and lysimeter study results (bromide, phosphorus, ammonium, and nitrate) were used as validation data sets. Results after studying distribution of soil characteristics in two sugarcane fields have shown that unlike in a field with Immokalee soil where total carbon and oxalate aluminum influenced most total phosphorus distribution, total carbon influenced most total phosphorus distribution in Margate field. Soil characteristics were observed to vary spatially and with depth. Sorption of phosphorus by soil from least was deionized water, simulated Florida rain, potassium chloride (0.01M KCl), and calcium chloride (0.005M CaCl2). The calculated linearized sorption coefficient (0.01M KCl) compared well with linearized sorption coefficient (fertilizer mixture). Negligible sorption of phosphorus was identified in E horizons sampled from two soil series (Margate and Immokalee). The similarity in movement behavior of chloride and nitrate for saturated flow experiment showed that both can act as tracers. Since ammonium, nitrate, and chloride fit a convective-dispersive model, there was absence of physical non-equilibrium in saturated flow experiment. For both saturated and unsaturated flow experiment, phosphorus was more retarded than ammonium. Phosphorus and ammonium concentrations below the water table were higher when the water table was set at 50cm than 30cm. For the lysimeter study, differences in highest concentrations of bromide for 0-15 cm and 15-30 cm from irrigated (20 cm from center of plant row) and non-irrigated zone (50 cm from the center of plant row) were attributed to bromide uptake. Since high phosphorus and nitrogen concentrations were observed within the root zone (0-30 cm) and increasing concentrations in tissues with time, nutrients were managed within the root zone and plants responded to applied nutrients and moisture. The significant results of this work were; (i) supporting electrolytes affect the sorption behavior of phosphorus in sandy soils and this has been shown in trend of linearized sorption coefficients; (ii) management of plant available nutrients within the root zone of sugarcane plants using drip irrigation; and (iii) a calibrated Hydrus 1D model for sugarcane production on sandy soils that can be used by modelers and farmers. (Abstract shortened by UMI.). |
