Evaluation of the performance of expanded black cotton soil as a hydroponic medium

Abstract

The traditional system of producing crops using soil (geoponics) is currently facing major challenges resulting in food deficits. An alternative is the adoption of soil-less culture (hydroponics) which is regarded as key in increasing production of vegetables, herbs and ornamentals. The study aimed at preparing a hydroponic medium from black cotton soil and rice husks. The process entailed moulding, firing, size reduction and performance evaluation. The aggregates were evaluated on the basis of dry bulk density and saturated hydraulic conductivity. The investigations revealed that the optimal conditions for preparing the medium were 750°C, 30 minutes and 9:1 for firing temperature, time and ratio of black cotton soil to rice husk (on a weight basis) respectively. These conditions resulted in 33% reduction in bulk density from 1.43 g cm-3 to 0.954 g cm-3 and

improvement in saturated hydraulic conductivity from 0.333*10-5 cm s-1 to 0.00385 cm s-1, which falls between the ranges for Sandy Loam and Loamy Sand based on estimated values by the RETention Curve (RETC) model. The Hydrus 1D version 4.16.0110 model was used to estimate the hydraulic parameters of the expanded black cotton soil. The optimized values were 0.1 cm3 cm-3, 0.55 cm3 cm-3, 0.01, 1.91, 0.00368 cm s-1 and -1 for Өr, Өs, ά, n, Ks and l respectively. The measured vs. simulated values for water retention, Ө(h) resulted in R2 of 0.83 and 0.0895 cm3 cm-3 for the RMSE which showed that the

model estimations could be applied to determine the water retention and hydraulic conductivity of the expanded black cotton soil aggregates at varied saturation. Further evaluation on performance of the medium was done by using it to grow tomatoes (cultivar Anna F1). Plant growth was assessed using stem elongation and enlargement; and root length density (RLD). The mean weekly stem elongation and enlargement rates for the crop grown on expanded black cotton soil were higher than those of the sampled black cotton soil (clay) by 4.42% and 9.69% respectively. However the mean RLD was 25654 m m-3 in black cotton soil compared to 18936 m m-3 for the expanded black cotton soil. This

reduction of 26% is however beneficial because it can allow the crops to be planted using smaller volumes of the expanded black cotton soil. The findings showed that the expansion of the black cotton soil by incorporating rice husks was achieved at lower firing temperature compared to commercially available expanded clay pellets fired at 1200oC