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.