Effect of composted Phalaborwa ground phosphate rock on performance of grain sorghum grown on variable soil conditions
Letsoalo, Maimela Solomon
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Grain sorghum (Sorghum bicolor [L.] Moench), is a cereal crop that is indigenous to Africa. The desire to reduce the negative impact of industrial wastes on the environment through the use of inorganic fertilisers and promote harmony between nature and the earth inhabitants calls for identification of viable and cheaper alternative non-hazardous fertiliser source for maintaining soil fertility and increasing crop yields on farmlands. This can also help farmers to manage on-farm wastes as wealth resources. It also minimizes the waste removal costs and serves as way of increasing income. This study assessed soil phosphorus (P) and other soil nutrients availability when using cheaper and locally available P-rich organic-based fertiliser sources for increased grain sorghum yields. Compost preparations, greenhouse and field trials were conducted at the North-West University experimental farm in Mahikeng. The P-enriched compost produced contained adequate levels of nutrients including P, which was 68.37 g/kg but with high level of Cd that was above the threshold level of 39 mg/kg it may pose serious threats for agricultural soils. The most common threats caused by Cd are stunting and chlorosis in plants. Evaluation of growth, yield, nutrient and nutritional contents of grain sorghum were carried out under greenhouse and field conditions following application of variable rates of the P-enrich composts. Two greenhouse trials consisted of factorial arrangements. The first greenhouse trial consisted of two soil textural classes (loam and a sandy loam) and seven compost rates (i.e. unamended control, 5, 10, 20, 40, 80, 160 t/ha) and inorganic NPK rate as a positive control while the second trial consisted of two soil types (Hutton and Coega) and seven compost tea rates. The compost tea rates included unamended control, 250 ml fortnightly, 250 ml weekly, 250 ml bi-weekly, 500 ml fortnightly, 500 ml weekly and 500 ml bi-weekly. In addition, a laboratory incubation study on phosphorus release from the P-enriched co-composted manures was conducted to quantify and compare P availability from co-composted GPR in two soils with variable textural characteristics. Results from the first greenhouse trial revealed that all measured sorghum growth parameters performed better under loam soil than the sandy loam. Higher compost application rates promoted early flowering first observed in the 80 t/ha rate at 49 days. However, biomass accumulation in the inorganic NPK fertilizer rate was higher (53.63 g) than any of the P-rich compost rates. However, the 27.48% grain protein content obtained at 40 t/ha P-rich compost treatment was significantly higher. Similarly, the 0.65% grain P content obtained from the 10 t/ha compost rate was the highest suggesting possible optimum compost rates. On the other hand, results of the compost tea trial revealed that compost tea application had beneficial effects on growth and yield of sorghum grain with the 500 ml bi-weekly application rate producing better growth than any other rates in both soils. However, the obtained highest grain yield of 16.93 g/ panicle was from the 250 ml weekly treatment in the Hutton soil. Higher P-use efficiency (PUE) values of 4.66 and 4.23, respectively for grain and biomass obtained from Coega soil compared to the -0.44 (grain) and -0.208 (biomass) in the Hutton soil. Compost tea application can be a useful fertilizer source for sorghum Results of the field trials revealed that 5 and 10 t/ha treatments gave the highest yields at 5490.1 and 5301.6 kg/ha, respectively. The highest total N (2.05%), crude protein (12.90%) and P (0.46%) contents from grain samples obtained from the 80 t/ha treatment. In addition, the 80 t/ha gave the lowest bulk density across all the treatments and resulted in the highest porosity. Although the 80 t/ha compost rate resulted in increased sorghum grain P uptake, reduced bulk density and favourably improved soil total porosity, the potential risks of exchangeable cation (EC) build-up following short and long term application of such rate makes it undesirable. Finally, the results of the laboratory incubation study revealed that P released over the 49-day period showed similar trends in both soils during the first 2 weeks and the third to the sixth weeks. However, the different P trend in loam soil decreased from the third week to the fifth week and the sandy loam soil showed a continuous increase in P from the first to the fourth week but decreased steadily starting from the fifth week. The final measured mineralized P in sandy loam (561.67 mg/kg) and loam (475.05 mg/kg) soils showed a P increase as a result of P-rich compost from the initial P values of 80 mg/kg (sandy loam) and 75 mg/kg (loam). However, there was non-significance effect on the cumulative P as affected by soil texture but the interaction between soil texture and compost rates had significance effect on cumulative P. The 10% compost tea rate containing 1574 mg/kg cumulative P in the sandy loam soil was non-significant while similar 10% rate containing 1236 mg/kg cumulative P in the loam soil was significant. The compost rate and soil texture played important role on compost mineralization. The findings in this study suggest the potential for possible improvement of soil P availability in sorghum fields using cheaper and locally available P-rich organic-based fertiliser source for increased sorghum yields.