Response of irrigated tomato (Solanum lycopersicum Mill) to mulch application rates

Rapid decline in available water for crop production has led to the adoption of irrigation schedules for meeting water supply throughout cropping seasons. Nonetheless, the loss of water from soil often results in spells of water stress between schedules, which adversely affect crop yield. Hence, the use of mulch in conserving soil moisture in irrigated farming is becoming popular among farmers. In this study, a two-year screenhouse pot experiment was conducted to evaluate the effects of Pennisetum purpureum (Pp) mulch on tomato (Roma variety) grown in daily irrigation (IF_daily), irrigation at 3-days interval (IF₃), and irrigation at 5-days interval (IF₅) conditions. The Pp mulch was chopped to 5 cm and applied on the soil surface of each experimental pot at 1 t ha⁻¹ (Pp₁), 2 t ha⁻¹ (Pp₂), 3 t ha⁻¹ (Pp₃), and 4 t ha⁻¹ (Pp₄). These rates were compared against a bare soil as control (Pp₀). The treatments were laid in a completely randomised design with four replicates. Tomato yield decreased by 53.6% and 26.6% in IF₃, and 86.2% and 65.0% in IF₅ compared with IF_daily in years 1 and 2, respectively. Among mulch rates, Pp₄ and Pp₃ increased tomato yield respectively by 107.5% and 99.9% compared with Pp₀, while Pp₂ and Pp₁ were similar in year 1. In year 2, mulch increased tomato yield by 84.1% (Pp₁) – 215.3% (Pp₄) and contributed substantially to tomato yield in IF_daily (R² = 0.99; p < 0.01); IF₃ (R² = 0.93; p < 0.01); and IF₅ (R² = 0.25; p < 0.05). However, withdrawing irrigation at 5 days interval was detrimental to tomato yield production.

Accelerating phytoextraction of petroleum hydrocarbon with organic stimulant

Phytoremediation has widely been recognised as an eco-system friendly and effective technique for soil remediation. However, this method is generally slow, and most plants used for phytoextraction are incapable of thriving in crude oil polluted soils with high concentrations of petroleum hydrocarbon. Hence, organic stimulants were developed for accelerating the phytoremediation of crude oil polluted soils by enhancing the growth of cowpea through nutrient supplementation, and increasing the bioavailability of petroleum hydrocarbon by saponification. Moringa and pawpaw seeds are high in plant available nutrients and contain saponin, a phytochemical that increases the bioavailability of contaminants. Although both seeds and moringa seed powder have been used for water purification, they have not been explored in soil remediation studies. For these reasons, pawpaw seeds were processed into powdery form to increase their surface area as organic stimulants for enhancing phytoremediation of crude oil polluted soil. This study investigates the performance of pawpaw seed powder, relative to moringa seed powder, in increasing the removal rate of petroleum hydrocarbon by cowpea under crude oil polluted soil conditions. The key functions of the newly developed organic stimulant include:•

Increased bioavailability of petroleum hydrocarbon for phytoextraction.

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Enhanced shoot production in cowpea.

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Increased capacity of cowpea to remove petroleum hydrocarbons from soils.

Organic stimulants for enhancing phytoremediation of crude oil polluted soil: A study on cowpea

Petroleum hydrocarbon (PH) contamination of soils remains a major threat to environmental health and food security. A two-years phytoremediation study was conducted on a crude oil polluted soil to assess changes in soil total petroleum hydrocarbon concentration (TPH_soil) following use of pawpaw seed powder (PSP), moringa seed powder (MSP) and their combination (PSP + MSP) as organic stimulants in cowpea cultivation. The stimulants were tested at different application rates (100, 150, 200 and 250 g m^-2), with the control (No stimulant) for their effectiveness in reducing TPH_soil and accelerating the removal rate (R) of PH from soil. The TPH_soil did not differ significantly (p < 0.05) among the treatments in year 1, but was highest in the control (11,600 mg kg^-1) and least in 200 g m^-2 PSP (7400.0 mg kg^-1). In year 2, mean TPH_soil varied significantly (p < 0.05) and remained highest in control (7100 mg kg^-1) but lowest in 150 g m^-2 PSP (2700 mg kg^-1). Application of 150 g m^-2 PSP gave the highest R (78.2%), followed by 150 g m^-2 PSP+MSP (77.4%), and least by the control (42.7%) over two years of study. The average fresh pod yield of cowpea over two years was highest in 250 g m^-2 PSP (2416.67 kg ha^-1), followed by 150 g m^-2 PSP (2173.34 kg ha^-1) and least in control (1302.22 kg ha^-1). There was significant negative association between TPH_soil and fresh pod yield (r = -0.403; p < 0.01). However, application of 150 g m^-2 PSP appeared most effective for enhanced phytoremediation of crude oil polluted soil and improvement of cowpea yield.