Abstract
Maize is the staple food in Swaziland but there have been shortfalls in production mainly due to the high cost of commercial fertilisers. One possible means of addressing the problem is the use of sunnhemp (<i>Crotalaria juncea</i> L.), a green manuring legume, which is cheap and easy to produce. A field experiment was conducted at the Malkerns Research Station during the 2014/2015 cropping season. The main objective of the study was to evaluate whether sunnhemp can be used as a substitute for inorganic fertilisers. The treatments were (1) maize with 40 kg/ha sunnhemp used as mulch, (2) maize with 80 kg/ha sunnhemp used as mulch, (3) maize with 120 kg/ha sunnhemp used as mulch, (4) maize with 40 kg/ha sunnhemp soil incorporated, (5) maize with 80 kg/ha sunnhemp soil incorporated, (6) maize with 120 kg/ha sunnhemp soil incorporated (7) maize with no fertilisers (8) maize with half rates of inorganic fertilisers and (9) Maize with recommended rates of inorganic fertilisers. The design of the experiment was a randomized complete block design and each treatment was replicated four times. Maize variety SC 403 was used. Data were collected on sunnhemp biomass, cost of placement and rate of mineralisation. Maize data were on growth, yield and yield components. The results on labour cost of placement show that soil incorporation was significantly (P < 0.01) higher than mulching with sunnhemp being 29.8 man-days/ha while it was 18.6 man-days/ha for mulching. The amount of mineralised sunnhemp was significantly (P < 0.05) higher in incorporated sunnhemp than from sunnhemp used as mulch. Maize grown with 120 kg/ha of sunnhemp yielded 7662.7 kg/ha while maize with 40 kg/ha of sunnhemp was lowest with 7251.1 kg/ha, a yield difference of 5.4%. Maize from soil incorporated sunnhemp yielded 7519.3 kg/ha while maize grown with sunnhemp used as mulch was lower with 7325.0 kg/ha, a yield difference of 2.6%. Maize grown with the recommended rates of fertilisers yielded higher than the rest of the treatments, it yielded 8405.7 kg/ha while maize grown with 80 kg/ha of sunnhemp later used as mulch was lowest with 6945.7 kg/ha. It is concluded that labour cost of sunnhemp placement was significantly (P < 0.01) higher for soil incorporation than mulching. Nitrogen mineralisation was significantly (P < 0.05) higher when sunnhemp was incorporated than when used as mulch. It is recommended that farmers grow their maize with 80 kg/ha of sunnhemp and use it as mulch seven weeks after planting.
Highlights
Nitrogen is an essential element for plant growth and development and is frequently the major limiting nutrient in most agricultural soils (Daughtry et al, 2000)
Sunnhemp canopy height was the highest (70.1 cm) in the treatment which was planted at 120 kg/ha
Crude Protein Content of Maize Stover The results show that crude protein content of maize stover was high in maize grown with 80 kg/ha of sunnhemp later incorporated into the soil
Summary
Nitrogen is an essential element for plant growth and development and is frequently the major limiting nutrient in most agricultural soils (Daughtry et al, 2000). Swazi farmers seldom apply the required amount of nitrogen fertilisers. This has led to the country relying on imports from the Republic South Africa to satisfy local demand. Further worsening the situation is the use of crop residues to feed livestock in winter. This means the nutrients are not being recycled and soil fertility keeps on declining. One possible means of addressing this problem is the use of Mzwandile P. Mabuza et al.: Effects of Inorganic Fertilisers and Sunnhemp (Crotalaria juncea [L.]) as a Green
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