Improved fallows or the rotation of fast growing nitrogen-fixing legume species with cereals have been shown to accumulate nitrogen and organic matter, recycle nutrients in the soil and improve soil physical and chemical properties, and increase crop yield compared to traditional fallows. However, the effect of soil nutrients added by fallow species on the incidence of pests, weeds and pathogens in the subsequent crop has not been assessed in southern Africa. In this study, we assessed the relationships between nutrients in the soil after fallows of crotalaria (Crotalaria grahamiana), pigeon pea (Cajanus cajan), sesbania (Sesbania sesban), tephrosia (Tephrosia vogelii), and their mixtures, and the incidence of soil insects, namely, snout beetles (Diaecoderus sp.) and termites, and weeds in eastern Zambia. Rotational fallows of sesbania+crotalaria, sesbania+tephrosia, sesbania+pigeon pea and tephrosia+pigeon pea increased infestation of maize by snout beetles as compared to the natural fallow or unfertilised maize grown continuously in monoculture. The beetles showed aggregated spatial distribution, influenced mainly by the nitrate and total inorganic nitrogen content of the soil. Termite incidence was higher in maize after a natural fallow and pure crotalaria, which had 11 and 7 times as much damage as maize planted after pigeon pea+tephrosia mixture. Total weed biomass in maize grown after a natural fallow was six times higher than in maize planted after pure sesbania fallows. The weed biomass was correlated positively with the potassium content of the top 20 cm soil and negatively with plant litter on the soil surface. It is concluded that organic inputs from pure sesbania, sesbania+pigeon pea, sesbania+tephrosia and tephrosia+pigeon pea reduce infestation by termites and weeds, and give maize grain yield comparable with the recommended rates of inorganic fertilisers. However, these fallows have the potential to increase infestation by snout beetles. This is the first study on the snout beetle in Africa, and we recommend more systematic investigation on its ecology in agroforestry systems.
Tag: crotalaria grahamiana
Diversity of plant-parasitic nematodes and their relationships with some soil physico-chemical charateristics in improved fallows in western Kenya
A survey was conducted in the food crop production area of western Kenya to characterise the nematode communities in different cropping systems and to monitor the effect of improved fallows on the abundance, diversity and community structure of plant-parasitic nematode assemblages. Soil samples were taken from short-term natural fallows (20 years), maize/beans cultivated fields and improved fallows with Crotalaria grahamiana, Sesbania sesban and Tephrosia vogelii. The Shannon diversity index and evenness were used to assess nematode diversity in the different cropping systems. The relationships between plant-parasitic nematodes and the soil physico-chemical properties were investigated with principal component analysis and co-inertia analysis.Planting trees/shrubs as improved fallows in croplands dramatically increased the populations of nematodes in the soil. However, the diversity of plant-parasitic nematodes was reduced. The distribution of plant-parasitic nematodes was correlated with that of some soil physical and chemical properties. Scutellonema spp. were dominant in the improved fallows where the highest values of magnesium and potassium were recorded. The plant-parasitic Dorylaimida (Xiphinema spp. and Paratrichodorus minor) were more abundant in the maize/beans plots and this abundance seemed related to high soil bulk density. Meloidogyne spp. and Rotylenchulusborealis had a strong positive correlation with soil organic matter and clay, and were abundant in the long-term natural fallows.The correlations between soil physico-chemical properties and nematodes indicate that, apart from the direct influence of the host plant, the soil characteristics play an important role in the abundance, distribution and structure of nematode communities. This validates the potential of nematodes as bio-indicator organisms of soil status.