The effect of soil surface temperature on the growth of millet in relation to the effect of Faidherbia albida trees. Agric. For. Meteorol., 60: 93-100.Faidherbia albida is an important tree species in traditional agroforestry cropping systems o f the Sahel. The enhanced crop growth under its canopy (the 'albida effect') has been attributed to increased soil fertility in the canopy zone, improved microclimate, particularly in relation to plant/soil water relations, and the fact that the tree sheds its leaves in the rainy season, which reduces shade and allows the crops to grow. The present studies demonstrate that shade-induced reduction of soil temperatures, particularly at the time of crop establishment, contributes to the better growth under these trees. Greater use of shade to reduce soil temperature to the benefit of crops is proposed.Vertical shade barriers were used to vary the soil surface temperature by varying the time that the soil was exposed to direct sunlight. Pearl millet (Pennisetum glaucum), sown in this range of environments, germinated and grew best in a partially shaded environment, but failed in fully exposed conditions; growth was inversely related to mean soil surface temperatures during seedling establishment. In a separate, concurrent study, at the beginning of the Sahelian rainy season, a nearly leafless tree canopy intercepted about half of the incoming radiation, which resulted in a decrease of up to 10°C in the maximum soil temperature at 2 cm depth (depending on position relative to the tree and time relative to rain) during the seedling establishment phase.
Variability in the growth of Faidherbia albida (Del.) A. Chev. (syn. Acacia albida [Del.]) in the Sahelian zone of West Africa is a common observation. The "albida effect" has been attributed to the amelioration of soil chemical, physical, and microbiological properties by the direct action of the tree, resulting in enhanced crop production beneath the mature canopy. Crop yields within fields without F. albida can also be highly variable within short distances. A study was conducted to observe the association between the variability in soil properties and the variable growth of young (2-yr-old) F. albida trees planted at the ICRISAT Sahelian Center located near Niamey, Niger. The results of this study indicate that the variability in F. albida growth is caused, in large part, by variability in soil properties across relatively short distances. Good growth sites had higher clay contents and base saturations, and lower exchangeable acidity than poor growth sites within the field. The tallest trees in the field were associated with the proximity to abandoned termite mounds and other microhigh sites. Seedlings planted on sheet-eroded sites caused by runoff from microtopographical high sites within the field grew poorly. The variability in F. albida growth in plantations within Niger is hypothesized to be due, in part, to preexisting soil conditions. These results also suggest that the albida effect might be partially caused by these preexisting "islands of fertility". This site-determined variable growth ofF. albida could be exploited with proper seedling placement strategies.
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