Soil fertility management

Trees improve soil fertility through recycling of nutrients from the deep soil horizons to the topsoil layers and by fixating atmospheric nitrogen (N). The recycling of nutrients by trees takes place either through capture of nutrients from the deep soil horizons or interception of nutrient leaching beyond the crop rooting zone by tree roots. These nutrients are then released to the topsoil horizons through litter and root turnover. These processes play an important role in the recycling of nutrients in agroforestry sys - tems [167] [167-169] and are critical for the ecological sustainability of improved fallow, woodlot and other agroforestry practices common in dryland areas. The recycling of phosphorus (P) by trees is usually limited by high P-fixation in acid soil, low mobility in the soil, and low foliar P concentration [168]. Consequently, external P input is necessary to sustain crop yield on P-deficient soils because the amount recycled by short-rotation fallows of 1-3 years is often lower than amounts extracted via crop harvest [170]. However, in areas with sub-soil P reserves, trees raise topsoil N and P to levels sufficient for maize production [171, 172]. Trees also accumulate other nutrients from the soil and may alleviate nutrient deficiencies, especially potassium (K) that can arise when sufficient levels of N and P are supplied [148]. Agroforestry transfers nitrogen from N-fixing trees to nearby crops. The transfer takes on the surface and below-ground through decomposition of pruning or litter, root and nodule turnover, roots exudates via mycorrhizal connections and via the buildup of soil organic matter [168, 170]. The amount of N fixed varies widely among species but for fast growing tree species like Sesbania sesban (L.) Merrill., Cajanuscajan L. Mill sp., Gliricidia sepium (Jaqua) and Tephrosia vogelii Hook. f., fixation can accumulate about 100–200 kg of N per hectare per year [148]. These amounts are substantial and can replenish soil N to levels sufficient to grow up to three subsequent maize crops on N- deficient sites [171].

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