Amelioration of a highly degraded tropical alfisol by planting 1. Changes in soil physical and chemical properties

This paper examines soil amelioration by planting 15 leguminous and graminaceous plant species, including herbaceous annuals, perennials and biennials, and woody perennials and biennials. Disturbed and undisturbed natural regrowth were planted with leguminous species, in some cases with fertilizer applied at planting (400 kg ha- of 15: 15: 15 NPK). The studies were made on two highly degraded sites in southwestern Nigeria which had been subjected to intensive mechanized cropping for a period of 10 years. Changes in soil physical and chemical properties were monitored from 1989 to 1991. Acacia dificilis, Brachiaria lata and Mucuna utilis had the lowest survival rates by the following growing season. Soil fertility and compaction levels differed between sites. Planting had no effect on the latter. The decreases in compaction (i.e. macroporosity) between 0.00 and 0.10 m depth at both sites one month before and five and 17 months after planting were 43, 59 and 61 per cent, respectively were attributed to exclusion of heavy machinery from the sites. Large decreases in fertility occurred at both sites and were attributed to a combination of nutrient extraction and to leaching. Between fallow species, exchangeable Ca, pH and the cation-exchange capacity (CEC) were greater and total acids lower for herbaceous cover compared with woody perennials, and was attributed to a higher Ca demand by the latter. Highest and lowest values of Ca, CEC and pH occurred in plots where plant material was returned to the soil (i.e. by cutting or die-back) and in cropped plots, respectively. Natural regrowth was as effective or better than planted species in improving soil physical and chemical properties. Therefore the use of exotic plant species for ameliorating highly degraded alfisols is unnecessary. Amelioration of highly degraded alfisols may be best effected by allowing natural regrowth t.0 occur while excluding all mechanized traffic from the site

Alleviation of soil constraints to crop growth in the upland alfisols and associated soil group of the West African Sudan savannah by tied ridges

The climate of the West African Sudan savannah (annual rainfall of 600–900 mm and a monomodal rainy season of 3–4 months) is characterized by frequent long- and short-term droughts. Crop growth in the Alfisols and associated soil groups is further constrained by soil compaction, low soil fertility, high soil temperatures, low soil water retention and available water holding capacity, and low water infiltration rate. Tied ridges, ridges with earthen bunds constructed at right angles to the self-same ridges at intervals of 1–4 m, can alleviate or circumvent the above constraints, and can conserve rainfall received on-site. Water runoff with tied ridging ranges from 0 to 15% of seasonal rainfall, whereas with either open ridging or flat planting 20–45% of seasonal rainfall is lost as runoff. Soil water content is, therefore, greater with tied ridging. Tied ridging also reduces surface bulk density, maintains soil fertility by reducing losses of soil nutrients in surface runoff and improves soil water retention and available water holding capacity. The water infiltration rate in furrows of tied ridged plots is lower than that with flat planting or open ridging. With tied ridging, however, rainwater is retained on site by the ‘ties’, whereas with open ridging or flat planting it is lost as runoff. Tied ridging increases depth of rooting and subsoil root density in maize (Zea mays), millet (Pennisetum americanum) and cotton (Gossypium hirsutum) in both wet and dry years, and in cowpea (Vigna unguiculata) in dry years. Root growth of bambara groundnut (Vigna subterranea) is unaffected by tied ridging. Cowpea subsoil root growth is not significantly affected by tied ridging in wet years although root proliferation occurs in the topsoil because of the high sensitivity of cowpea to transient waterlogging. Vegetative growth and dry matter production of maize, millet, cotton, bambara groundnut, cowpea, groundnut (Arachis hypogaea) and sorghum (Sorghum bicolor) are increased by tied ridging in both wet and dry years. Grain yields of maize, millet and sorghum are increased by tied ridging, as is lint production of cotton. Cowpea grain yield is increased only in dry years. Grain yields of bambara groundnut are not significantly affected by tied ridging. Yield responses of groundnut to tied ridges are variable. Growth and yield inhibition may occur in waterlogging-sensitive crops such as cowpea and cotton during wet years. In general, greatest yield and growth increases from tied ridging occur in drought-sensitive cereal crop species and cultivars. Furthermore, strong and positive responses to tied ridges may be obtained consistently over a long period of time only in upland environments where long- and short-term droughts are frequent, soil compaction and temperatures are high, and available water-holding capacity and water infiltration rates are low

Contributory factors to soil spatial variability in an ultisol 1. Burning vegetation residues in heaps during land clearing

The effect of burning vegetation residues in heaps following in situprimary burning (not in heaps) during land clearing on soil properties was studied in a Typic Kandiudult in Southem Cameroon. Burning in heaps resulted in very poor or absence of plant growth during the following season. In relation to sites where burning was limited to primary burning, bulk density, penetrometer resistance, mean weight diameter of soil aggregates, pH, Bray‐l‐P, exchangeable Ca, K, and Na, and effective CEC, and soil temperatures on warm, sunny days were greater, and organic C, total N and total acidity were lower in the topsoil of sites where burning took place in heaps. The more compacted nature of the soil in such sites predisposes them to localized accelerated runoff and erosion. In general, subsoil physical and chemical properties were unaffected by burning in heaps, except for exchaneable K and Na which were greater. The absence of or poor plant growth in sites where burning occurred in heaps was attributed primarily to an imbalance in soil micronutrient availability induced by the rapid increase in pH

Effect of hedgerow species in alley cropping systems on surface soil physical properties of an Oxic Paleustalf in southwestern Nigeria

Effects of hedgerow species on surface soil physical properties were studied in an on-going trial established since 1981 on an Oxic Paleustalf in southwestern Nigeria. The experimental treatments were Leucaena leucocephala, Gliricidia sepium, Alchornea cordifolia and Acioa barterii hedgerows planted at 4 m interhedgerow spacings and a control (no hedgerows). Plots were sequentia-ly cropped with maize (Zea mays) and cowpea (Vigna unguiculata). Soil physical properties, i.e. particle size distribution, bulk density, apparent pore size distribution and water infiltration, were monitored during the dry season in January and the main growing season in June 1989. Soil temperature was monitored at approximately monthly intervals from February to June 1989. Soil properties of alleycropped plots were superior to those of the control. Soil compaction was highest in the control and lowest with L. leucocephala. Between the hedgerow species, within-season increases in soil compaction were largest with A. cordifolia. Lowest soil temperatures were observed with L. leucocephala and A. barterii. Frequency, quantity and quality of prunings produced by the hedgerow species appeared to have a major beneficial effect on soil physical properties