Published on International Journal of Agriculture & Agribusiness
Publication Date: April 27, 2019
A. T. Kugedera
Department of Livestock, Wildlife and Fisheries, Great Zimbabwe University
Journal Full Text PDF: The Use of Agroforestry Practices in Soil Conservation.
The objective was to review the use of agroforestry practices in soil conservation. The paper highlights how different agroforestry practices can be used to conserve soil and discusses the major concern of each and every practice used by farmers. The paper seeks to come up with the best agroforestry practices to conserve soil in both smallholder and commercial farming areas. Secondary data was used to come up with a full review of how different agroforestry practices can be used to conserve soil. The most sustainable agroforestry practice to conserve soil for smallholder farmers was indicated.
Keywords: Agroforestry, practices, soil & conservation.
Agroforestry has been proposed as a sustainable land use that can mitigate soil erosion and promote the economic welfare of small farmers. Soil conservation index has been developed to cater for soil disturbances. Agroforestry practices enhance soil conservation through conserving moisture and fertility. Soil conservation is largely equals to maintenance of soil fertility which requires control of erosion, maintenance of organic matter, soil physical properties, nutrients and avoidance of toxicities (Young, 1989).
2. AGROFORESTRY PRACTICES USED TO CONSERVE SOIL
Agroforestry practices conserves soil through fertility enhancement. Practices such as improved fallow where agroforestry species such as Sesbania sesban, Gliricidia sepium, Cajanus cajan and Tephrosia vogelii which improves nitrogen availability in the soil through nitrogen fixation. These species fixes nitrogen and makes it available in the soil thus improving soil fertility (Nyamadzawo et al., 2004). These agroforestry species are deep rooted and are able to mine nutrients from underground that is they practice nutrient mining making these nutrients available in the soil (Chikowo, 2004; Nyamadzawo et al., 2004). The presence of these agroforestry species also reduces effects of wind, extremely hot temperatures during dry season and effects of intensive rains at the start of new rainy season and improves cover on soil leading to reduced effects of soil erosion (Mafongoya et al., 2005). Shrubs such as Sesbania sesban are able to recycle plant nutrients from lower soil strata and build up the soil organic matter (Lal, 2001). Improved fallow improve soil physical properties, such as better infiltration and aggregate soil stability, which reduce soil erosion and enhance the ability of the soil to store water (Kwesiga et al., 2003; Nyamadzawo et al., 2004). The incorporation of tree leaves and twigs as biomass in the soil improves organic matter input in the soil and act as mulch. This improves infiltration rates thereby reducing surface runoff thus controlling soil erosion (Mafongoya et al., 2005). Biomass transfer also improves soil structure and texture because macro-organisms such as termites which decompose the twigs and leaves creates airspaces within the soil hence improves soil porosity leading to improves infiltration and conserves moisture (Kang et al., 1991).
2.1 Traditional Agroforestry
Traditional agroforestry relied most on indigenous tree species such as Brachystegia spiciformis where farmers collect biomass from these trees and incorporate in their fields. In Masvingo province where most areas are associated with Miombo woodlands they use biomass from either Brachystegia spiciformis or Julbernadia globiflora in their fields and homegardens to improve soil texture and nutrient content. In West African, for example they used Faidherbia albida in semi-arid and sub-humid regions as trees in improved fallows. Faidhebia albida is a leguminous tree which fixes nitrogen and practices nutrient mining. This improves soil fertility by recycling back lost nutrients to the soil through foliage. Leaves of Faidherbia albida are easily decomposed and if used in farm lands and home gardens improve soil structure and texture.
2.2 Alley cropping
Alley cropping (Hedgerow inter-cropping) improves the physical nature of the soil environment (Lal, 1989a; 1989b). The addition of mulch from agroforestry species such Sesbania sesban and Calliandra calothyrsus used in hedgerow inter-cropping lowers soil temperature, reduce evaporation and improve soil fauna activity, soil structure resulting in better infiltration, reduce runoff and improves water use efficiency (Lal and Greenland, 1986). On sloping land, the tree rows act as a physical barrier to soil and water movement, resulting in significant reductions in erosion losses. An important benefit of hedgerow inter-cropping in soil conservation is the addition of large amounts of organic material from pruning as mulch or green manure. The leaves and twigs from agroforestry species used are rich in nitrogen and this improves nitrogen content in the soil (Kang et al., 1985; Kang and Ghuman, 1991). Organic matter added improves soil structure and texture. The inclusion of Thitonia diversifolia in hedgerow inter-cropping improves phosphorous levels in the soil. Prunings have favourable effects on soil physical and chemical properties, on microbiological activity and hence on soil productivity (Kang and Ghuman, 1991).
Hedgerow inter-cropping have significant positive effects on soil fertility parameters such as organic Carbon levels, total Nitrogen and extractable Phosphorous levels over a range of climatic and soil conditions (Gutteridge, 1990). Hedgerow inter-cropping is one of the most widely agroforestry technique used in Haiti with contour hedgerows of leguminous nitrogen-fixing shrub species such as Leucaena leucocephala (Isaac et al., 2000). Shrubs species are periodically coppiced and prunings are scattered as mulch. The higher levels of organic matter help to slow erosion due to runoff by increasing soil porosity and infiltration as well as enhance soil fertility. Mulch cover also maintains soil moisture during dry periods (Isaac et al., 2000, 2003).
The traditional of planting Leucaena leucocephala in contour hedges for erosion control and soil improvement in Southeast Asia, especially Indonesia has shown high results for soil conservation. Lopping and prunings from such hedgerow species could also provide mulch which aid in preventing sheet erosion between trees (Li et al., 2001). The presence of plant cover act as mulch also reduces the impact of rain drops in the soil and thus minimizes splash and sheet erosion (Chen et al., 2003). The use of Tithonia diversifolia, a roadside shrub may reduce the problem of phosphorous deficiency in soils (Togo, 2001). Most African soils are phosphorous deficiency. The use Tithonia diversifolia biomass with phosphate rocks showed significant increases in phosphorous levels in the soil. Tithonia diversifolia is widely used in Northern and Western Africa where some farmers grow it as alley cropping and some use its biomass as biomass transfer. In Tanzania it was shown that Tithonia diversifolia can be used as a sole agroforestry species to increase phosphorous levels (Togo, 2001).
Trees used in agroforestry practices might have access to soil phosphorous (P) from unsuitable pools inaccessible or exploited by crops (Hands et al., 1995). Many trees in the tropics used as windbreaks and shelterbelts such as Jacaranda mimosifolia forms associations with ectomycorrhizal (ECM) or vesicular-arbuscular mycorrhizal (VAM) fungi (Chen et al., 2003). This association may enhance P uptake by the tree through the extensive proliferation of mycorrhizal hyphae, which results in the effective exploration of large soil volume. Mycorrhizal fungi can also contribute to the synthesis and production of phosphatase enzyme and organic acids that ameliorate and upgrade availability of soil P (Handreck, 1997). The growing of woody perennials in agroforestry systems for either fodder or windbreaks plays an important protective role in soil. Woody perennials such as Acacia anguistissima, A. nilotica and A. polyacantha improve and conserve soil functions (Nair, 1984). These Acacia species are leguminous and fixes nitrogen from atmosphere into the soil, thus improving N content in the soil. Addition of organic matter through litter fall and dead and decaying roots modifies soil porosity and increase infiltrations rates leading to reduced erodibility of soil and improving the efficiency of nutrient cycling within the soil-plant system (Nair, 1984). The role of woody perennials in agroforestry as live fences, shelterbelts and windbreaks prevents soil from adverse effects of wind. These agroforestry practices also promote growth of annuals which reduces surface runoff and increase infiltration rates (Isaac et al., 2003). Leaf falls from windbreaks and shelterbelts acts as mulch which improves soil structure due to increased microbial activity. Decomposition of leaf fall from trees such as Tephrosia candida, Cajanus cajan and Acacia senegal increases soil porosity (Chen et al., 2003).
Agroforestry practices have long term productivity in soil conservation if proper incorporation of woody species is done well in land use areas (Nair, 1984). The evaluation of agroforestry practices such as Taungya, plantation forestry, integrated systems involving plantation crops and multiple purpose trees showed positive effects in conserving soil. The productivity and protection of soil could be anticipated by proper incorporation of appropriate woody species in land use systems (Nair, 2009). The conservation of soil water by Acacias is more useful during drought years as compared to normal and above normal years of rainfall in the Blue Nite region conditions (Bukhari, 1998). Acacia species such as Acacia senegal conserves soil moisture before sowing of annuals and after harvesting of annuals so that soil structure and texture cannot be damaged. Deep rooted woody perennials such Acacia polyacantha can add to the utilization of available soil water (Li et al., 2001).
Incorporation of woody perennials such as Acacia senegal into farming systems can increase the overall biomass productivity (Ong et al., 2000; Livesley et al., 2004) and reduce evaporation rates, thus improving availability of soil water. The presence of Acacia senegal enhances ecosystem productivity (Gaafar et al., 2005). The concepts and practices of soil amelioration by use of agroforestry practices have been extensively reviewed by several authors (Lal, 2001). Numerous studies have reported higher soil organic material (SOM) content in topsoil under trees than in open areas (Lal, 2002). There is higher soil organic carbon under tree than in open grassland in a humid savanna in Ivory Coast (Lal, 2002). The use of Eucalyptus species and Pinus caribaea Moerlet in plantations on sandy soils in Congo showed an improvement in SOM. The use of Acacia senegal in plantations on sandy soils Sudan showed a restore of soil fertility as well as in traditional fallow systems (Gaafar et al., 2005).
Trees having deep roots can potentially intercept and prevent nutrients from leaching down the soil profile and capture nutrients accumulated in the subsoil below the rooting depth of annual crops (Gaafar et al., 2005). Nutrients taken up by trees from below the rooting zone of annual crops will later return as an input when transferred to surface soil through leaf litter, root and pruning of tree leaves and branches (Mapa and Gunasena, 1995). Trees in agrisilvicture restores soil fertility through decomposition of leaf litter and also enhance the action of soil fauna which is essential for SOM and plant residue decomposition (Buresh and Tian, 1998). Fertiliser tree fallow is an agroforestry practice used in Zambia to improve soil organic matter and nitrogen. This practice also conserve soil many ways since agroforestry species used are also able to reduce toxic substances in the soil through neutralisation with allelopathic chemicals they produce (Ajayi et al., 2005). Fertiliser trees are fast growing tree species that accumulate N in leaf and root biomass and recycle it into the soil (Mafongoya et al., 2005). Sesbania sesban was initially used for this practice to improve soil fertility in Zambia. They produce easily decomposable biomass, are compatible with cereal crops in rotation and are adapted to the climatic and soil conditions of the Miombo woodlands (Kwesiga and Coe, 1994).
2.3 Fertiliser tree system
Fertiliser trees act as a break crop by smothering weeds and improving the soil’s physical and chemical properties (Nyamadzawo et al., 2004). The technology is based in the fact that although nitrogen is scarce in the soil it is very abundant in the atmosphere (Ajayi et al., 2005). Fertiliser trees transform nitrogen from the atmosphere and store it in the roots, the branches and the leaves. Decomposition of leaves of fertilizer trees such as Tephrosia candida, Calliandra calothyrsus and Cajanus cajan improves soil nitrogen thus conserving soil nitrogen content (Ajayi et al., 2005). Fertiliser tree technology improves soil infiltration and reduced runoff. It also maintains greater soil moisture during dry periods (Ajayi et al., 2005). Fertiliser tree fallows improves soil physically, chemically and microbiologically. Soil physical properties can be improved by adding large quantities of litter fall, root biomass, root activity, biological activity and root macropores (Rao et al., 1998). Plots that had fertiliser tree fallows were found to have improved soil particle size, improved infiltration and water retention capacity and reduced soil runoff (Chirwa et al., 2004). The higher number of larger soil aggregates means that there is less nutrient seepage deep into the soil, that there is greater potential for organic matter and carbon build-up and that there is less erosion potential (Rao et al., 1998; Chirwa et al., 2004). Fertiliser tree fallow also sequester soil carbon improving C: N ratio (Chirwa et al., 2004).
The other major avenue of soil improvement with agroforestry is through soil conservation. Land degradation caused by erosion, salinity, and fertility depletion, and advancing deserts which is a looming danger facing humanity (Nair, 2007; Kumar and Nair, 2007). The potential of agroforestry to reduce the hazards of erosion and desertification as well as to rehabilitate such degraded land and conserve soil and water has been well recognized (Simons and Leakey, 2004; Nair, 2007). Grass–shrub–tree buffers (agroforestry design) are superior to grass buffers in reducing sediment losses in the temperate zone. Direct or supplementary use of trees and shrubs such as Sesbania sesban to control soil erosion is now a widespread agroforestry practice in both tropical and temperate regions (Buresh and Tian, 1998).