**5. Soil and water conservation by cover cropping**

Soil conservation is an important issue in sustainable management, especially on hillsides. Cover crops or living mulch provide important benefits in soil and water conservation. The primary function of alley cropping on sloping lands is erosion control and soil conservation [51,54] . Two forms of soil erosion exist: sheet and rill erosion. Sheet flow is the removal of a relatively uniform thickness of soil and is usually caused by rain-splash, surface runoff, and wind. In rill erosion, water flows with soil particles in small channels [58]. Soil erosion decreases water availability, infiltration rates, water-holding capacity, nutrients, organic matter, and the depth of the soil. Soil erosion not only causes plant nutrient loss but also SOM loss. The latter affects field capacity and soil aggregation structure. Soil erosion has a negative effect on the productivity of soils (Table 4) [59]. The eroded soil typically contains about three times more nutrients than the soil left behind and 1.5–5 times more organic matter. The major costs to a farm associated with soil erosion come from the replacement of the lost nutrients and reduced water-holding ability, accounting for a productivity loss of 50-75% [60].


**Table 4.** Soil fertility effects of erosion [59].

Soil erosion is connected to water erosion. Water erosion increases the amount of runoff, so that less water can enter the soil matrix and become available to the crop. In severely degraded soils, water infiltration may be reduced by as much as 93%, and so water conservation is linked to soil conservation [61]. Increased SOM content can enhance field capacity and consequently reduce soil erosion. Another effect of reducing soil moisture losses is that the soil cover reduces evaporation in fields. Vegetation acts as a buffer to the soil because rain-splash is an important detaching process in soil erosion. Raindrops striking bare soil have the ability to throw soil particles through the air over distances of several centimeters [62]. A vegetative cover also contributes to slope stability. In Nigeria, in land with a 14% slope and under total rainfall of 1412 mm during a 3-month study period, maize alley cropping with contour hedgerows of *Lucaena leucocephala* and *Gliricidia sepium* established at a 6-m interhedgerow spacing with prunings used as mulch effectively contained erosion by 85% and 73%, respectively [63]. The aboveground components of the plant, such as the leaves and stems, absorb some of the energy of falling raindrops, running water, and wind. The belowground components, the root system, contribute to the strength of the soil, holding soil particles in place. Living mulch can reduce soil and water erosion significantly, as the presence of the canopy slows down raindrops, reducing surface runoff and enhancing water filtration. A well-developed root system holds soil particles together, reducing soil erosion. Moreover, evapotranspiration of plants produces a drier soil environment due to the capable of withstanding a higher intensity and longer duration of rainfall compared with a slope that lacks vegetation [63].
