**6. Conclusions**

biological N2 fixation by cowpeas replenished the available N to both crops and also for

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].

**Phosphorous ( kg ha–1)**

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,

**Plant-available water (%)**

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

subsequence crop [57].

300 Organic Fertilizers - From Basic Concepts to Applied Outcomes

**Erosion level Organic matter**

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

**(%)**

Slight 3.0 67 7.4 Moderate 2.5 66 6.2 Severe 1.9 43 3.6 Agriculture systems have evolved over long periods as a consequence of modifications of climate, agricultural technology, and socioeconomic conditions [64]. In recent decades, cropping systems, both in developed and developing countries, have become increasingly simplified with markedly reduced diversity in vegetation patterns over time and across the landscape. Concomitantly, a large increase has occurred in the use of synthetic fertilizers and pesticides [65]. Consequently, agriculture is suffering stress from environmental issues, such as nitrate leaching to groundwater, nitric oxide release to the air, and CO2 from the fertilizer industry being released into the atmosphere [66]. Thus, the establishment of an adequate crop system that is economically acceptable, environmentally sustainable, and technically practi‐ cable is the task of agronomists and farmers.

Organic agriculture is regarded as a sustainable agricultural system, taking into considera‐ tion soil fertility conservation, which covers soil physical properties, plant nutrient availabil‐ ity, and erosion control, as its key issues [67]. In organic agricultural practices, biological N fixation has received increased attention from agricultural agronomists and producers. Al‐ though the economic value of biological N fixation by legumes varies widely, when the cost of production of the legumes is taken into consideration, opportunities still remain to plant legumes as a short-term rotation crop or an intercrop or living mulch [68]. Seeds of grain legumes can be used as a fast nitrogen available fertilizer in organic production at low tem‐ peratures in early spring [26]. Intercropping management has long been practiced and has played a crucial role in sustainable agriculture. One of the main benefits of an intercropping system is the rational utilization of natural resources. The management of intercropping is primarily according to natural laws within an agroecosystem. The regulation that major crops cannot cover the entire soil surface during the whole growing season provides chan‐ ces for other plants growing as mixture crops, temporarily or spatially. Although competi‐ tion between intercrops is always observed, a well-managed polyculture system can provide a higher total production than all of the crops planted as pure stands. Crop diversity, in both time and space, appears to be a critical element of sustainable agroecosystems that require few external inputs. Green manure, a major measure for the self-sufficient maintenance of soil fertility, offers organic matter to soil and fixing N2 into organic form in the case of le‐ gumes.
