Soil management

Climate change impact

Soil nutrient depletion has been one of the leading reasons for the environmental damage caused by the expansion of cocoa plantations into virgin forest lands. These forests provide the cocoa plant with very fertile soil, low weed pressure and a favorable microclimate. CSC is focused on increasing the organic matter content of the soil and improving its water holding capacity thereby increasing the resiliency of the farm to droughts and strong winds. Improving soil structure, nutrient content, and water capacity increases adaptation to environmental risks. Adequate soils facilitate replanting of cocoa and reduce farmers motivation to abandon their cocoa farm and encroach on forest land.

Description of practices

Soil management starts at establishment, conversion of forested land should occur through slash and mulch techniques instead of slash and burn. Mulching, has been found to be beneficial for young cocoa trees, increasing root and shoot size (Carr and Lockwood, 2011). Mulch species should be grown alongside cocoa to reduce the costs of this practice (Carr and Lockwood, 2011). Furthermore, systematic weeding and cutting of cover crops reduces nutrient competition (Akrofi-Atitianti, et al. 2018). The remains of pruning and husks of cocoa can be transformed into compost tea which improves soil nutrient content and facilitates the distribution of nutrients across the farm. Soil nutrient analyses improve inputs selection and management methods. When using fertilizer, yields may not increase if the level of shading is too high or if the plantation is poorly maintained (Schroth, et al. 2016). Shade trees increase soil water retention, by decreasing soil temperatures and evapotranspiration (Schroth, et al. 2016), and prevent nutrient leaching caused by prolonged and intensive rainfall. Drainage systems and protective layers of ground cover reduce soil erosion during floods and heavy rainfall (Dohmen, et al. 2017).

State of the art

Converted tropical forests, sloping, young, and full-sun farms are very susceptible to getting washed out of nutrients (Hartemink, 2005). Full-sun cocoa production systems rely heavily on chemical fertilizers to raise yields. Furthermore, carbon breakdown caused by land clearing is assumed to be the leading cause of soil organic matter loss, erosion, acidification, and mineralization (Tondoh, et al. 2015). The quality of the soil in tropical ecosystems is strongly linked to the plant residue inputs and litter residence times which protect soil and provide food for soil organisms (Tondoh et al 2015). Shade trees protect the soil in cases of extreme rainfall, by reducing raindrop impact on soil and runoff when their litter is left on the plot. However, the results of a field trial in Bolivia showed no difference in field moistures between different production systems (Alfaro, et al. 2015). The water-holding capacity of the soil depends on its composition. In particular, the organic-matter content of the soil will determine this capacity (Smith, 1964).

Hartemink, 2015

Outlook

Practices that improve soil adequacy for replanting and increase yields have a distinct potential in mitigation (less deforestation) and productivity (higher yields). Medina and Laliberte (2017) note that there is still significant uncertainty in terms of adequate levels of mineral nutrients for better cocoa production. The relationship between available potassium levels and drought tolerance in cacao is promising but requires further study. More research is required into which species are most adequate for use in mulching in different regions and into the nutrient competition of companion tree species. The availability of nutrient-specific fertilizers could reduce costs for farmers and improve yield outcomes in the future.

Productivity: Soil fertility declines are a leading factor of decreasing cocoa yields in mature cocoa farms (Vaast and Somarriba, 2014). The employment of chemical fertilizer is often not an option for cocoa smallholders, either because of lack of access or its high cost. Mulching, compost teas, and other soil management practices may help increase cocoa farm productivity.

Adaptation: Extreme climatic events and less favourable weather are expected to lead to soil erosion, loss of soil fertility, and water competition. Increasing soil organic content and the water holding capacity is the main objective of CSC adaptation with respect to the characteristics of cocoa farm soils. 

Mitigation: By maintaining and improving the characteristics of the soil for cocoa production across its life-cycle the costs of replanting on the same plot are reduced. If successful replanting can be performed, encroachment into native forestland will be less likely and the carbon liberation of land transformation avoided. Some operations, like organic mulching, can also mitigate greenhouse gasses by generating carbon sinks.

The effect of land conversion on nutrient composition and the properties of the soil is unclear and may be dependent on localized factors and the original characteristics of the land (for a list of contradicting studies see Dawoe, et al. 2014). Moreover, the quantification of nutrient loss and gain through litter is difficult, there are widely differing estimates. With increasing fertilizer costs (Jezeer, et al 2017) the CSC practices of agroforestry, pruning, and mulching can be cost-efficient alternatives for increasing soil organic matter. Agroforestry systems have superior water retention and water quality than monoculture systems (Jacobi, et al. 2014). Shade trees also reduce soil erosion caused by raindrop impact and create root channels to minimize the negative effects of heavy rainfall. Lastly, cocoa husks left on plantation to decompose increase the number of natural enemies and pollinators (Forbes, et al. 2017).