1: Less soil moisture on hill, 2: increased soil moisture in hollow valley
The soils of different microclimates vary according to their slope exposure, aspect, and shade cover. In general, shaded slopes facing away from the sun will be more moist and more acidic. The diagram above shows the tendency in hollow valleys for excessive water to collect and a tendency for these areas to have more acidic soils.
The organic carbon levels and the microbial activity have a direct bearing on the pH of the soil as well as the water content. The microbes decompose organic matter in the soil and leaf litter. In dry soils the microbes are not active. Microbial activity increases as temperature increases, when there is plenty of organic carbon available as a food source and when the pH of soil is closer to neutral (pH 7).
Microbial activity is important because microbes release nitrogen and other nutrients for plants to absorb. The availability of nutrients depends on their solubility. In an acidic environment, the solubility of essential nutrients such as phosphates of iron, manganese, and aluminium, is reduced (Pavan and Chaves, 1996 & Edison Martins PauloI; Enes Furlani Jr. 2010).
A study conducted in Brazil by Krystal de Alcantara Notaro et al, (2014) found that the pH of the natural forest soil was more acidic than both plantation and agroforestry systems used for coffee farming. This is most likely due to the absence of any pH correction materials, such as gypsum, in the forest. Coffee plants prefer an acidic soil between 5 and 6 pH (FAO). Where the pH drops below 5, key nutrients such as nitrogen become less available to the plant.
Aerobic vs. Anaerobic Soils
The reason plants such as coffee require soil with good drainage, without waterlogging, is that the soil needs to breathe. Such soil is described as aerobic. ‘Unfavourable anaerobic conditions occur in waterlogged subsoils that leads to a build up toxic hydrogen sulphide and reduced iron and manganese.