This refers to the ability of the soil to supply essential plant nutrients and soil water in adequate amounts and proportions for plant growth and reproduction in the absence of toxic substances which may inhibit plant growth.
Soils are composed of five main components:
Soils differ because they have different proportions of these components and because the mineral particles have been affected to different degrees by weathering. Age of soil minerals, prevailing temperatures, rainfall, leaching and soil chemistry are the main factors which determine how much a particular soil will weather. Vanuatu soils, because they are geologically young, are often less weathered than soils of neighbouring Pacific countries.
The major and micro or trace elements are made available to plants by breakdown of the mineral and organic matter in the soil. Availability of these nutrients depends on how much is present, the form in which it is present in the soil, the rate at which it is released from organic matter or mineral particles and the soil pH i.e. its acidity or alkalinity.
The proportion of nutrients held on the clay and humus particles influence deficiencies e.g. potassium(K), calcium(Ca) and magnesium(Mg) are held on the surface of clay particles and are directly taken up by plant roots or from the soil solution. An excess of K can create a deficiency of Ca or the reverse can occur. Acid soils high in Manganese(Mn) often cannot supply enough Cobalt(Co) for rhizobium bacteria with a consequent effect on nitrogen(N) fixation by legumes. Also on very acid soils Manganese and Iron(Fe) make phosphorus(P) unavailable to plants by 'fixing' it in insoluble complexes. The chemical relationships influencing soil fertility are complex and affected by the parent material from which soil develops, the type of clay present, the proportions of different sized particles, e.g. sand, silt, clay, which also have important effects on soil structure. Detailed discussion of this topic is outside the scope of this Bulletin.
Vanuatu is fortunate in having large areas of relatively fertile soils. However in areas where grazed pastures are found, the VPIP has identified areas where deficiencies of potassium (K) occur, especially on coralline soils and deficiencies of copper (Cu) and possibly other trace elements on areas of Efate and Santo. Soil phosphorus (P) levels also vary between different soils and are too low for good legume growth on Erromango, North Tanna, Montmartre and environs on Efate, interior areas of the Santo plateau and Middle Bush, Santo. In the Pacific low soil P is usually the main soil fertility factor contributing to legume loss in a pasture.
However, pasture species also respond in their adaptation to different levels of soil fertility. For example green panic, guinea grass, glycine and siratro provide maximum production on the most fertile soils, whereas signal grass, koronivia and the legumes, sensitive (Mimosa pudica), hetero, seca and Cook stylo grow well on lower fertility soils. Koronivia grass in fact tolerates low fertility best of all the native and introduced grasses. The adaptation of a range of pasture species evaluated by the VPIP in Vanuatu to different environments is presented in tabular form at the end of the Bulletin.
Grazed pastures can have a substantial influence in improving soil fertility by increasing the organic matter content and nutrients available to plants through nutrient cycling. The evidence for the beneficial aspects of grazed pastures under coconuts is discussed later. For any farming system to be sustainable it should maintain soil organic matter levels at least 3%.
Generally permanent improved grass/legume pastures on soils without fertility limitations have been associated with increased soil organic matter levels and increased soil micro and macro fauna activity (compared with other agricultural activities) and this usually improves soil structure which leads to improved soil moisture and aeration relationships.
Twenty year old improved pastures in Vanuatu have surface soil bulk densities lg/cc higher than what exists in adjacent, undisturbed regrowth bush. However on certain types of heavy clay soils under high stocking rates soil compaction can be considerably higher than what prevailed in the initial bush situation. In Vanuatu most soils are derived completely or in part from coralline or ash parent material and are therefore very permeable and there is no evidence to suggest the rainfall, run-off and groundwater recharge characteristics from pasture lands are any different from undisturbed bush with the same soil type, length and degree of slope.