The igneous rocks are classified into: lavas (olivine basalt, augite andesite and hornblende andesite) from the molten magma rivers pouring from the craters; and pyroclastic rocks (agglomerates, ashes and tuffs) explosively ejected from volcanos as solid particles of varying sizes from fine dust or rounded pebbles, to the large boulders that can be seen around the island.

These lavas and pyroclastic rocks were worn down into the soil parent materials by the humid, tropical climate. Significant erosion and weathering varies according to the rock type and the intensity of rainfall, and that is defined by the island’s geography.

The two types of soil parent materials provide a broad base for soil classification. Lava-derived soils are mostly red (with some grey clay) and are usually found in the central mountainous areas. Pyroclastic-derived soils vary from dark grey to black and are usually found on the coastal belt. The valley floor soils are darker in colour, deposited in river valleys (alluvial) and accumulated at the foot of steep slopes (colluvial).

Soils can be further classified based on particle size or texture. Clay is the smallest in diameter (< 0.002 mm); then silt (0.002 – 0.005 mm); followed by sand as the largest (0.005 – 2.0 mm).

A clay soil is one that is over 50% clay, with its associated poor drainage rendering it difficult to work for crops. But clay is an important constituent of a good soil because of its ability both to deliver important ions and nutrients to plant roots, and for its minerals to buffer soil acidity and prevent excessive acidity, which hinders plant growth. A loam soil is ideal, as it contains a balance of particle sizes – 20% clay, 40% sand and 40% silt.

About 85% of Grenada’s soils are classified as clay loams. With moderate water retention capabilities and high fertility, they are ideal for our agricultural purposes. Heavy, hard-to-work clays account for just 12%, while the light, sandy loams – ideal for crops under irrigation – a mere 3%.

There are three major types of clay loams – Woburn, Capitol and Belmont – that account for three quarters of the island’s soils. Dark-brown to grey Woburn clay loam, derived from volcanic ash and agglomerate rocks, tends to be neutral with high erodibility resulting in shallow depths and low moisture retention.

The brick-red Capitol clay loam, derived from lavas, tends to be acidic . It is usually found on steep slopes over deeply weathered, igneous rocks predominantly to the south of the island.