Grenada’s soils are derived from the igneous rocks created during the island’s violent volcanic origin. Apart from raised beaches and reefs exposed in the north of the island, there are some sedimentary limestone rocks in isolated areas.
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).
CHART SHOWING THE PERCENTAGES OF SAND, CLAY AND SILT IN THE BASIC TEXTURAL CLASSES.
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 Grenadas 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.
The dark grey to black Belmont clay loam, derived from volcanic ash and agglomerate rocks, is well-drained. It tends to be alkaline, and is found to the north of the island. It is considered the island’s most fertile soil.
The Capitol and Belmont clay loams classes dominate the distillery region. A Capitol subsection of Palmiste clay loam is a yellowish-brown clay loam derived from tuffaceous shales. It is moderately well-drained and acidic, with a high natural fertility. The reddish-brown Concord clay loam, derived from volcanic ash and agglomerate, is moderately well-drained. The Belmont subgroup of La Tante and Plains clay loams, however, are alluvial soils with highly localised distribution. The Belmont subgroup of La Tante and Plains clay loams, however, are alluvial soils with highly localised distribution.
- Hardy, J. A MCDonald and G. Rodriguez. 1932. Studies in West Indian soils.
- L. Ternan, A. G. Williams and C. Francis. 1989. Mountain Research and Development
Of the clays, the greyish-brown Perseverance clay is the most common (8%). It’s derived from volcanic ash and agglomerate rocks mostly found near the coast. It’s heavy, poorly drained and difficult to work. The brownish-black Hartman clays of colluvium/alluvium origin, are mainly found in the southwest of the island on gentle slopes. Similarly, the brownish-black Hope clay is also of colluvium/alluvium origin and is poorly drained.
Sandy loams are the most desirable soils for agriculture but are few and far between. Plains sandy loam is recognised as one of the best island soils, made up of alluvium/colluvium material, naturally fertile, light and well drained. The Bonair sandy loam is also made up of alluvium/colluvium material, and usually occurs at river mouths where, subject to periodic flooding, it tends to be heavily stony, and thus quick to drain.