Agriculture Reference
In-Depth Information
appearance and grade. Mangosteen trees may also require protection from full
sun during early establishment as well as from wind. Windbreaks for these
crops are standard practice (see Chapter 3).
Soils
Using the US classifi cation system, soils are separated into ten groups, based
on parent material, soil age and the climatic and vegetative regime during
formation. Tropical soils are diverse, having formed from dif erent parent
material and climatic conditions (Fig. 2.5). These soils have formed in areas
where the soil temperature at 50 cm dif ers less than 5°C between the warm
and cool season. The parent rock materials are as dif erent in the temperate
zone as in tropics; erosion and deposition are similar; soil formation can have
been from recent volcanic or alluvial fl ood plains to 1 million years old. The
dif erences in temperate regions lie in soil-forming factors such as glaciation
and movement of loess, which have not occurred in the tropics (Sanchez and
Buol, 1975).
The majority of tropical soils are found in US soil orders oxisols, aridisols,
alfi sols, ultisols and vertisols and are spread widely throughout the tropics (Fig.
2.5). The soil orders are separated on the presence or absence of diagnostic
horizons or features that indicate the degree and kind of the dominant soil-
forming process. It is very dii cult to make generalizations about tropical soils
other than they have less silt than temperate soils and that surface erosion
and deposition have been more signifi cant. There are greater volcanic deposits
in the tropics and a larger proportion of younger soils than in the temperate
region. Only a small proportion (2-15%) of the tropics has so-called lateritic
soils (oxisols and ultisols), defi ned as soils that have high sesquioxide content
and harden on exposure (Table 2.4). The red colour of tropical soils does not
mean that they have low organic matter. For example, the average organic
carbon content in the top 1 m of a black North American mollisol is 1.11%,
while red, highly weathered tropical oxisols may have 1.05%, the reddish
temperate ultisols 0.4% and tropical ultisols 0.66%.
The more intensively farmed, fertile soils of the tropics cover about 18% of
the area and are alfi sols, vertisols, mollisols, and some entisols and inceptisols
(Table 2.4). These soils generally developed from alluvium and sediment and
are high in calcium, magnesium and potassium (Table 2.5). This gives them
a high base status with no acidity problem. Phosphorus defi ciency can be
readily corrected. Larger groups of tropical soils (oxisols, ultisols and others)
are of low base status, highly leached and cover 51% of the tropics (Table
2.4). Phosphorus defi ciency can be signifi cant as it is fi xed by the iron and
aluminium oxide in these soils, which also often have aluminium toxicity
problems, with sulfur and micronutrient defi ciencies (Zn, B, Mo, S). However,
they have good physical properties. The high-base soils (aridisols) in tropical
