Agriculture Reference
In-Depth Information
Soil evaluation
The soil on the selected site should be appropriate for the species you plan to
grow to improve the chances of success. Soil preparation and management,
including liming to change the pH, provide some fl exibility with a soil that has
a good structure. If the soil is improperly treated it will not provide the best
conditions for plant growth and production will not be the ideal.
The results from the physical analysis of the soil will determine its
texture and structure, water retention and drainage. These characteristics, if
unfavourable, can be very serious production constraints and are dii cult and
expensive to modify. Most soils considered for tropical fruit crops have good
topsoil and subsoil structures. Since fruit trees often have deep root systems,
poor subsoil conditions, such as a perched water table, will not allow root
growth below it. Other subsoil problems, in addition to a high water table, are
associated with the existence of rocks or hard soil layers. The presence of a
heavy soil layer above a sandy layer or the opposite can lead to dii culties for
water drainage. An easy way to determine this is to dig three or four trenches
to 2-3 m scattered over the planned site to evaluate the soil profi le.
Soil chemical analysis will tell you the soil's nutrient content, pH, salinity
and alkalinity levels, organic matter content and the presence of elements that
could be toxic. Soils with undesirably low pH (<4.5) are frequent in the humid
tropics. This acidity is corrected by incorporating lime during land preparation
and often during the life cycle of the crop. Liming should raise the pH to
adequate levels of around 5.5-6.5. Some crops can do well in a wide range of
pH values while others need a narrower range. A frequent problem in tropical
soils is low calcium and phosphorus levels and high aluminium availability,
in addition to magnesium, zinc and boron defi ciencies. These defi ciencies
can be corrected by an appropriate fertilization programme and liming. In
arid tropical zones, soils with high pH and low phosphorus availability, with
defi ciencies in iron, copper, manganese and zinc due to the nutrient being in
a non-soluble, unavailable form, are common and most can be corrected by
fertilization. Soils with high salinity or alkalinity should be avoided since they
can be very costly to correct. The soil organic matter content should also be
adequate and be maintained or increased if necessary by the incorporation of
manure or green manure crops. Soil organic matter improves moisture and
nutrient retention, maintains soil structure and enhances biological life in the
root zone (see Chapter 2 for more information).
Irrigation water
The potential irrigation water's quality needs to be determined. The salinity
and sodium level in relation to calcium and magnesium (sodium adsorption
ratio or SAR) are evaluated, as these parameters can cause problems with
