Soil Management: Soil Water Management
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Water management practices include drainage and irrigation of the land, and soil moisture conservation practices. Adequate soil moisture is required for good growth and yield of plantation tree crops. Hence, good responses to irrigation have been reported (Hutcheon et al ., 1973; Kee and Chew 1993, Lim et al ., 1994). However, excessive water such as high water table can reduce crop productivity substantially (Lim et al ., 1994).
Soil moisture conservation measures or irrigation will be beneficial in the following areas :
- inadequate rainfall of less than 1700 mm per year,
- poor rainfall distribution pattern,
- somewhat excessively to excessively drained soils,
- very shallow soil or soil causing restricted rooting.
As irrigation is frequently impossible due to inavailability of water and high capital costs of installation, water conservation measures should be aimed primarily at maintaining maximum use of rainfalls on the plantations. This includes minimising run-off and erosion, and maintaining or improving infiltration of water into the soil. Therefore, soil and water conservation practices are complementary to a large extent.
Irrigation, despite giving good yield responses, should only be implemented if the following conditions can be met :
- regular severe moisture stress is limiting growth and yield,
- adequate water with salinity less than 1000 mhos cm-1 can be ensured during dry season,
- the irrigation system is easy to maintain,
- an economic system of irrigation is possible.
The other extreme of water management is when excess water occurs in the area. Proper drainage systems are essential to prevent prolonged flooding which is detrimental to crop production.
The primary aim of drainage for plantation tree crops is to maintain the water table at 75 cm and not less than 50 cm from ground surface at most times. To achieve this, a good outlet with sufficient capacity for the water discharge requirements is vital. Otherwise the excess water could still be contained within the planted area. The direction of field and main drains should be in the line with the flow direction of the water. The intensity and dimension of drains depend largely on the expected amount of water to remove during the wet months. Cheong and Ng (1974) proposed higher intensity of field drains for clayey soils compared to sandy soils. The water level in the drains may be controlled using water gates, weirs and stops.
In sandy podzols, perched water table may occur due to poor percolation of water. Scupper drains which break through the hard-pan (spodic horizon) are required to remove the stagnant water before planting. Similarly, in compacted soil with poor infiltration rate, aeration drains have been found to be beneficial.