EFFECT OF THE SYSTEM OF RICE INTENSIFICATION (SRI) ON ...

White & Seng (1997) state that not only the Prateah Lang soil but also most other rice soils in
Cambodia have a low CEC and organic C content hence making it difficult to preserve supplies of
N, K and water in the soil.
For RPF growing rice on the given sandy loam Prateah Lang Soil groups or similar lowland rainfed
soils, applications of OM could change the C:N ratio. For instance will amendments with OM with
C:N ratios of 25:1 or higher decrease the soil N content as micro organisms cannot locate enough N
in the soil and they therefore will be forced to take up all the available N in the soil solution (Brady
& Weil, 1999a). As the N content is so low in the Prateah Lang soil - the decomposition of OM
might be slowed down as micro organisms cannot find enough quantities of N (Brady & Weil,
1999a).
It is well recognized that applications of organic inputs to lowland rice soils such as rice straw,
animal manures, rice stubble and green manure can increase N content in soils (Kundu & Ladha,
1999). It is estimated that 1t of dried rice grains will provide app. 1.5 t of rice straw which contains
9 kg N and thereby a free nutrient source (Kundu & Ladha, 1999). Straw is however often removed
in resource poor areas to use for animal fodder or burned in situ (Dobermann & Witt, 2000) which
is also the case for the target farmers in this study. The stubble and the roots will however still be
present in situ and is hence another source of N and other nutrients. Only few farmers use chemical
fertilizers. Most of the chemical fertilizers used in Cambodia are used in irrigated rice systems
(Nesbitt & Phaloeun, 1997).
The rainfed lowland rice which is the most common rice cropping system in Cambodia - and in
many other south Eastern Asian countries (Wade et al., 1999). The shift between anaerobic and
aerobic conditions due to a fluctuating water table will lead to oxidation and reduction of the soil
which can result in losses of gaseous N, other nutrients are immobilized and changes in pH (Wade
et al., 1999). The very changing water table may give rise to losses of N – as drying of flooded rice
fields result in large losses of N through nitrification and denitrification (Seng et al., 1999). The
RPF not only in this study but in Cambodia and many other locations are kept in a difficult situation
with the low soil fertility and few means of changing this.
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