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Acid sands of South-eastern Cambodia: Their origin, properties and management for upland crops

Hin, SarithORCID: 0000-0003-0671-623X (2019) Acid sands of South-eastern Cambodia: Their origin, properties and management for upland crops. PhD thesis, Murdoch University.

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Deep sand and sand on clay profiles occupy a large proportion of the Cambodian landscape. However, the diversity and the factors affecting origin and formation of sand profiles of Cambodia has not been explored and analysed in detail. Moreover, currently there is limited knowledge about the edaphic properties of these sands particularly those that are limiting to agricultural production. The present study concentrated on origin and properties and some management methods to ameliorate acidity of sands in south-eastern Cambodia.

From texture analysis of 19 profiles sampled along toposequences in four study areas (Kampot, Tramkak, Ponhea Krek, and Rolea Bíer/Tuek Phos districts), sand fractions ranging from 63 to 200 μm and 200 to 600 μm were predominant in most profiles, except for Kampot 4, where the fraction was dominated by sand grains from 200 to 600 μm and five profiles from Kampong Chhnang province, where the three medium-coarse sand fractions (63-200, 200-600 and 600-2000 μm) were equally dominant. Clay content was generally < 100 g/kg within 0-80 cm depth but mostly increased from 60 to 160 cm depth. Sub-angular and sub-rounded grains were the major shapes of sand grains in most profiles (20-80 %). Sub-angular shapes of sand grains in Tramkak and Kampong Chhnang generally decreased from the site at the base of mountain to the lower part of the toposequence. In contrast, percentages of sub-rounded grains were higher in the profile in the lower part of the toposequence. Moderately spherical sand grains were the major class in all profiles, ranging from 40-75 % of grains. Based on the above analysis and their proximity to the mountains, the sand profiles sampled along toposequences in four study areas of south-east Cambodia showed the major influence of in situ weathering of siliceous parent materials and to a lesser extent colluvial transport of sands. There was no evidence of long range transport of the sand grains or of any aeolian processes in re-working of the sands.

All the sands were strongly acid with pH (CaCl2) around 4. When pH was 4.2 or greater there was limited exchangeable Al while at pH 4 or less exchangeable Al comprised 30% or more of the effective cation exchange capacity (ECEC). The DPTA extractable Mn varied from low and potentially deficient values to potentially toxic levels. Almost all the topsoils had low levels of extractable P, K, S, Zn and B that suggest deficiency risk for crops.

The pot experiments on mung bean response to lime application on acid sands of two contrasting sands from Ponhea Krek1 (PK), and Kampong Chhnang 4 (KC) showed that near maximum growth of mung bean (90-95 % of maximum) was achieved at 0.60-0.76 t of lime /ha in PK and 0.4-0.65 t of lime /ha in KC. Lime at 1 t/ha reduced leaf Mn concentrations from toxicity levels to suitable values for mung bean growth, but resulted in marginal or deficient levels of leaf P.

Despite relatively short duration of the experiments, there was evidence of mobility of exchangable Ca from lime-treated topsoil to the untreated subsoil. In addition, pH increases up to 25 cm below the depth of lime incorporation were recorded even at CaCO3 or Ca(OH)2 rates of 1 t/ha. These results suggests that subsoil amelioration of acidity may be feasible after shallow surface application of lime on weakly buffered sands subject to monsoonal rainfall.

Field experiment results also showed strong response of mung bean grain yield to lime application on the two acid sands (Tramkak 1-TK, PK). To achieve 90-95 % of maximum grain yield required 0.6-0.7 t of lime/ha in TK and 0.8-0.9 t/ha in PK. In contrast, incorporating lime only within subsoil had limited effect on mung bean growth and grain yield. Given the heavy and frequent rainfall in the wet season, mixing 1 t of lime/ha within 10-15 cm depth should be an effective method of lime application on acid sands to achieve optimum mung bean yield and over time to alleviate subsoil acid constraints.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor(s): Bell, Richard and Newsome, David
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