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Edaphic factors limiting rice responses to applied inorganic fertilisers in rainfed lowland soils in southeast Cambodia

Seng, Vang (2000) Edaphic factors limiting rice responses to applied inorganic fertilisers in rainfed lowland soils in southeast Cambodia. PhD thesis, Murdoch University.

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Soil-water interactions with nutrients are central to understanding constraints to rice production in the rainfed lowlands. Proper identification of the nature of these constraints may lead to increasing farm productivity through the identification and use of effective soil amendments and an appropriate application of fertilisers. This thesis has made a significant contribution to the understanding of soil water x phosphorus interactions in the rainfed lowland rice ecosystems, by a series of experiments on two rainfed lowland soils of southeast Cambodia.

The most common physical and chemical properties of the two Cambodian rainfed soils: the Black clay soil (Kandic Plinthaquult) and the Sandy soil (Plinthaquult) were determined. The soils were strongly acidic (pH, CaCl2 = 4.0), low in total N (<2.0 g/kg), Olsen-extractable P (<2.5 mg/kg), organic carbon (<15 g/kg), effective cation exchange capacity (ECEC<4.0 cmolc/kg), and they possessed high Al saturation of the ECEC (70-80 %). Changes in their chemical properties caused by flooding and temporary loss of soil-water saturation, and the consequences of these changes for the growth and grain yield of rice (Oryza sativa L.) were also examined.

It was concluded from a field experiment in southeast Cambodia that low rice yields and a weak response to the application of inorganic fertiliser were most likely due to a temporary loss of soil-water saturation restricting nutrient availability and uptake by the rice plants. In subsequent glasshouse and field experiments, the P uptake and growth of rice plants grown under fluctuating soil-water regimes were closely related to changes in soil pH and Eh. Changes in soil pH and Eh corresponded closely to varied soil-water regimes. This controlled P uptake and shoot dry matter yields, presumably by affecting P availability. Continued or temporary loss of soil-water saturation decreased rice growth significantly from that in the continuously flooded soils. This was related to substantial decreases in P uptake, apparently caused by decreased P availability resulting from oxidation of soil after a loss of soil-water saturation. The decreases of shoot dry matter under a temporary loss of soil-water saturation were associated with two mechanisms at least.

Firstly, when the soils were subjected to a loss of soil-water saturation the reactions of phosphates with Fe released during prior flooding resulted in a decline in the availability of native and added P fertiliser. Secondly, during the loss of soil-water saturation, exchangeable Al levels were high enough to account for the decrease in P uptake by the rice plants.

The addition of straw ameliorated the effects of a temporary loss of soil­water saturation on growth of rice because it stimulated soil reduction and lessened changes in soil pH and Eh. It resulted in increased shoot dry matter yields through increased P uptake by the rice plants. The beneficial effects of straw were attributed in part to the direct addition of P, N or K to the soils. In addition, the stronger soil reduction it induced restricted the formation of Fe bound phosphates and increased the plant-availability of P.

In the rainfed lowlands, fluctuations in soil-water regimes vary greatly in timing, intensity, and duration. The availability of P is expected to vary correspondingly. More information is needed to manage the effects of fluctuating soil-water regimes on P supply to increase productivity over the range of soil types, cultural practices and seasonal conditions found in rainfed lowland ecosystems.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental Science
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Bell, Richard, Nesbitt, Harry and Willett, Ian
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