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Nutrient management of rice seedlings in the rainfed lowlands of Cambodia

Ros, Chhay (1998) Nutrient management of rice seedlings in the rainfed lowlands of Cambodia. PhD thesis, Murdoch University.

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This study is concerned with low-input rainfed lowland rice cultivation systems in Cambodia where soil fertility is poor, rainfall uncertain, and fertiliser use limited. The investigation comprised a field survey and a series of experiments in the field and glasshouse. The aim was to determine whether improving vigour and/or nutrient concentration or content of rice seedlings would enhance plant growth and increase grain yield. It further aimed to determine the factors controlling these responses.

A survey of 15 farms in the Phnom Penh municipality showed that farmers generally applied higher rates of fertilisers, especially cow manure, to the nurseries than to the mainfields. This lead to a greater yield, and higher nutrient concentration in seeds of plants harvested from the nursery area than for plants harvested from the non-nursery area. However, levels of nitrogen (N), phosphorus (P), and potassium (K) in the soil remained low in both the nursery and the mainfield. Low levels of plant available nutrients, therefore, remained as a serious constraint to rice growth. Subsequent field and glasshouse experiments evaluated the effect of fertiliser addition to seedlings in the nursery on subsequent growth in the nursery and on final yield of plants after transplanting to the mainfield.

Applying N or P fertiliser to the nurseries, increased grain yield by 10-15%, regardless of water regimes and fertiliser rates applied after transplanting. Fertiliser application to the mainfield at transplanting could not overcome the loss of yield experienced with seedlings which were not fertilised. The traditional photoperiod sensitive rice cultivar grown responded to fertiliser added to the nursery regardless of age at transplanting (4, 6, or 9 weeks after sowing).

Having established that consistent responses to nursery application of N and P were obtained in the field and glasshouse experiments, the remainder of the study examined the mechanism of the response. Three main mechanisms were considered: nutrient concentration, nutrient content and seedling vigour. Nutrient concentration was tested by sowing seed with a range of P concentrations, and nutrient content by coating seeds with a range of P fertiliser.

Plants grown from seeds with a high P concentration had greater shoot and root dry matter than plants grown from seeds with a low P concentration. Similarly, coating rice seed with phosphate fertiliser or soaking rice seed in a solution containing phosphate, increased shoot and root dry matter. Coating rice seed with rock phosphate fertiliser strongly increased early plant growth and had no harmful effect on seed germination percentage. The other seed treatments reduced seed germination.

Thus by various means, it was possible to increase nutrient content and growth of the seedlings. However, the total content of nutrients in the seedlings at transplanting was still low compared to final nutrient uptake at harvest. It was concluded, therefore, that the main benefit of the nursery fertiliser application, and of other means of enhancing seedling growth were due to increased vigour of the seedlings after transplanting. The remaining experiments examined the hypothesis that increased vigour of transplanted seedlings was the main benefit of nursery fertiliser application.

Vigour of the seedlings was altered by pruning leaves or roots or both leaves and roots. Removal of 60% of roots at transplanting depressed subsequent growth and removal of 30% of leaves strongly depressed growth. Depressing seedling vigour and nutrient content before transplanting by completely submerging seedlings for 24 h or 48 h/week for 4 weeks impaired plant growth, shoot and root dry matter, and nutrient content of the plants after transplanting.

The results suggest that increasing vigour of transplanted seedlings was the main factor associated with the post-transplanting response to nursery treatments. However, the role of increasing nutrient content could not be entirely ruled out.

A final experiment investigated the distribution and redistribution of 13C and 15N in rice seedlings during an 8-day post-transplanting period. The response of growth and redistribution of 13C and 15N to root pruning and leaf clipping were investigated. Redistribution of 13C to roots was unimpaired by root pruning supporting results showing that the relative growth rate of roots, and the length of new roots after transplanting was not restricted by pruning. Root pruning did however temporarily depress shoot growth after transplanting and this was associated with depressed redistribution of 15N to newly forming leaves. By contrast, leaf clipping strongly depressed new leaf growth after transplanting but only weakly impaired new root growth.

In conclusion, applying fertiliser to the nursery or applying nutrients to seeds by coating or by sowing seeds with high nutrient concentrations, are possible methods to increase the vigour and nutrient content of transplanted seedlings and hence increase the growth and grain yield of the transplanted rice plants. By contrast, pruning leaves should be avoided, and care should be taken to minimise loss of roots from seedlings during pulling.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science and Engineering
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 and White, P.
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