U. R. Sangakkara

Agronomic regeneration of soil fertility in tropical Asian smallholder uplands for sustainable food production

Smallholder cropping systems are a vital component in the agricultural sectors of tropical Asia, especially in the developing nations. These systems are important for producing food and providing a livelihood to a major proportion of the populations of Asia. While the rice systems, usually cultivated under puddled conditions are considered stable, the upland or highland units, which provide a range of food, fibre and feed commodities, are generally situated on marginal lands. Low soil fertility, erosion, sub-optimal crop management and subsistence farming conditions characterize these units. Hence, these units require improved but low cost strategies to reverse the trend of lower productivity, loss of sustainability and most importantly, the loss of livelihood for the farming populations. Among the many strategies available, the regeneration of soil fertility through agronomic measures would be a useful and easily adoptable method of enhancing productivity of tropical Asian upland small-holding farming units. The methods available include the revival of age-old traditions such as adding organic matter directly to soils, green manuring and mimicking forest ecosystems through agrofor-estry. Agronomic measures using plants to enrich the soil and regenerate its supporting capacity are presented in the present paper. Relevant examples are cited to highlight the potential of these methods for regenerating productivity of the upland smallholder farming units and more importantly the agro-based livelihood of a large majority of the populations of tropical Asia.

How does soil fertility affect yam growth

Abstract Yams (Dioscorea spp.) are known to be very demanding in terms of soil fertility. The influence of deleterious soil fertility on the growth of yams, however, has not yet been studied. A field study was carried out in Côte dIvoire, in a fertile forest site and in a low fertility savanna site, submitted to identical climatic conditions, to determine how soil fertility affects Leaf Area Index, Leaf Area Ratio, Crop Growth Rate, Tuber Bulking Rate, Radiation Interception, Radiation Use Efficiency and final tuber yields in D. alata and D. rotundata, grown in the presence of adequate fertilizers. While germination and establishment of both species were not affected by the site, growth parameters of D. alata were superior to those of D. rotundata. This resulted in higher yields of D. alata, when compared to D. rotundata. Only the radiation use efficiency was higher in D. rotundata than in D. alata. Both species reached higher Crop growth rate and Tuber bulking rate and produced higher fresh tuber yields at the forest site than at the savanna site, reflecting the known demand of yams for high fertility soil. Correlation analyses showed significant relationships between the mean Leaf Area Index measured over the entire growth period, Tuber Growth Rate calculated between tuber initiation and harvest, and fresh tuber yields, suggesting that the development of leaf area is determining the growth rate of the tuber which itself determines the fresh tuber yield. The decrease in D. alata tuber yields at the savanna site was explained by a decrease in Leaf Area Index, while the decrease in D. rotundata tuber yields at this site was explained by a decrease in Radiation Use Efficiency. Both effects might have been the consequence of a suboptimal nitrogen and potassium nutrition at the savanna site.

Early Growth of Crotalaria (Crotalaria juncea), Tithonia (Tithonia diversifolia), and Maize (Zea mays) as Affected by Soil Fertility and Phosphorus Fertilizer under Pot and Field Conditions

Green manures are important in tropical cropping systems and are planted in degraded soils, thus affecting early growth. Pot and field experiments evaluated the impact of soil fertility on early growth of two important tropical green manures (Crotalaria juncea and Tithonia diversifolia) when compared to that of maize (Zea mays) with high and low levels of phosphorus fertilizer. Growth of tithonia was not affected by soil fertility level irrespective of phosphorus fertilizer, indicating its suitability for degraded soils. Crotalaria was affected by soil fertility (root growth was stimulated by phosphorus), indicating its potential for soils with some degree of fertility. Maize was significantly affected by soil fertility and phosphorus fertilizers, indicating the requirement of fertile soils for the successful development of a good root and shoot system. The importance of field studies in the tropics is also presented as the diverse field conditions reduces significant effects found in pot studies.

Root distribution patterns of white yam (Dioscorea rotundata Poir.): a field study

Abstract The productivity of white yam (Dioscorea rotundata Poir) must increase to sustainably meet the demand of the increasing populations in the developing world where this is a staple crop. Although this could be achieved through the use of mineral fertilizers, reports indicate limited effects of these inputs on tuber yield. We hypothesized (i) that D. rotundata has a small and shallow root system and (ii) that this root system does not respond to mineral fertilizer application. Two field experiments were conducted in Côte dIvoire in year 1 and in year 2 to test these hypotheses. In the first field experiment we measured biomass production, as well as root density during plant growth in fertilized and unfertilized plots while in the second experiment, we analysed the distribution pattern of roots in horizontal and vertical root profiles in fertilized plots. The root system of D. rotundata consisted of seminal, adventitious and tubercular roots. Only the adventitious roots remained alive until the end of the growth cycle. The root length density was very low with a maximum of 0.25 cm cm−3. No roots were observed in the 15–30 cm horizon at 50 cm from the plants crown. The horizontal and vertical root maps revealed that roots were mostly distributed in clumps, and there was a good correlation between the two methods of root sampling for this species. Fertilizer application had no significant effect on plant biomass, fresh tuber yield or on root growth. However, thinner and longer roots and higher tuber yields were observed in year 1 than in year 2. The different weather conditions and more specifically the higher soil temperature might explain the results obtained in year 1. Therefore, it could be recommended to farmers not to fertilize D. rotundata.

Nitrogen Fertilizer Affects Growth, Yield, and N Recovery in Cassava (Manihot esculenta L. Crantz)

Cassava is a very important tuber crop in Asia. As fertilizer application, especially nitrogen (N), is important to sustain yields, the impact of different rates of N fertilizer and their time of application on growth, yield, and N-recovery efficiency was evaluated over two major seasons in Asia. Applying N at planting increased leaf area index (LAI), especially at greater rates. Split application of 90 kg N at planting and 45 days later induced growth of more tubers, resulting in good yields, when compared to single or three split applications at 45 and 90 days after planting. Split application on two occasions also developed the greatest N-recovery efficiency by both shoots and roots. The correlations between N efficiency and yield parameters were significant and negative (P = 0.05). The importance of two split applications of moderate rates of fertilizer to cassava is presented.