Intercropping under coconuts in Sri Lanka

Coconut is a major commercial crop of Sri Lanka. Growing a number of other crops in association with coconuts is a widespread practice in all coconut-growing areas of the country. The rationale for the practice is that other crops can profitably be grown between or under the coconuts during the different growth stages of the palms and thus the overall productivity of the land under this long-duration crop can substantially be increased. The paper gives a concise account of the practice in Sri Lanka indicating the crops most commonly grown as intercrops, arrangement of different crops and early research results on the productivity of the intercrops and their effect on coconuts. Adequate supplies of water and labour are the two major inputs needed for the success of the system. Drought, lack of funds, price instability, lack of technical know-how on intercrop management and problems of timely availability of inputs are the major constraints experienced by farmers in expanding intercropping. Research on both biological and socio-economic aspects is needed to overcome these constraints and extend this potentially attractive system.     

A knowledge-based expert system for planning and design of agroforestry systems

Computer-based Expert Systems that use knowledge, facts, and reasoning techniques to solve problems, normally requiring the abilities of human experts, are increasingly being used in many activities. The United Nations University (UNU) Agroforestry Expert System (AES) is a first attempt to apply this technique to agroforestry. UNU-AES is a prototype Knowledge-Based Expert System (KBES) designed to support land-use (agricultural, forestry, etc.) officials, research scientists, farmers, and individuals interested in maximizing benefits gained from applying agroforestry management techniques in developing countries. This prototype addresses the options for alley cropping, a promising agroforestry technology which has potential applicability when used under defined conditions in the tropics and subtropics. Alley cropping involves the planting of crops in alleys or interspaces between repeatedly pruned hedgerows of fast-growing, preferably leguminous, woody perennials. The primary benefits from this technique include nutrient enrichment, soil improvement, and erosion control. UNU-AES, which is the first known attempt at the application of expert system procedures in the field of agroforestry, uses a total of 235 decision rules to develop its recommendations. With the inclusion of more climatic and socio-economic data and improved advisory recommendations, UNU-AES can be expanded to provide advice on alley cropping in more diverse geographical and ecological conditions and eventually address other agroforestry techniques.     

Dynamics of Particulate Organic Matter following biomass addition from fallow-improvement species in southern Mali

Knowledge about the nature and turnover rates of soil organic matter (SOM) is a key to soil fertility maintenance, especially in traditional farming systems. Particulate organic matter (POM), rather than total SOM content, is now considered a better measure of the soil's fertility. Fractionation of SOM, based on particle-size distribution after dispersion, was carried out on soil samples collected from a field experiment at N'Tarla, Mali (12.25° N latitude, 5.42° W longitude, Typic Plinthustalf). Biomass of off-site grown Gliricidia sepium (Jacq.) Walp. (gliricidia), Pterocarpus erinaceus Poir.(pterocarpus), and Stylosanthes hamata (L.) Taub.(stylosanthes) was applied to a maize (Zea mays L.) crop for two consecutive years. A significant decrease was noted in the soil's POM and POM-N contents after harvest of maize in both years as compared with the initial value measured at the beginning of the experiment. Small POM fraction (0.053 to 0.250 mm, POM₅₃) contributed more to total POM than large POM fraction (0.250 to 2.000 mm, POM₂₅₀). The POM N content was better correlated with maize grain yield than POM total weight, and the N content of POM₅₃ was better related to maize grain yield (R² = 0.55) than that of POM₂₅₀ (R² = 0.20). Total SOM was poorly correlated with maize grain yield (R² = 0.14). The results indicate that the quality of the applied biomass is of greater value than its quantity in sustaining maize grain yield, and POM₅₃ might be a better indicator of soil fertility status than POM₂₅₀ and total POM. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of mulching with multipurpose-tree prunings on soil and water run-off under semi-arid conditions in Kenya

The effect of adding leaf mulches ofGrevillea robusta, Cassia siamea andGliricidia sepium on the rate of soil and water run-off from a crop field were studied during two cropping seasons in an alfisol under semi-arid conditions at Machakos, Kenya. The experiment was conducted in a randomized complete block design with three replications; two rates of mulch of each species (2.24 t and 4.48 t, on dry matter basis, per ha) and a no-mulch control constituted the seven treatments. Soil and water run-off losses after each major rainfall event and the changes in ground and crop cover were measured. Rainfall erosivity and changes in soil bulk density and infiltration rate were also determined.Soil losses from the plots with mulches ofCassia siamea, Gliricidia sepium andGrevillea robusta were significantly lower than those from the control (p=0.10). Over the two seasons, the cumulative soil losses from plots mulched with cassia, gliricidia and grevillea were 11%, 57% and 81% lower than that of the control plot. Similarly, water run-off losses from cassia, gliricidia and grevillea mulch plots were 28%, 48% and 58%, respectively, lower than that of the control plot. Thus, cassia was found to be better than gliricidia and grevillea in reducing both soil and water run-off losses.Soil bulk density did not change while infiltration rate at the end of the experiment was significantly more than in the beginning; however, there were no significant differences in these soil physical properties among the treatments.     

Multipurposes tree prunings as a source of nitrogen to maize under semiarid conditions in Zimbabwe

Time of pruning application and mixing prunings of varying qualities could be important management options to increase the rate of nitrogen recovery from multipurpose-tree prunings that are used as a source of nitrogen to crops. A field experiment was conducted in the semiarid zone of Zimbabwe to test this hypothesis, using prunings of calliandra (Calliandra calothyrsus) and leucaena (Leucaena leucocephala), alone and in mixtures, and maize (Zea mays) as the test crop. Time of pruning application significantly improved N uptake, N recovery, and grain yield of maize. Applying prunings of calliandra at maize planting was significantly better in terms of N uptake, N recovery, and grain yield than applying them four weeks after planting. However, with leucaena, time of prunings application had no significant effects on N recovery. Mixing prunings of leucaena and calliandra had no effect on maize N uptake, N recovery, and grain yield. Split application of available prunings during the crop growth cycle had no effect on N recovery compared to one-time application of entire amounts of prunings at planting.     

Biophysical interactions in tropical agroforestry systems

The rate and extent to which biophysical resources are captured and utilized by the components of an agroforestry system are determined by the nature and intensity of interactions between the components. The net effect of these interactions is often determined by the influence of the tree component on the other component(s) and/or on the overall system, and is expressed in terms of such quantifiable responses as soil fertility changes, microclimate modification, resource (water, nutrients, and light) availability and utilization, pest and disease incidence, and allelopathy. The paper reviews such manifestations of biophysical interactions in major simultaneous (e.g., hedgerow intercropping and trees on croplands) and sequential (e.g., planted tree fallows) agroforestry systems. In hedgerow intercropping (HI), the hedge/crop interactions are dominated by soil fertility improvement and competition for growth resources. Higher crop yields in HI than in sole cropping are noted mostly in inherently fertile soils in humid and subhumid tropics, and are caused by large fertility improvement relative to the effects of competition. But, yield increases are rare in semiarid tropics and infertile acid soils because fertility improvement does not offset the large competitive effect of hedgerows with crops for water and/or nutrients. Whereas improved soil fertility and microclimate positively influence crop yields underneath the canopies of scattered trees in semiarid climates, intense shading caused by large, evergreen trees negatively affects the yields. Trees in boundary plantings compete with crops for above- and belowground resources, with belowground competition of trees often extending beyond their crown areas. The major biophysical interactions in improved planted fallows are improvement of soil nitrogen status and reduction of weeds in the fallow phase, and increased crop yields in the subsequent cropping phase. In such systems, the negative effects of competition and micro-climate modification are avoided in the absence of direct tree–crop interactions. Future research on biophysical interactions should concentrate on (1) exploiting the diversity that exists within and between species of trees, (2) determining interactions between systems at different spatial (farm and landscape) and temporal scales, (3) improving understanding of belowground interactions, (4) assessing the environmental implications of agroforestry, particularly in the humid tropics, and (5) devising management schedules for agroforestry components in order to maximize benefits. This revised version was published online in June 2006 with corrections to the Cover Date.     

Characterization of Intergeneric Hybrids of Saccharum using Molecular Markers

The potential use of RAPD markers in identifying genuine intergeneric hybrids of Saccharum was examined in 12 hybrids of Saccharum with Erianthus spp., Sorghum bicolor and Zea mays. In the three groups of hybrids examined, RAPD markers could precisely identify the true hybrids of Saccharum officinarum × Sorghum bicolor and Saccharum officinarum × Zea mays. Though Erianthus had a very distinct RAPD profile and as many as 107 Erianthus-specific bands were identified, a decisive profile that can identify all the hybrids could not be obtained. The strong molecular differentiation between S.officinarum and Erianthus was not adequately resolved in the RAPD profiles of their hybrid progenies which suggest that more refined molecular techniques will be required for the characterization of Saccharum–Erianthus hybrids.     

Analysis of Genetic Diversity Among Saccharum spontaneum L. From Four Geographical Regions of India, Using Molecular Markers

Saccharum spontaneum L. a wild relative of sugarcane is the most variable and diverse among the Saccharum species. This species had been successfully exploited in sugarcane improvement programmes since 1915 and most of the present day commercial varieties are derivatives of interspecific hybrids involving S. spontaneum. The S. spontaneum germplasm available today in the World collections is diverse and represent different geographical groups. In the present investigation, an attempt was made to characterize 40 S. spontaneum clones collected from 4 different geographical areas in India using 20 random, 2 ISSR and 2 telomere primers. Of the 491 bands generated by these primers 83.9% were polymorphic. The genetic diversity estimated based on these markers was found to be moderate (48.9%). The pair-wise genetic distance between the accessions ranged from 29.8 to 60.0. The accessions from Arunachal Pradesh were found to be the most diverse, while Tamil Nadu accessions showed relatively less diversity. Diversity between Tamil Nadu and Kerala collections was found to be low, while the diversity between the Orissa group and the rest was found to be high. The collections from Mayurbanj and Cuttack regions of Orissa were found to be distinct. Arunachal Pradesh accessions, being more diverse, are a potential source for exploitation in sugarcane breeding programmes.