Effect of tree legumes in hedgerows on soil fertility changes and crop performance in the semiarid highlands of Rwanda

The soil fertility improvement potential of five tree legumes as alley hedges was studied on a Ultic Haplustox soil at a semi-arid highland site in Rwanda from 1983 to 1989. Tree species tested were C. calothyrsus Meissn., C. spectabilis DC, L. diversifolia (Lam.) de Wit., L. leucocephala (Lam.) de Wit. and S. sesban (L.) Merr.; they grew to a height of 1.01 to 2.15 m and 2.85 to 3.37 m, after 1 and 1.5 years respectively. S. sesban could not withstand intensive pruning (four times per year) and 83% of the trees died after the six prunings. In contrast, C. calothyrsus lost only 28%, both species of Leucaena 9 to 10%, and C. spectabilis none of the trees after 18 prunings in 4.5 years. Above-ground biomass production increased steadily with the age of the trees. Mean values for 1984–89 were: 5.6 to 7.3 Mg/ha dry weight of leaf biomass and 3.7 to 5.0 Mg/ha of wood. Mean annual nutrient addition to soil through leaf biomass was: 72 to 119 kg/ha of N, 2 to 3 of P, 47 to 94 of Ca, 8 to 19 of Mg, and 29 to 60 of K, and it was equivalent to an application of 10 Mg/ha/year of cattle manure. Tree mulch application reduced the rate of soil fertility decrease due to cropping; percent increases over control were 2 to 20 for soil C, −3 to 7 for K, 4 to 51 for Ca, 7 to 31 for Mg, and 3 to 47 for exchange capacity in mulched plots. Combined application of leaf biomass and manure increased the soil pH (water) by 0.4 to 0.5 units, K by 0 to 0.7 mmol, Ca by 10.8 to 17.6 mmol and Mg by 0.5 to 2.3 mmol and exchange capacity by 10.5 to 34.0 mmol above the level of soils cleared from the savanna vegetation. Mean nutrient export from the site in woody stems ranged from 17 to 28 kg/ha/year of N, 1 to 2 of P, 13 to 16 of Ca, 2 to 5 of Mg, and 17 to 34 of K. Grain yield increase of beans (Phaseolus vulgaris L.) and sorghum (Sorghum bicolor L.) to tree foliage application was the largest in C. spectabilis alleys, followed by the others. Crop yield increases due to tree mulch use were less in manured plots. Maize {Zea mays L.) response was poor. Tuber yield of sweet potato (Ipomoea batatas (L.) Lam.) decreased when planted in alleys, due to the lower tuber yield per unit crop area in alleys than in the control and a 14 to 32% yield fall off at the tree/crop interface

Area harvests equivalency ratio for measuring efficiency in multiseason intercropping

Both area and time factors have to be considered to quantify resource-use efficiency in multiseason intercropping. The land equivalent ratio (LER), commonly used as an indicator of efficiency, is not suitable because it considers only the area factor to estimate intercrop advantages. The area time equivalency ratio (ATER) unrealistically assumes continuous crop growth throughout the year, thus it underestimates the advantages of intercrops. To avoid these problems, some authors used the mean value of LER + ATER as an arbitrary compromise. This paper proposes a new concept called area harvests equivalency ratio (AHER). This ratio incorporates the time factor in the form of number of possible harvests of each component crop in a system that could be obtained during the full intercrop period, if each component was monocropped. Four cases of intercrops were used to compare the above concepts. The AHER has proved to be a better efficiency indicator than others, except when a component species occupies the land for one season plus a fraction of the succeeding season (l.5, 2.5 seasons). In this particular case, other concepts are also disappointing. The problem with any attempt to quantify resource-use efficiency is that there is no “true value” for yield advantages in intercrops. However, intuitively, AHER does seem to be nearer to the “true value” than others

Five years of research on improved fallow in the semiarid highlands of Rwanda

Alternative technologies that maximize the use oflocal resources and improve the sustainability of crop production in Rwanda are needed. Promising species for the semi-arid highlands, where the research reported in this paper was conducted, include mucuna (Mucunaprwiens) for fallow periods of 1 4- 1 8 weeks, sunnhemp (Crotalaria ochroleuca), pigeonpea (Cajanus cajan) and desmodium (Desmodium distortum) for fallow periods of 20-24 weeks, and Sesbania sesban and Tephrosia vogelii for long fallows ofa year or more. In crop rotations, these legumes contribute 17-240 kg N/ha to associated non-legume crops. However, two seasons of growth are needed for there to be a marked effect on crop yields and the beneficial effect of the legumes lasts only for one or two seasons after the incorporation of biomass. The legumes also supply nutrients recycled from the soil

Spécial érosion : réhabilitation des sols et GCES

Le Rwanda connaît depuis une cinquantaine d'années une phase active d'érosion parce que la pression démographique pousse les paysans à cultiver des pentes toujours plus raides et plus hautes, des sols épuisés et fragilisés. De nombreux projets ont tenté de lutter contre l'érosion : leur succès est de courte durée car les processus d'érosion sont multiples, le référentiel technique n'est pas au point, les conditions écologiques sont très variées (altitude de 800 à plus de 3000 m) et les implications foncières, sociologiques, politiques et économiques sont complexes. Les résultats de la recherche sur de nombreuses parcelles confirment les risques graves de décapage (1 à 3 cm par an) des horizons humifères (par l'érosion en nappe et en rigole et par le travail du sol) sur les pentes raides (20 à 60%) des collines cultivées. Cependant, les recherches suggèrent quatre solutions pour stabiliser les versants : le paillage, les haies vives alternant avec de gros billons couverts en permanence, les terrasses radicales (risques de glissement) et la végétalisation permanente (forêt avec sous-bois, prairie ou verger avec plantes de couverture). Mais la "conservation des sols" pour elle-même ne satisfait pas les paysans car elle n'augmente pas la productivité de la terre ni du travail. Une nouvelle série d'essais a été mise en place pour vérifier s'il est possible non seulement de stabiliser le sol et la production, mais encore d'améliorer sensiblement celle-ci avec les amendements organiques disponibles sur place. Les résultats confirment que la culture entre des haies d'arbustes permet de maîtriser le ruissellement (Kram < 2% mais Krmax = 35% en cas d'orage sur sol humide) et l'érosion (E < 2 t/ha/an au bout de deux ans). Les haies produisent 3 à 9 t/ha/an de biomasse fourragère et 2 à 4 t/ha/an de petit bois, matière organique qui jointe aux résidus de culture forme une biomasse équivalente à la litière déposée par la forêt naturelle. (Résumé d'auteur