Purely annual crop-based production systems have limited scope to be sustainable under upland conditions prone to infestation by Imperata cylindrica if animal or mechanical tillage is not available. Farmers who must rely on manual cultivation of grassland soils can achieve some success in suppressing Imperata for a number of years using intensive relay and intercropping systems that maintain a dense soil cover throughout the year, especially where leguminous cover crops are included in the crop cycle. However, labour investment increases and returns to labour tend to decrease in successive years as weed pressure intensifies and soil quality declines. Continuous crop production has been sustained in many Imperata-infested areas where farmers have access to animal or tractor draft power. Imperata control is not a major problem in such situations. Draft power drastically reduces the labour requirements in weed control. Sustained crop production is then dependent more solely upon soil fertility management. Mixed farming systems that include cattle may also benefit from manure application to the cropped area, and the use of non-cropped fallow areas for grazing. In extensive systems where Imperata infestation is tolerated, cassava or sugarcane are often the crops with the longest period of viable production as the land degrades. On sloping Imperata lands, conservation farming practices are necessary to sustain annual cropping. Pruned tree hedgerows have often been recommended for these situations. On soils that are not strongly acidic they may consistently improve yields. But labour is the scarcest resource on small farms and tree-pruning is usually too labour-intensive to be practical. Buffer strip systems that provide excellent soil conservation but minimize labour have proven much more popular with farmers. Prominent among these are natural vegetative strips, or strips of introduced fodder grasses. The value of Imperata to restore soil fertility is low, particularly compared with woody secondary growth or Compositae species such as Chromolaena odorata or Tithonia diversifolia. Therefore, fallow-rotation systems where farmers can intervene to shift the fallow vegetation toward such naturally-occurring species, or can manage introduced cover crop species such as Mucuna utilis cv. cochinchinensis, enable substantial gains in yields and sustainability. Tree fallows are used successfully to achieve sustained cropping by some upland communities. A variation of this is rotational hedgerow intercropping, where a period of cropping is followed by one or more years of tree growth to generate nutrient-rich biomass, rehabilitate the soil, and suppress Imperata. These options, which suit farmers in quite resource-poor situations, should receive more attention.
Tag: cover plants
Dinitrogen fixation by the legume cover crop Pueraria phaseoloides and transfer of fixed N to Hevea brasiliensis—Impact on tree growth and vulnerability to drought
Rubber tree plantations (Hevea brasiliensis) are expanding into marginal areas with low soil fertility and long dry seasons with a high risk of soil erosion and drought damage to trees. Introducing an N2-fixing legume cover crop in rubber plantations may reduce runoff and soil erosion as well as increasing the availability of nutrients but may also increase competition for water. This study quantified the effect of the legume cover crop Pueraria phaseoloides on N, P and K nutrition, water status and growth of young rubber trees (three years old in 2007) over a four year period (2007-2010). The plantation was located on a toposequence with a range of soil depths and water storage capacities in northeast Thailand.The legume aboveground biomass production and its nutrient content and decomposition rate were measured and the N2 fixation was estimated using the abundance of 15N (15N) in the legume. Measurements were taken of the tree stem girth and height and tree leaf predawn water potential, nutrient content and greenness. The transfer of N2 fixed by the cover crop to the trees was estimated using 15N in the tree leaves.The annual biomass production of the legume was 8Mgha-1 year-1 and the N accumulation by the legume was 250kg N ha-1 year-1. The natural abundance method applied to the aboveground components of the legume gave N2 fixation rates varying from 85 to 93% depending on the year. The leaf 15N was similar in the three non-legumes (H. brasiliensis, Vetiveria zizanioides and Praxelis clematidea) used as reference plants for estimating the N2 fixation. The higher level of N and the much lower leaf 15N values for the rubber trees intercropped with P. phaseoloides, compared to rubber trees growing without a legume cover crop, showed that there was a relatively high transfer of fixed N from the legume to the trees, varying from 39% to 46% of tree leaf N depending on the year. Neither N2 fixation nor N transfer varied significantly along the toposequence. At the bottom of the toposequence, both the nutrient (N, P and K) and water status of trees was significantly improved with the legume cover crop, doubling the tree girth at seven years of age (tree girth: 28cm, tree height: 700cm). However, at the top of the toposequence with low water storage capacity, the legume cover crop improved tree nutrition and growth but reduced the trees’ ability to survive intense drought.These results raise concern about the resilience to drought of the rubber tree/. P. phaseoloides system, since the positive effect of the legume on rubber tree nutrition and growth may increase the risk of water stress and tree mortality. With future changes in climate, an increasing number of areas will be concerned by the question of optimizing the tradeoff between N inputs and water availability. © 2015.
Guidelines for integration of legumes into the farming systems of East African highlands
Grain legumes are major protein sources for animals andhumans. Given that farmers export both grain and stoverfrom the fields, the amount of residue left to the soil is toosmall to have a profound effect on soil fertility. Participatoryresearch was conducted to evaluate the performance of sixlegume cover crops (Vetch, Stylosanthus, Crotalaria, Mucuna,Canavalia, and Tephrosia) and two food crops (Pea andCommon bean) in southern Ethiopian Highlands, one of theAfrican Highlands Initiative (AHI) sites called Areka, to beused for soil fertility improvement. Besides evaluating thebiomass productivity of legumes, the objective of this researchwas to learn about the perception of farmers to LCC, feedand food legumes, to identify socio-economic factors affectingadoption and also to identify potential niches for theirintegration. For short term fallow (three months or less),Crotalaria gave significantly higher biomass yield (4.2 t ha-1)followed by Vetch and Mucuna (2 t ha-1), while for medium-term fallow (six months or more) Tephrosia was the best performing species (13.5 t ha-1) followed by Crotalaria (8.5 tha-1). The selection criterion of farmers was far beyondbiomass production, and differed from the selection criteria of researchers. Farmers identified firm root system, early soil cover, biomass yield, decomposition rate, soil moisture conservation, drought resistance and feed value as important biophysical criteria. Soil moisture conservation was mentioned as one important criterion and decreased in order of Mucuna (22.8%), Vetch (20.8 %), Stylosanthus (20.2 %), bare soil (17.1 %), Crotalaria (14 %), Canavalia (14 %) and Tephrosia (11.9 %), respectively. The overall sum of farmers’ ranking showed that Mucuna followed by Croletaria are potentially fitting species. However, Vetch was the most preferred legume by farmers regardless of low biomass, due to its’ early growth, high feed value and fast decompositionwhen incorporated into the soil. The most important socio-economic criteria of farmers for decision-making on whichlegumes to integrate into their temporal & spatial niches ofthe system were land productivity, farm size, land ownership,access to market and need for livestock feed. These indicatorswere used for the development of draft decision guides forintegration of legumes into multiple cropping systems of EastAfrican Highlands.