 |
 |
|
|
 |
 |
|
|
|
|
|
||
|
 |
 |
 |
|
|
 |
|
|||||
|
 |
|
 |
 |
 |
|
|
 |
|
|
    |
|
ILEIA Newsletter Vol. 11 No. 3 p. 14 Soil conservation and weed control: friends or foes?Koen van Keer and Francis TurkelboomDespite more than 20 years of local research and development activities by a multitude of GOs an NGOs, adoption of soil conservation practices in the highlands of Northern Thailand remains rather poor. There are several socio-economical as well as agro-technical reasons behind this failure, some of them now being revealed by studies using on-farm or farming systems approaches (Anderson 1995; Turkelboom et al 1995). One aspect which so far has been overlooked, however, is the seriousness of weed problems and their interference with soil conservation issues. Ever since farmers started to cultivate the highlands, weeds have been a drag on them. The current agrarian crisis which afflicts the highlands – and has its origin in land scarcity – is worsening the weed problems. Recent surveys, all based on farmers’ perceptions, indicate that weeds are the first or second most important agricultural constraint in various swidden systems in Northern Thailand (SFC 1993; Anderson 1995). Grasses, such as Imperata cylindrica, Thysanolaena latifolia and Microstegium vagans are problematic during the fallow period and force farmers into deep (15-20 cm) hoe tillage when preparing their fields. Broadleaf weeds, mainly pantropical Compositae such as Ageratum conyzoides, Crassocephalum crepidioides and Conyza sumatrensis are particularly irksome during the cultivation period. They can attain exorbitant densities of several hundred to more than thousand individuals per square meter. The situation is so desperate that farmers are increasingly turning to the unorthodox practice of spraying salt, an ultimate and efficacious method to control broadleaf weeds in upland rice, however with unknown consequences for soil, water and crops other than rice (Van Keer, 1995). Soil conservation practices have been tested on-station and extended to farmers in the form of complete "packages". All these packages promote, among several other conservation measures, no-burning, no- or minimum-tillage and permanent land use. Few researchers seem to have questioned the implications such recommendations might have for the weed problem. Farmers, on the other hand, still stick to burning, deep soil cultivation and fallow rotations, mainly as a response to the weed problems. Fire: a hot topic The use of (controlled) burning for field clearing provokes controversial opinions among agronomists and farmers. While some almost pathetically condemn the practice, others take a more carefully balanced stand. Farmers unanimously praise fire for the control of weeds, insect pests and diseases and for its (short-term) beneficial effects on soil fertility. Burning is undoubtedly the fastest and easiest way to clear a field and will always be cheaper than alternative methods for clearing and pest or soil fertility management. The positive effects of fire on weed control and plant nutrient availability, at the expense of a net loss of C, N and S from the system are well documented. Other aspects, such as effects on pests and diseases, effects of burning small amounts of biomass (from degraded fallows or crop residues) and mid- and long-term soil fertility effects which also deal with losses by leaching and erosion are less well known. Unlike the controversy about the sustainability of spotwise burning for field preparation, greater consensus seems to exist about the negative effects of uncontrolled burning. Wildfires, often a consequence of careless burning practices, are dangerous and retard the process of vegetative regeneration in fallow lands. Whenever a farmer decides to burn his field he should take all possible precautions to prevent the start of wildfires. No-tillage, no rice? Conservation tillage systems (no- or minimum-tillage) were originally designed for advanced agricultural systems in the north. Later on they were also recommended for soil conservation in tropical regions, despite the knowledge that weed problems increase if tillage operations are reduced. Conservation tillage cannot go without increased manual weeding or (high) inputs of herbicides. Minimum soil disturbance was (and still is) a characteristic feature of strict cyclical swiddening systems with long (more than 8 years) fallow and short (1 year) cultivation periods (ref). However, in areas with high land pressure deep tillage is the only low-input method available to farmers when grass infested fallow land has to be prepared for upland rice cultivation. Moreover, it is very effective to control the dominant grass species (Van Keer, 1995). The impact of tillage on soil erosion seems as controversial as the fire issue previously discussed. Tillage makes the soil more erodible and increases the risk for the generation of concentrated flow (gullies). On the other hand it can increase the roughness of the soil surface and water infiltration capacity. The net result on soil erosion depends, in consequence, on factors such as soil type, preceding fallow type, landscape position, field history, method and intensity of soil cultivation and rainfall characteristics (Turkelboom et al., 1995). Last but not least we should also look into the effects on crop yields. Farmers claim that deep tillage gives better rice yields, a statement which has been confirmed by research results (Van Keer, 1993). Alley fields as weed gardens Alley cropping systems were triumphantly presented as a stable alternative to shifting cultivation, also in Northern Thailand where they are the most tested and extended soil conservation practice. It was believed that, besides their ability to control soil erosion, they could replace some of the functions of a fallow and as such facilitate the transition towards permanent land use. In reality, we observed that alley fields are left fallow as well, very often because of weed problems (SFC, 1993). Recent publications on alley cropping are more critical and mention several drawbacks of the system (Ong, 1994; Turkelboom et al., 1994). This paragraph focuses only on the drawbacks related to weed management. The instructions for use by delivery of an alley cropping kit advise the farmer to apply the cuttings as a mulch to the crop in the alleys, for the sake of soil conservation and weed control. Most farmers, however, place the cuttings within or along the hedgerows. Mulching seems impractical for upland rice which leaves little space in between individual hills. Farmers also claim that mulching before sowing hampers rice seedling emergence. Corn might be a better candidate for mulching, but large amounts of mulching material are needed to effectively suppress weeds. No-burning is another alley cropping dogma, difficult to accept by farmers struggling with weeds (SFC, 1993). It is not uncommon to see controlled burning in small plots in between the hedgerows. The worst scenario is when vegetative buffer strips become a weed problem themselves. This was reported for several grass species that were introduced for soil conservation purposes and later became serious weeds, not only in the fields where they were planted but all over the region (Wongprasert and Thaiyanunt, 1992). Useful waste The weed flush at the onset of the cropping season necessitates early and complete weeding in upland rice fields. Unfortunately this happens in a period when poor crop cover and high rainfall intensities make the soil extremely prone to erosion. One of the best ways in which farmers can make good use of weeds is to use them for the establishment of trash lines. Trash lines are buffer strips made by piling up weed and crop residues along the contour lines, as an alternative to burning, piling up in individual heaps or throwing away in an adjacent fallow. If trash lines contain sufficient organic residues they can be very efficient for erosion control (Turkelboom et al 1995). Their efficiency and utility can be further improved by fortification with stakes or stones and/or interplanting with crops such as sweet potato, corn, tarot, chili, lemon grass, banana, papaya, etc. The advantages of trash lines in comparison to the permanent hedgerow systems mentioned above are their easiness of establishment, mobility, low labour requirement, lack of competition with the main crop and direct profit if interplanted with useful crops. They are often temporary but can evolve towards permanent buffer strips if desirable. Another practice which can sometimes be seen is the use of weed residues as "gully plugs", to prevent further incision of existing erosion channels. Fallow management Farmers resort to fallowing when a build up of weed makes further cultivation of a field unprofitable. As land pressure forces farmers to shorten the fallow periods and leads to fallow degradation, the concept of improved (planted) fallow comes to the fore as a possible solace. Planting fast growing (nitrogen fixing) shrubs/trees on fallow land could suppress the weeds and improve the soil in a shorter period than natural fallows would do. A conversion of all fallow land into improved fallows would be undesirable however, as so-called degraded fallows (in terms of weed infestation, biomass production and soil fertility) can be of high practical or commercial value to the local community. Examples of such fallow types are grass fallows dominated by Imperata cylindrica or Thysanolaena latifolioa, used for thatching roofs and making brooms respectively. Weed control for soil conservation The examples presented in this article illustrate that the interactions between soil conservation and weed control are important and complex. Both aspects need to be integrated when looking for sustainable cropping practices in a tropical highland environment. Soil conservation programmes risk to be rejected by farmers if they are too rigid and neglect the weed problems. However, if the programme tackles weed issues at the same time, it is more likely to be successful. Whether or not to burn or till are decisions we should leave up to the farmer. Whether the advantages outweigh the disadvantages or vice versa depends on each specific field situation and on the farmer’s goals and tools available to reach these goals. What we can do as researchers and extension workers is to find out and advise how to maximize the benefits and minimize the damages. The almost instinctive practice of yearly burning could be changed into a more "opportunistic" use of fire, only when no other options are available. This could be the case for a fallow with a large amount of biomass or for fields heavily infested with weeds or other pests. Farmers should use all possible means to keep the fire restricted to their fields, despite the extra labour input this will require. Deep soil cultivation might be necessary to control grasses or to prevent fertile ashes from being washed away. Wherever possible it should be avoided in erosion risky conditions or combined with erosion prevention measures, like the establishment of trash lines. A well considered fallow management, preferably coordinated at village level, could combine the advantages of natural fallows and improved fallows for maximization of weed control, soil conservation and profits from fallow products. If farmers and researchers combine their efforts, the seemingly contradictive objectives of soil conservation and weed control can be reconciled, which will contribute to more sustainable ways of highland farming. Koen van Keer and Francis Turkelboom, Soil Fertility Conservation Project, PO Box 4, Mae Jo, 50290 Chiangmai, Thailand. References Back to Top |
|