Soil Technology最新文献

Understanding the basic processes and factors that are responsible for inducing land degradation, particularly soil erosion and associated phenomena is critical to the conceptualization, design, and implementation of productive, stable, and sustainable agricultural systems. This is particularly so on steeplands where the potential for soil erosion and runoff water losses is high. The productivity and degradation hazards on these lands are determined by the site's climate, soil and topography. However, their uniqueness lies more with their topographic constraints than with other factors. Use of steeplands is an increasingly common situation in the tropics because of high population pressures and continuing encroachment on hilly lands. Erosion potential and actual erosion in these settings may exceed tens or even hundreds of tons of soil loss per hectare per year; thus the selection and design of cropping systems, land management systems, and water management systems must be tailored to attain effective runoff and erosion control in order to avoid their detrimental impacts both on-site and off-site. Contrary to the customary arguments for the ‘long-term’ nature of erosion impacts; enhancing the conservation-effectiveness of rainfed farming on tropical steeplands can be shown to provide both short- and long-term benefits to the farming system, the overall economy, and the environment. Productivity-enhancing crop and soil and water conservation management approaches (biological measures) may be more important than structural measures in imparting long-term sustainability. Incorporating indigenous knowledge into project design should be emphasized to assure the farmer's involvement and cooperation in planning, implementing, and maintaining conservation measures.

Hedgerows and residue cover have been reported as successful practices to reduce soil and water losses on steep lands. However, there is a lack of information on the efficiency of hedgerow types, hedgerow-mulch interaction and the effect of soil moisture and slope (gradient and length) on the overall performance of these practices. Results from four experiments using Vetiver and other grasses under simulated field conditions in Venezuela are presented. The experiments were conducted in the mountain coastal range of Venezuela, at 1800 m above sea level, where intensive horticultural systems occupy steep lands. A double nozzle rainfall simulator, yielding an average intensity of 55.6 mm h⁻¹ was used on 5 m length and 0.5 m wide plots, on an Aquic Paleudult soil with different soil moisture conditions and two 15% and 26% slopes, in order to evaluate the efficiency of five 50 cm wide hedgerows: Vetiver (Vetiveria zizanioides), Lily (Agapanthus africanus), Fern (Nephrolepis sp.), Lemon grass (Andropogon citratum) and no hedgerow; and five levels of pine needles residues: 0, 0.5, 1.5, 3 and 5 Mg ha⁻¹. Upland flow was simulated using a flow equal to an 80% rainstorm runoff as obtained under wet and very wet conditions, in order to imitate equivalent slope lengths. Hedgerows and mulch were good conservation measures, especially when they were combined. Vetiver grass and Fern were the more efficient hedgerows because of their highly dense vegetative structure. Residue requirements are less when the slope is protected by hedgerows. Larger intervals between hedgerows would be possible provided there is mulch between hedgerows. A table is presented as a guideline to design hedgerows spacing considering rainfall erosivity, soil erodibility and crop-management intensity.

Surface seal development and failure substantially affect soil erosion processes. A flume study was conducted to determine surface seal failure of a variable charge subsoil by rainfall and surface flow under different slope steepnesses. Two experiments were conducted: (1) an alternating sequence of a rainfall-surface flow for a 2.6% slope bed involving increasing rainstorm intensities and surface flows, and (2) a 2 h rainstorm of 55.4 mm · h⁻¹ intensity followed by surface flow on a soil bed with slope steepnesses of 8.9, 12.5 and 16.5%. The results indicate that seal failure on this soil is a local ‘catastrophic’ phenomenon, in which failure occurs in the form of a cavity that rapidly grows under local flow conditions. Observations suggest that seal failure was caused by a combination of surface flow hydrodynamics and several subsurface factors such as pore pressure changes, vented soil air, and return flow.

African tropical mountains are often overcrowded because the climate is healthy and favorable to intensive agriculture. Consequently the density of population in the mountains of Rwanda and Burundi has reached an exceptional level (150 to 800 inhabitants/km²) that leads to delicate problems of soil protection against runoff and various types of erosion on steep cultivated hillslopes. Previous measurements on runoff plots have shown that sheet and rill erosion risks have reached 300 to 700 t/ha/year on 20 to 60% slopes with regional rainfall erosivity (R_usa = 250 to 700), very resistant ferrallitic soils (K = 0.01 to 0.20) and traditional farming systems (C = 0.8 to 0.3). Curiously, the runoff rate (10 to 30%) is relatively moderate so that it is possible to restrict erosion with a natural or leguminous fallow, a pine plantation (litter effect) or by mulching coffee, banana or cassava plantations. The problem is now to produce enough biomass to mulch the whole surface with the help of agroforestry. A new strategy (GCES = land husbandry) was suggested to meet the major farmer problems: what should be done to increase the soil productivity rapidly and protect the rural environment? A part of the answer is to be found in the efficient management of water, organic matter and soil fertility restoration (Roose et al., 1988). This strategy was first tested in 9 runoff plots (5 × 20 m) on a 23% slope of a very acid ferrallitic soil (pH = 4). Three types of living hedges (leucaena, calliandra, calliandra + setaria) twice replicated, were compared with the international bare standard plot and with the regional farming system (maize + beans during the first season, and sorghum during the second season). After 2 years, living hedges reduced runoff to less than 2% and erosion to 2 t/ha/year: they produced fire wood and high quality leguminous forage (3 to 8 kg/m) and return to the soil as much as 80 to 120 kg/ha/year of nitrogen, 3 kg/ha/year of phosphorus, 30 to 60 kg/ha/year of calcium and potassium, 10 to 20 kg/ha/year of magnesium. Thanks to agroforestry it was possible to reduce erosion hazard but not to restore the soil productivity. Without 2.5 t/ha/3 years of lime to increase the pH up to 5 and reduce the aluminium toxicity, without 10 t/ha/2 years of farm manure and mineral fertilizers to nourish the crops, the yield remains very low (800 kg/ha/season of cereals). Thanks to agroforestry and a mineral fertilizer complementation, erosion hazard was controlled and the productivity of soil and labour intensified more than 3 times.

In the Venezuelan Andes the traditional way of cultivating coffee has been beneath shade, using the variety Coffea arabica var. arabica. However, since 1975 the government has promoted a change from this type of cultivation to ‘sun coffee’ which implies the elimination of shade trees and the replacement of old coffee trees with varieties such as C. arabica var. bourbon and var. caturra. In order to analyze the possible consequences of this change to the soil erosion in the sloping coffee plantation, a study was carried on in the region of Canaguá, in the State of Mérida. The results were obtained from studying the successive loss of the mineral fraction and runoff, measured in plots situated in (1) a 16–17 year old shade plantation, (2) the same plantation in the first and second years following its transformation to a ‘sun plantation’, and (3) a seven to ten year old sun plantation. The results show that: (a) Despite the larger losses in the sun plantation, both types of management show very low erosion after the plantation has become established, (b) the loss of the mineral fraction less than 4 mm represents the greatest difference between erosion in the sun and shade plantation in full production, (c) the losses of < 4 mm fraction during the first year following implantation of sun coffee are four times greater than those found in the old shade plantation, and (d) there is a positive relation between erosion and human activities within the plantation, contrasting with a non significant correlation between erosion and runoff and rainfall erosivity.

In the past, the watershed sediment yield estimation, in rivers where hydrometric information is available, has been done using the sediment rating curve, which is obtained fitting theoretical functions to the water and sediment discharge relationship. Once this relationship is defined, it can be used along with the flow duration curve of a given year (or runoff-frequency data) to obtain the annual sediment yield prediction. Sometimes, specially in recent years, a better estimation has been developed using the same procedure but replacing the flow duration curve for the daily flow series, transforming each daily flow in suspended sediment discharge, by means of the sediment rating curve, and accumulating this information to monthly and annual levels. This procedure is judged adequate for large watersheds, where the differences among the mean daily flow and the maximum and minimum streamflow during the day are not very large. Generally the equations that fit best the sediment rating curves are highly nonlinear, therefore, in small watersheds where the differences among mean daily flows and the extreme instantaneous flows during the day are important, working with mean daily flows (and even worse with flow duration curves) introduces large errors in sediment yield estimations. In addition to this, there also exists other problems related to the previous procedure, namely the extrapolation errors, that can be very important. Also the quality of the basic information is of paramount importance in this type of analysis. This paper discusses all the above problems analyzing case studies related to two important Venezuelans reservoirs: Dos Cerritos in the Tocuyo River and Cumaripa in the Yaracuy river. An evaluation is made of the effects of these sources of errors in the sediment yield estimation of both cases.

Water erosion is the major threat to soil and water conservation in the steep lands of the tropics. Besides surface erosion on gentle to moderate slopes, mass movements are common on steep slopes. In addition to the negative effects on productivity and crop production risks, in many tropical regions, offsite effects of sedimentation, floodings and landslides are also rooted in accelerated soil erosion. The prediction of water erosion by direct measurements in erosion plots, or by using empirical models has not generally given satisfactory results in the tropics, specially when mass movements are the potential erosion processes. Modeling the surface soil hydrological processes, under the prevailing conditions of climate, use, management and cropping in two selected sites of Venezuela, resulted in fairly accurate simulations of both the soil surface and landslide erosion processes and their main effects. The model SOMORE, used for such simulation, is based on easily available climate and soil input parameters, and produces as the main output the soil moisture regime in a daily basis, including the average soil moisture at root depth, and the water losses by surface and subsurface runoff, and by internal drainage. The output of the model is used as the basis for the selection, with a probabilistic approach, of the best alternatives of use and management of soil and water resources for each combination of soils, climate and topography.

The purpose of this study was to document the fractal nature of three soils of Mexico with contrasting genesis and marked differences in morphology and to estimate the fractal dimensions of their sets of aggregates and pores. These dimensions were estimated along lines and were called linear fractal dimensions. A single, ‘ideal’ fractal dimensionality was detected in the three soils studied. The soil linear fractal dimensions, calculated from macro and micromorphological data, had larger values than the dimension of the Cantor fractal dust model, but were less than unity. It was shown, that the fractal structure of the soil pore space could not be described by the same dimension as that of the aggregates. The linear fractal dimensions of soils of distinct genesis, were significantly different on all scales compared, but the differences fluctuated between 0.4% and 9.1%.

This paper describes the design of a ventilated ‘open’ chamber system for continuous measurement of fluxes of water and carbon dioxide from the soil surface. The system consists of four units: a chamber, an air switching unit, an infrared gas analyzer and a data logger. The chamber has the shape of a rectangular ‘box’ lying on the soil surface covering a ground area of 0.68 m⁻². The wind speed through the chamber can be regulated with a fan from zero up to a maximum of about 0.9 m s⁻¹. The soil evaporation measured by the chamber showed good agreement with evaporation measured by a balance. In situ measurement of evaporation by the chamber agreed well with evaporation estimated by a physically-based soil model. The soil water content under the chamber showed good agreement with the simulated values as well as with the water content measured outside the chamber. The magnitude of soil CO₂ efflux was in good accordance with other data reported in literature. The main error with this technique was found in the measurement of the air flow through the chamber.