Enhancing the agronomic productivity of degraded soils in north-east Thailand through clay-based interventions.

Although North-east Thailand occupies one-third of the arable land in Thailand, income pcr capita for the region is no more than 40°;() of the national average and poverty affects 37% of the population. A major reason for this high level of poverty is the relatively limited natural resources of the region. The soi Is of North-east Thailand are dominated by sandy, light-textured soils with low organic matter and low clay. Consequently, they have a low water holding capacity, cation exchange capacity (CEC) and, hence, limited bu ffering capacity against both man-made and natural stresses. Using a paired site analysis, the degree of chemical degradation between an undisturbed (Dipterocarp forest) and disturbed (agriculture system) was assessed. It was estimated that the amount of soil organic carbon lost in the 0-10 cm depth interval ranged from 3.84 to 10.11 t ha-I. This resulted in a dramatic decline in the CEC of the soil. Using a saturation index (Su) that quantifies degradation based on CEC, the effects of changed land use resulted in Su values ranging from 52.9-90.3% dearly indicating the impact ofagricultural practices on a fundamental property ofthese soils. In an effort to remediate the chemical attributes of these degraded soils and enhance productivity, a series of field based experiments have been initiated in the Chiang Yuen area of North-east Thailand. Two structured field trials on rernediating soil chemical degradation included the following treatments: • current farmer practice • termite mound soil • composted leaf litter • locally available lake dredged material • loca lly sourced bentonite • waste bentonite from vegetable oil processing • soil slotting, The trial land was planted with forage sorghum and two consecutive crops were harvested over the 2002 growing season. Dry matter production ranged from 0.14 to 0.22 t ha-I in the control treatments of each trial-with the plants having almost completely failed because ofdrought-to 8.4 and 10.0 t ha-·I for the termite mound material and local bentonite plus leaf litter compost respectively. These dramatic increases in productivity are probably due to increases in CEC, plant nutrient supply and water holding capacity of the soil. The application of locally resourced high-activity clay materials at a ratio of2: I offers a potential way to increase the producti vity ofdegraded light textured soils within the first growing season. This approach to soil rejuvenation could potentially be used to enhance food security at the household level and allow the development of conservation based fanning systems. 1 International Water Management Institute (IWM1), Kasetsart University, PO Box 1025, Bangkok 10903, Thailand 2 Khan Kacn University, Khan Kaen, Thailand International Water Management Institute (IWMI), Pretoria, South Africa. 4 IRD, Bangkok, Thailand, 5 CSIRO Land and Water, Townsville, Australia. 'Corresponding Author: a.noble@cgiaLorg ALTHOUGH North-east Thailand accounts for onethird ofthe arable land in Thailand, income per capita for the region is no more than 40% of the national average and poverty is at a high level affecting 37% of the population (Matsuo, 2002). A major reason for this high level is the relatively limited natural resources of the region. Inherent fertility of soils is low, water resources scarce and rainfall patterns and distribution erratic (Panichapong, 1988). The soils of North-east Thailand are dominated by sandy, lighttextured soils with low organic matter and low clay (Ragland and Boonpuckdee, 1987). Consequently they have a low water holding capacity, CEC and hence limited buffering capacity against both anthropogenic and natural stresses. Although the annual precipitation is 800-1400 mm, most of it falls during the six-months rainy season and is often erratic and poorly distributed resulting in seasonal drought periods. Before the 1950s the region was dominated by climax Dipterocarp forest. These were highly productive ecosystems characterised by tight nutrient cycling through organic matter and efficient water use. Increased demand for arable land associated with population growth led to indiscriminate clearing of these forests and a decline in the fertility status of the soils and consequently, productivity. Continuous production of export crops such as rice, kenaf (rosella), cassava, and sugarcane has resulted in a rapid decline in the inherent fertility status of these soils, with an associated loss of productivity. Rice is frequently grown in this region on land that has an undulating topography. Initially, farmers establish rice crops on such land at the bottom of depressions and subsequently progress up the slope towards the upland (Limpinuntana, 1988). Rice production in the bottom or lower paddies is considerably more stable than that grown in the upper paddy fields. Accordingly, rice can only be grown in the upper paddy fields for one to three years out of five because of insufficient water for transplanting (Limpinuntana, 1988). These upland soils have been extensively leached and eroded and consequently have a low inherent fertility, low CEC, sandy texture, low water holding capacity, low organic matter content and are acid. Consequently, farmers have moved towards low-input and long-duration crops such as cassava and kenaf. To maintain the productivity of soils, farmers traditionally apply cattle manure and composts derived from household waste and leaf litter to both upland 148 and lowland fields along with sparing amounts of inorganic fertil iser. The production of composts predominantly relies on the cycling of plant materials from areas in close proximity to the field and household organic waste products which may not be sufficient to provide adequate levels of nutrients for enhanced productivity. In addition, the effects of these organic amendments may not be long lasting because of rapid mineralisation and therefore need regular routine additions. For more intensive highinput systems on these light-textured soils, farmers have resorted to rehabilitating them by adding locally available termite mound material. This clearly demonstrates the ability of traditional farmer knowledge in perceiving and implementing strategies to address the issue of declining fertility associated with their production practices. These materials are commercially excavated from large mounds that are the products of termite activities (Macrotermes spp.). Farmers will apply up to 7200 t ha" ' to small plots where intensive vegetable production is undertaken. In recent years the dredging of lowland reservoirs throughout Thailand to increase water storage capacity has been undertaken by local councils. The dredged materials, which have relatively high organic carbon and clay contents, become a waste material that requires disposal. These materials are often used in the construction industry as backfill. Recently, farmer groups have investigated the potential role of these dredged materials in improving the productive capacity of their soils. Their main attraction is that they are relatively cheap (US$I.Oo-3.00 per 6 t truck load, which is significantly cheaper than any chemical fertilisers with equivalent nutrient levels). This practice has rapidly expanded and is generally confined to areas close to the source. However, this practice can damage the soil if the dredged material contains high levels of iron pyrite (acid sulphate). This produces acid in an oxidising environment. In addition, once the process of dredging ceases there will no longer be a supply of this resource. As farmers traditionally recognise the value of clay materials in restoring the nutrient and water holding capacity ofdegraded soils, a possible improvement to current practices is the use of high-activity clays. Bentonites are naturally occurring 2: I layer silicate clays that have a high permanent negative charge due to isomorphous substitution that occurred during formation. As a result of this, they have a high CEC which is often dominated by essential cations such as Ca Mg 2+. When bentonites are added to soils they are able to increase the nutrient holding capacity of the soils and therefore reduce potential losses of nutrients through leaching (Noble et al. 2000). The focus of the current study is on quantifying changes in surface charge characteristics associated with changed land management under two contrasting cropping systems: rice and cassava. Through the construction of charge fingerprints and the subsequent use of the Saturation Index (Su) (Noble et al. 2000) an estimation of the degree of charge diminution that the soils had undergone from a 'benchmark' state, in this case remnant Dipterocarp forest, can be achieved. A field trial was established at Chiang Yuen, North-east Thailand, to evaluate selected strategies that are currently used by farmers to remediate declining productivity, including the use of high activity clay as a soil amendment. Materials and methods Assessment of degradation Six paired sites covering both upland and lowland cropping systems in North-east Thailand were selected for subsequent analysis (Table I). The selection of sites was based on the criteria of: • the existence of Dipterocarp forest in close proximity to an agricultural production system • a well defined boundary separating the two production areas • the same soil type in both areas • little topographical difference (i.e. slope) between the two areas. Samples were collected at five points in each area along a transect at right angles to the boundary scparating the two systems. Sampling points were 5 m apart and at each point three augur holes (10 cm diameter) were made and soil samples collected at depths of 0-1 0, 20-30, and 50-70 cm and bulked to form depth-specific, composite samples. Three ofthe sites were from cassava-based systems (C l-C3) and three from rice-based systems (R I-R3).