Assessing land suitability for crop diversification in Cambodia and Australia

• Department of Agriculture and Food, Western Australia, Australia
• Cambodia Agricultural Research and Development Institute, Cambodia

Crop diversification can alleviate poverty by increasing income and improving nutrition for farm households. In the lowlands of Cambodia there is potential to diversify from the traditional wet season rainfed rice system by double cropping (with rice and legumes) using the abundant shallow groundwater for irrigation. Similarly, in the uplands, relatively large areas of land are available for the expansion of cropping and could be used to grow field crops and fruit and nut trees. To ensure that crop diversification is effective, information is needed on soil and climate constraints, and potential environmental impacts, to assess the suitability of land for particular crops. The land resource studies and soil maps needed for assessing land suitability are generally not available for Cambodia.

This project aimed to facilitate crop diversification in Cambodia and Australia by assessing land suitability for growing crops such as legumes and field crops in lowland and upland areas of Cambodia respectively, and pulses in southern Australia. In an 18-month extension, the aim was to ensure that the land capability classification developed was robust and therefore could be more widely applied in Cambodia, and to deliver the tools of land suitability assessment to key stakeholders in Cambodia.

In Cambodia, the first task for the project team was to assess the land capability in the study areas, using data on climate and soil constraints, and growth requirements for a range of crops. Scientists used software developed in previous ACIAR projects to combine data from research, land resource surveys, assessment of crop requirements, farmers' and scientists' knowledge and empirical studies, to determine overall land suitability for particular crops. The focus was on food crops that can be used to alleviate poverty and improve nutrition in farm households, with the highest priority assigned to crops already well established in Cambodia, such as maize, soybean and mung bean.
In Australia, project members determined land suitability for pulses by combining the expertise of farmers' practice with technical information from research, taking into account both economic and biophysical factors. Farmer-led pulse growers associations were involved throughout the project and are helping to disseminate the results.

Soil-landscape models were developed for five different parts of Cambodia with contrasting geology and geomorphology:
Banan district, Battambang province - featuring soils on limestone and other sedimentary materials, which are common in northwest Cambodia
Kong Pisei district, Kampong Speu province - featuring soils influenced by mixed sedimentary rocks of the Srang mountains
Ou Reang Ov district, Kampong Cham province - featuring soils on basaltic materials which are common in eastern Cambodia (a new soil group identified for this terrain)
Ponhea Krek district, Kampong Cham province - featuring soils on basaltic materials plus associated sandy materials of the old alluvial plain
Tram Kak district, Takeo province - featuring soils on sandy materials from the Damrei Romneal mountains.
The acquired knowledge from the five districts added greatly to the understanding of upland soils of Cambodia, and provided the first semi-detailed soil-landscape maps of upland areas, supported by soil profile descriptions, plus chemical and mineralogical analysis data. The importance of geology for the properties of upland soils was evident from the present study.
Limiting factors for non-rice crops were rated for eight soils. The most severe limiting factors were: soil acidity, waterlogging, low nutrient retention, low organic matter, surface crusting, low soil water storage and erosion risk. The research team created the first national land resources database in Cambodia describing 194 soil profiles of which 67 were detailed full profile descriptions. Detailed chemical analysis was completed on 37 profiles and mineralogy on 18 profiles. The team captured additional soil profile records from previous projects, making a total of 259 profiles in the national database, of which 138 are full profile descriptions. The database, held and maintained by CARDI. has the capacity to allow data entry in Khmer, also to produce soil profile reports in Khmer using the in-built lexicon of soil terms in Khmer.
Relative performance of maize, soybean, peanut, mungbean and sesame was determined across eight soils in two seasons and two years. Peanut was the most reliable crop across all soils and seasons of planting, largely due to reliable emergence and drought tolerance. Soybean was very poor in the early wet season due to drought. Overall, DSSAT (The Decision Support System for Agrotechnology Transfer) modelling simulations suggested that yields were lower for the peanut sown in the early wet season from March to April than for the crops sown from May to July, and the yields tended to decline when the crop was sown after mid-August. Sowing peanut after mid-August likely results in water stress at the time when the crop reaches anthesis in September/October and thus yield reduction occurs due to the significant decline of rainfall from mid-October.
For soybean and maize, the simulations suggested that average yields were lower for the crops sown in the early wet season from March to May than for those sown from June to July, and the yields tended to decline when the crop was sown after mid-August. Sowing in mid-July was most reliable for simulations based on three years' rainfall. In the early wet season, simulations indicated that yield potential was as high as the main wet season, but only achieved when rainfall distribution was favourable. Conversely, low rainfall in the early wet season (as occurred in 2002) produced very low yields of maize and soybean.
Rainfall analysis indicated that only seven out of 41 sites across Cambodia had recorded average rainfall within the range of annual rainfall values reported previously. March rainfall in Cambodia appeared to be a strong predictor of the total amount of rain during the rest of the year for most locations with sufficient rainfall records to analyse.
A methodology for land capability classification was introduced to Cambodia for use with non-rice crops. It is flexible enough to be expanded over time to include other crops including fruit trees. The team ranked soils according to relative crop yields: Kein Svay and Kompong Siem soils > Toul Samroung > Prey Khmer, Ou Reang Ov and Kompong Siem calcareous > Labanbsiek > Prateah Lang. Rankings varied for individual crops. The team identified significant variability in soil acidity within four soil groups - Prey Khmer, Prateah Lang, Labansiek, and Ou Reang Ov and it concluded that the variable occurrence of soil acidity stands was a serious barrier to the development and adoption of productive and sustainable cropping systems on these soils in Cambodia.
Profitability of the crops was estimated using crop yields obtained in the on-farm trials. Break even yields were: 2 t/ha for maize; 0.4-0.5 t/ha for mungbean; 0.8-1.0 t/ha for soybean; 1.2 t/ha for peanut. Early wet season cropping was unlikely to produce profit at the yield levels obtained except with peanut. Main wet season cropping was much more profitable because of higher yields, and thus there were more crop options on most soils.
Maps of land capability produced for field peas, chickpea, lentil and faba beans identified areas not previously targeted for pulses development in Western Australia. The maps have already been adopted within the pulses program at Department of Agriculture to target areas for its pulses R&D. The methodology used for mapping capability was subsequently used within the agency to predict effects of climate change on major grain commodities, and will be used in a new GRDC project on 'Break crops' (oilseed, pulses, oats, lupins) in rotations.