Rice production in Cambodia faces challenges due to water scarcity and improper fertilizer use, which raise environmental concerns. Integrating biochar and water-saving irrigation methods offers a promising solution for farmers. This study evaluated how biochar and chemical fertilizers affect rice yield, quality, and nutrient loss under different soil types and irrigation methods. A column experiment was conducted using clay (S1) and sandy loam (S2) soils, with two irrigation methods: alternate wetting and drying (AWD) and continuous flooding (CF). The treatments included a control (T0), chemical fertilizer only (T1), chemical fertilizer supplemented with 4 tons per hectare (t ha⁻1) of biochar (T2), and chemical fertilizer supplemented with 6 t ha⁻1 of biochar (T3). Across both soil types and irrigation methods, T3 resulted in the highest dry grain yield and protein content compared to T1. This biochar at 6 t ha⁻1 was effective in both clay and sandy loam soils, though its efficiency varied slightly; for instance, under AWD irrigation, rice yield increased by approximately 35

Cambodia plans to expand its rice sector and become a prominent rice exporter. A key concern is that soil fertility is a crucial factor affecting rice production, and nutrient leaching into the environment can lead to reduced nutrient uptake and lower rice yield. Carbonized waste biochar has gained recognition not only as a potential soil fertility enhancer but also as a significant nutrient leaching reducer. It is currently being introduced in many regions. The study was to evaluate how a combination of chemical fertilizers and rice husk biochar affects nutrient leaching into the topsoil layer and plow sole of soil columns during direct seeding with continuous flooding, and to assess their combined effects on rice growth and yield. In the leachate from these two soil layers, except for ortho-phosphate (PO 4 3− ), the combination of CHEM + BIO2 or + BIO4 treatment (chemical fertilizers + biochar at a rate of 2t ha −1 or + biochar at a rate of 4t ha −1 ) significantly decreased ammonium (NH 4 + ) and nitrate (NO 3 − ) levels more than CHEM alone, particularly in the plow sole, suggesting that their combination and biochar sorption capacity are beneficial for nitrogen use by plants. CHEM + BIO2 had varying effects, whereas CHEM + BIO4 led to a significant increase in rice yield, plant biomass, tiller number, panicle length, grains per panicle, and grain weight per panicle. These findings suggest that incorporating biochar amendments in rice production can reduce N leaching. However, there is no evidence to support its effectiveness in reducing P leaching. Therefore, further studies are needed to determine the usefulness of this approach.

Soil fertilization is one of the top rice crop production issues. Nutrient leaching in rice crop production, leading to lower plant uptake and low yield, poses a challenge for Cambodian farmers and is becoming a key concern for the environment. Carbonized organic waste, called biochar, is known as a potentially valuable input to enhance soil properties. It has been introduced in many regions. A soil column-based experiment was conducted to evaluate the effect of biochar on the leaching of N and P and rice growth in link with soil structure. Two types of columns were built, using disturbed and undisturbed soil. Four rates of nutrient input including chemical fertilizer, chemical fertilizer + 2 t/ha of biochar, chemical fertilizer + 4 t/ha of biochar, and control were applied to the plantation of rice cultivar (locally named Sen-Pidor) in both conditions. Leachate (NH4-N, NO3-N, and PO43−) and rice growth were collected at 7-day intervals, while the grain yield and biomass of plant were collected at the mature stage. Our primary results showed that the leachate and rice growth were not significantly different between both conditions. However, the leaching of NH4-N and NO3-N in the column with chemical fertilizer + 4 t/ha of biochar was lower than the column with chemical fertilizer, while PO43− leaching was the same measured from both rates. Remarkably, the plant height was the highest under the disturbed condition with chemical fertilizer + 2 t/ha of biochar, whereas under the undisturbed condition it was the highest with chemical fertilizer + 4 t/ha. In addition, biochar amendment at the rate of 4 t/ha enhanced rice yield by 32.17% in comparison with the column using chemical fertilizer, and by 52.77% in comparison with the control. Additionally, biochar amendment at the rate of 4 t/ha had a great impact on the biomass of plants compared to the column without biochar contact. Our results indicate that biochar amendment has the potential to minimize N leaching, but not P leaching, while enhancing rice yield and biomass of plant.

Hand-held hoses and watering cans are widely used by smallholder farmers to irrigate vegetables in Cambodia and Laos. Overwatering is common. Technology change (e.g., low-pressure drip irrigation) has been used to improve irrigation efficiency but can be unaffordable for many smallholder farmers. The purpose of this study was to identify an appropriate method of predicting crop water demand, develop and field-test improved irrigation schedules for smallholder leafy vegetable farming based on that method, and then develop extension tools to communicate the schedules to smallholder farmers. Improved irrigation schedules for leafy vegetables were developed based on a crop water use prediction technique that is well established (the Penman–Monteith method) but beyond the capacity of smallholder farmers to implement without access to simple aids. Compared to conventional practice, the method approximately halved water and labour use and improved irrigation water productivity 2–3 fold in field research and demonstration trials. Simplified extension tools to assist smallholder farmers with practice change were developed. This work showed that significant efficiencies could be gained through improved irrigation scheduling without changing application technology.

Land development is rapidly occurring on sand-dominant soils that cover substantial areas of the Lower Mekong Basin (LMB). Sands are at risk of degradation on sloping uplands where agriculture is expanding and on lowland landscapes where intensification of cropping is occurring. Sandstone and granitic geology explain the prevalence of sand-dominant textures of profiles in the LMB. However, the sand terrains in uplands of Cambodia and Southern Laos mostly have not been mapped in detail and the diversity of their edaphic properties is poorly understood. On high-permeability sands, lowland rainfed rice crops are drought-prone, while nutrient losses from leaching are also a risk. Furthermore, waterlogging, inundation and subsoil hardpans are significant hazards that influence the choice of crops and forages for lowland soils. Soil acidity, low nutrient status, hard-setting and shallow rooting depth are significant constraints for crops and forages on sands in the lowlands. Land use change in the lowlands to alternative field crops and forages on sands is contingent on their profitability relative to rice, the amounts and reliability of early wet season rainfall, and the amounts of stored water available after harvesting rice. Low soil fertility and soil acidity are limitations to the productivity of farming systems on the sand profiles in uplands, while erosion, low soil organic matter levels and water balance are concerns for their sustainable use. Site-/soil-specific fertilizer and lime management, land suitability assessment and the use of conservation agriculture principles (minimum tillage and crop residue retention) can overcome some of these constraints.

Growing vegetables after rice harvest allows Cambodian farmers to use land that would otherwise be unproductive between rice crops. Producing vegetables on these soils is limited by low soil pH, low cation exchange capacity and limited nutrient retention capacity. Soil pH in the top 20 cm is generally low (pH 5.5 H2O) and may limit the availability of nutrients. Farm-based trials in Siem Reap and Kampot provinces assessed the effect of lime and fertiliser on leafy vegetable crop growth and yield. At lime-only sites, lime was applied at rates of 0.5, 1.0 and 2.0 tonnes per hectare (t/ha) in conjunction with farmer practice fertiliser rates. For sites with lime and fertiliser treatments, combinations of farmer practice and optimal fertiliser rates, no lime and 2.0 t/ha of lime were applied. Two consecutive crops were planted at one site to examine the residual effect of lime on soil pH and crop yield. At lime-only sites, all crops responded to lime application with yield increases of up to 100%. For sites that assessed combinations of lime and fertiliser, the treatment of lime and optimum fertiliser rates showed the highest yield increase (92%). Application of 2.0 t/ha lime increased soil pH by approximately 1.0 unit. This effect was still evident after a second crop of Bok Choy. For the 0.5 t/ha lime treatment, an initial soil pH increase of 0.4 units had reduced to 0.2 units after the second crop. The first crop yield was higher than the second crop yield. Long-term field trials are needed to examine residual lime effects.

AbstractThe area of dry-season rice (Oryza sativa L.) has rapidly increased in Cambodia owing to the large-scale development of irrigation infrastructure. But little is known of potential productivity and adaptive crop management. The objective of our study was to evaluate potential yield and nutrient requirements of dry-season rice in Cambodia, and the economic feasibility of soil-specific management recommended by the government. Field experiments were conducted on four soil types (Bakan, equivalent to Alfisol; Krakor, Inceptisol; Prateah Lang, Plinthustalfs; and Toul Samroung, Endoaqualfs) in four provinces (Battambang, Kampong Thom, Pursat, and Siem Reap) during the 2016 and 2017 dry seasons to compare 14 (2016) and 8 (2017) N-P-K combinations. Grain yield ranged from 1.0 to 5.5 t ha−1 in 2016 and from 1.3 to 6.7 t ha−1 in 2017. Potential yield from the experiments was 6–7 t ha−1 on Toul Samroung soil, 5–6 t ha−1 on Bakan soil, and 3–5 t ha−1 on Prateah Lang and Krakor soils. A rate of 140-60-60 kg ha−1 of N-P2O5-K2O was more than enough to achieve the best yields on any soil group. On the other hand, modest application rates in soil-specific management (44–78 kg ha−1 of N, 23–28 kg ha−1 of P2O5, 0–30 kg ha−1 of K2O) proved reasonable for resource-poor farmers in Cambodia, since the treatment always provided >75 % of the highest economic profit in high-input plots.

Abstract Rice is widely grown in rainfed lowlands during the wet season in the Mekong region. Limited nutrient availability is a common constraint on crop yield, and the optimal rate of fertilizer application depends on the soil type. The objective of our study was to evaluate rice productivity and the economic feasibility of various nutrient management regimes in Cambodia. We conducted field experiments on three soil types (Prey Khmer, Prateah Lang, and Toul Samroung, equivalent to Psamments, Plinthustalfs, and Endoaqualfs, respectively) in four provinces (Battambang, Kampong Thom, Pursat, and Siem Reap) during the 2016 and 2017 wet seasons to compare nine (2016) and seven (2017) N–P–K combinations. Grain yield ranged from 0.9 to 4.8 t ha−1 in 2016 and from 1.0 to 5.2 t ha−1 in 2017, depending on soil type and nutrient management. The Prey Khmer soil contained around 80% sand, and rice yield responded most weakly to nutrient management. The moderate fertilizer input in the current soil-specific recommendation was effective on this soil type. However, on more fertile soils with a higher clay content and a higher cation-exchange capacity (Toul Samroung and Prateah Lang), an additional 20 kg N ha−1 combined with adding 15 kg ha−1 of P2O5 or 20 kg ha−1 of K2O significantly increased yield and economic return. Although P and K use during Cambodia’s wet season is uncommon, our results demonstrate the importance of these nutrients in improving the country’s rice production.

Basaltic terrain occupies significant areas of eastern Cambodia and occurs in pockets elsewhere in the north, north-east and north-west. Basaltic soils are likely to be prominent in the development of upland cropping; however, the nature and properties of these soils are poorly understood. In studies in Ou Reang Ov district of Kampong Cham province, a previously undescribed brown gravelly clay loam soil was found to be prevalent. In Ou Reang Ov district it comprises about 13 % of the basaltic terrain. It is morphologically distinct from the rice soils described for basaltic terrain (Labansiek and Kompong Siem Soil groups) and occurs on the slopes of basaltic hills and plateau. The Ou Reang Ov Soil group is a newly proposed member of the Cambodian Agronomic Soil Classification. The Ou Reang Ov Soil group is well drained and unsuited to padi rice. Indeed the gravel content of the soil makes field crops prone to drought. Apart from sub-soil acidity in some profiles, other soil chemical properties were generally favourable. Indeed the extractable P levels on Ou Reang Ov soil were generally high. Kaolin and quartz were the dominant minerals in Ou Reang Ov soil although the soil texture suggests that much of the quartz is in silt and clay size fractions. Smectite clay minerals were prevalent even though this soil does not exhibit cracking at the surface when dry. Although occurring on slopes, the Ou Reang Ov soil is relatively resistant to erosion. Overall it is considered to have fair to good capability for cropping, if drought tolerant crops are selected.

Rice is the most important food security crop in Asia. Information on its seasonal extent forms part of the national accounting of many Asian countries. Synthetic Aperture Radar (SAR) imagery is highly suitable for detecting lowland rice, especially in tropical and subtropical regions, where pervasive cloud cover in the rainy seasons precludes the use of optical imagery. Here, we present a simple, robust, rule-based classification for mapping rice area with regularly acquired, multi-temporal, X-band, HH-polarized SAR imagery and site-specific parameters for classification. The rules for rice detection are based on the well-studied temporal signature of rice from SAR backscatter and its relationship with crop stages. We also present a procedure for estimating the parameters based on "temporal feature descriptors" that concisely characterize the key information in the rice signatures in monitored field locations within each site. We demonstrate the robustness of the approach on a very large dataset. A total of 127 images across 13 footprints in six countries in Asia were obtained between October 2012, and April 2014, covering 4.78 m ha. More than 1900 in-season site visits were conducted across 228 monitoring locations in the footprints for classification purposes, and more than 1300 field observations were made for accuracy assessment. Some 1.6 m ha of rice were mapped with classification accuracies from 85% to 95% based on the parameters that were closely related to the observed temporal feature descriptors derived for each site. The 13 sites capture much of the diversity in water management, crop establishment and maturity in South and Southeast Asia. The study demonstrates the feasibility of rice detection at the national scale using multi-temporal SAR imagery with robust classification methods and parameters that are based on the knowledge of the temporal dynamics of the rice crop. We highlight the need for the development of an open-access library of temporal signatures, further investigation into temporal feature descriptors and better ancillary data to reduce the risk of misclassification with surfaces that have temporal backscatter dynamics similar to those of rice. We conclude with observations on the need to define appropriate SAR acquisition plans to support policies and decisions related to food security.

Siliceous sedimentary formations underlie much of Cambodia, consequently there is a propensity for sandy surface soils. Only the soils fringing the Tonle Sap lake, those of the alluvial plains along the major rivers (especially the Mekong), and soils developed on basalt deviate from the characteristic of sandy soils. Substantial areas of sandy, high permeability soils are used for lowland rainfed rice production. Due to their inherent high hydraulic conductivities, standing water in rice fields of the deep sandy soils drains rapidly after rainfall predisposing rice crops to drought and high rates of nutrient leaching. However, loss of soil water saturation may limit rice yield by inhibiting nutrient uptake more often than drought, per se. Prospects for growing field crops in sandy lowland soils are contingent on the amounts and reliability of early wet season rainfall or on amounts of stored water after harvesting rice. Apart from drought, waterlogging and inundation are significant water-related hazards that influence the growing of field crops in lowland soils. In addition, soil fertility constraints in the early wet season and dry season will likely differ from those encountered by rice due in part to the different soil water regime they encounter. In particular soil acidity, low nutrient status, hardsetting and shallow rooting depth have been identified as significant constraints for field crops. Vast areas of sandy upland soils occur in Cambodia but are only poorly described. Low soil fertility is likely to limit upland farming systems on the sandy uplands and erosion is a concern for their sustainable use. There is a need to hasten the pace of research and resource assessment of these uplands so that land suitability assessment and sustainable farming systems are available to guide the expansion of agriculture in these areas. 1 Office of Soil and Water Sciences, Cambodian Agricultural Research and Development Institute, P.O. Box 01, Phnom Penh, Cambodia 2 School of Environmental Science, Murdoch University, Murdoch, Western Australia 6150 3 Department of Agriculture of Western Australia, BaronHay Court, S. Perth, WA 6151 (April to July) and main wet season (July to October) or the amounts of stored water after harvesting rice. Apart from drought, waterlogging and inundation are significant water-related hazards that influence the growing of field crops in lowland sandy soils (White et al. 1997; Bell and Seng 2004; Bell et al. 2005). In addition, soil acidity and low nutrient status have been identified as significant constraints for crops on sandy soils in Cambodia. Vast areas of sandy upland soils occur in Cambodia but are only poorly described, and at present not extensively used for agriculture. Low soil water storage, and low soil fertility, including soil acidity, are likely to limit upland farming systems on the sandy uplands and erosion is a concern for their sustainable use. There is likely to be pressure to develop agriculture on these sandy uplands over the next 20 years. There is a need to hasten the pace of research and resource assessment of these sandy uplands so that land suitability assessment and sustainable farming systems are available to guide the expansion of agriculture in these areas.

Sandy soils occupy a large proportion of the Cambodian landscape, and improved understanding of these soils is critically important to support agricultural development. This study aims to identify variability in soil particle size distributions and acidity among sandy soils from different parts of southern and eastern Cambodia. Soil samples were collected from different layers at 8 sites within 4 study provinces and analysed for particle size distribution, soil pH and exchangeable Al. Clay and silt were minor fractions and comprised similar amounts at most sites. Clay fractions generally increased at about 1 m depth. Soils at a site close to the beach and sites nearby coarse grained granite mountains contained very high percentages of coarse sand (up to 87 %). At other sites, fine sands were dominant fractions. Very low pHCaCl2 values (< 4) in whole profiles were found at 2 sites in the coastal area, close to the beach and sandstone mountain and at one site in eastern Cambodia. Highest whole profile exchangeable Al (0.44-1.13 cmol/kg) were seen at a site close to sandstone mountain and a site in eastern area.

Upland agriculture is expanding rapidly in Cambodia, but in-depth studies of the soils have been confined to rice soils. In Battambang province, soils associated with limestone have been identified with distinctive properties for non -rice crops. These soils were characterised in Banan district through soil surveying, soil chemical analysis and crop production trials. Alkaline soils on the pediments and gently sloping plains surrounding limestone hills have low crop productivity on account of iron deficiency and alkalinity related constraints. It is proposed that these dark clay soils developed on limestone and related parent rocks be assigned to a calcareous phase of the Kompong Siem Soil group to distinguish them from related soils developed on basalt. The Kompong Siem calcareous phase is a very dark grey coloured soil with a high proportion of carbonate nodules. When it occurs on sites suitable for wetland rice, its color and texture should lead to identification as the Kompong Siem Soil group in the Cambodia Agronomic Soil Classification. However, on hill slopes, Kompong Siem calcareous phase is not suited to rice and hence such soils cannot presently be keyed to Kompong Siem Soil group.

The acceleration of crop diversification in Cambodia in lowlands and in uplands could be facilitated by a process for the assessment of land capability for non-rice crops. Maize, soybean, mung bean, sesame and peanut appear to be the crops of most interest initially for land capability assessment, together with cassava and sugar cane. Land capability was determined for maize, soybean, mung bean, sesame and peanut in Banan district, Battambang province. Limiting factors for crops were identified, and land qualities rated for the soil types identified previously in a soil survey of the district. The main Soil groups on Banan district are Toul Samroung, Kein Svay and Kompong Siem. The calcareous phase of Kompong Siem soils that occurs on the lower slopes of limestone hills has low capability for non-rice cropping with alkalinity and waterlogging being the most likely limiting factors. By contrast, the Kein Svay soil has high capability for non-rice crops especially on the high levee banks. Toul Samroung soil, where it occurs in slightly elevated land within the rice plains, has fair capability for non-rice crops, but it is not recommended for non-rice crops in the main wet season due to inundation risk.