Improving water and nutrient management for double cropping in Cambodia

Growing dry-season crops after rice on hard-setting soils can pose serious challenges to Cambodian farmers seeking to diversify and intensify with greater efficiency, profitability, and sustainability.

Fried frog legs and duck tongues are still sizzling on the hot plate plonked down in front of me. My Cambodian dinner acquaintances smile broadly. I sometimes question whether the serving of these local delicacies is an act of hospitality or the blunt end of a wry sense of humor testing the reaction of foreigners trying hard not to offend.

Cambodians love their food and love eating it with company. No one should eat alone, they say. It’s not surprising they appreciate sitting down with friends, family, and workmates to multiple plates piled with steaming greens, succulent meats, and a hefty serving of rice, as the horrific past of hunger, community dislocation, and nationwide devastation lingers in the not-too-distant past (see Revisiting the “Killing Fields”).

Rice culture and cultivation
The staple food of Cambodians is rice. Only a few kilometers out from the capital city of Phnom Penh are verdant rice paddies offering a tranquil reprieve from the city’s hustle and bustle.

Rice-based farming systems have been the mainstay of rural livelihoods in Southeast Asia for centuries. However, trends suggest an increasing diversification and intensification¹ within these systems, which are opening up new opportunities for farmers as well as highlighting existing production constraints.

Puddling or wet cultivation is the traditional method of land preparation for establishing rice─a practice that destroys soil aggregates, breaks the capillary pores through which water enters, disperses clay particles, and lowers soil strength in the puddled layer.^2–4 Repeated puddling leads to the formation of a dense layer of soil or hardpan below the topsoil layer that helps retain water in flooded rice fields. The hardpan has high bulk density, which reduces water loss through drainage and allows the field to maintain standing water required in growing the rice crop.⁵

Going beyond rice
In continuous rice culture, the formation and maintenance of the compacted lower soil layer that reduces water infiltration can be considered an investment in infrastructure. In the rice–nonrice system, however, the destruction of surface soil structure and the development of the hardpan can impose serious liabilities on the establishment and performance of crops grown after rice.

In Cambodia, hard-setting soils are widespread and strongly influence crop production during the dry season as these tend to limit the retention, movement, and plant use of water and nutrients.⁶ This is due to two contrasting unfavorable physical conditions at different water contents, namely, the complete breakdown of large, air-dry soil aggregates into microaggregates with sudden immersion in water (slaking)^7,8 and an extremely hard structureless mass formed during drying⁹, with a positive correlation between hard-setting and bulk density.¹⁰

Irrigation or rainfall after sowing can cause the soil surface to collapse while the subsequent drying may harden the surface, preventing the seedlings from emerging11 or impeding the root growth of established plants.⁹

Growing dry-season crops after rice on hard-setting soils can pose serious challenges for farmers in Cambodia. Further understanding how land formation techniques (leveling, grading, and raised-bed construction) for improved water and nutrient management and efficiencies affect soil structure and the behavior of nutrient and water dynamics is required to develop refined and integrated management practices and realize the potential of high-value non-rice crops.

Dry-season farm productivity
The Australian Centre for International Agricultural Research (ACIAR) project Improving water and nutrient management to enable double cropping in the rice-growing lowlands of Lao PDR and Cambodia is seeking to increase dry-season farm productivity through irrigation design, better use of residual soil moisture, and clever nutrient strategies. A multi-year maize trial conducted on a hard-setting lowland Prateah Lang soil with the Cambodian Agricultural Research and Development Institute (CARDI) is investigating soil-water-plant dynamics with irrigation frequency, volume, and application method on soils amended with agricultural residues (rice straw, cattle manure, and biochar).

Land leveling, grading for slope, and raised-bed construction are used to improve water and nutrient management and efficiencies. Leveling can assist in water application as field gradients can reduce waterlogging in poorly drained soil types.¹² Crops grown on beds and furrow irrigated may experience slow wetting through capillary action, thereby reducing soil aggregate disruption.^6,13 However, despite land formation to improve irrigation water application, movement, and drainage, these practices risk exacerbating inherent hard-setting tendencies where there has been significant loss of soil organic matter as a result of topsoil removal (resulting in the loss of more labile fractions, which is an indicator of soil productivity and health)¹⁴ or significant disruption of the topsoil has taken place in the land-forming process.

The retention and incorporation of organic amendments in hard-setting soils can be an important factor when land-forming on these soil types. The nutrient- and water-holding capacity of soils can be increased by adding organic amendments,^15,16 thereby enhancing soil fertility and increasing crop productivity.^17,18 Crop residue mulch, livestock manure, and biochar have been found to suppress weeds,^15,19 elevate soil fertility,^20,21 increase crop nitrogen uptake,²² lower soil temperature²³ and tensile strength,²⁴ induce occlusion,²⁵ reduce soil water evaporation,¹⁵ enhance lateral and vertical soil water movement, and improve crop root movement for access to nutrients and water.²⁶

There is no misconception of the challenges that face the agricultural industry in this part of the world as farmers look to new horizons in production, water management, and nutrient strategies. It is therefore important that research on improving water and nutrient management for double cropping in Southeast Asia be undertaken as farmers seek to diversify and intensify with greater resource-use efficiency, profitability, and sustainability.

After a long, hot day in the field, there is no greater reward than sitting down with my Khmer colleagues, reflecting on the good work achieved and discussing the obstacles still to be overcome. The food on the table might be new and exotic; however, in the company of an ever-effervescent group laughing loudly at lost translations and giving detailed explanations of how the food is grown, it is hard not to enjoy the tastes and experiences of Cambodia.

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Ms. Molesworth is a PhD candidate and works at the Centre for Regional and Rural Futures investigating soil-water-crop dynamics in Australia and Cambodia.

References:

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