Soil erosion on sloping oil palm land in Malaysia: where cover crops work, and where they need help

Malaysia has little flat, premium agricultural land left for oil palm expansion. Growth in planted area over the past two decades has relied increasingly on marginal land: hill slopes, degraded logged areas, and catchment fringes that present significantly greater challenges for sustainable soil management than the alluvial flats and gentle undulations where the industry built its foundation. As more of Malaysia's oil palm area grows on steeper, more erodible terrain, soil erosion has shifted from a secondary management consideration to a major threat to plantation productivity.

Bare soil on slopes sheds a large share of rainfall as surface runoff and loses far more topsoil than covered ground, and that topsoil carries nitrogen, phosphorus, potassium and organic matter into waterways and drainage channels. Plot studies show how large the difference can be. In a young rubber plantation on sloping ground in Côte d'Ivoire, sowing a Pueraria legume cover in the inter-row reduced runoff by 88% and soil loss by 98% compared with bare soil, measured on 1 m² plots over three years; adding logging residues as well almost eliminated runoff and soil loss (Perron et al., 2024, Science of the Total Environment 913: 169335, https://doi.org/10.1016/j.scitotenv.2023.169335). Small plots tend to show large relative reductions, and rubber is not oil palm, but the direction of the effect is clear.

The economic cost of topsoil loss

Topsoil holds most of a soil's organic matter and a large share of its nitrogen and phosphorus, built up over decades. Every tonne that erodes takes those nutrients with it, and they have to be replaced with extra fertiliser. Over a full crop cycle, persistent erosion thins the productive topsoil, reduces the rooting depth available to the palm and raises the cost of keeping the soil fertile compared with well-protected sites.

Erosion also carries sediment into rivers, irrigation channels, and mill water treatment systems, creating downstream costs and potential regulatory liability. For RSPO-certified operations, erosion control is a specified certification requirement, and evidence of unchecked slope erosion risks audit failure and loss of premium market access.

How ground cover reduces erosion: the physics

Ground cover plants reduce soil erosion through three complementary mechanisms. The first is raindrop interception: plant leaves, stems, and litter intercept raindrops before they strike the soil surface, dissipating kinetic energy that would otherwise detach and splash soil particles. On bare soil, falling raindrops create micro-craters, detach soil particles, and create a surface seal that reduces infiltration: initiating the runoff that carries detached particles downslope.

The second mechanism is root reinforcement of soil aggregates. Plant roots physically bind soil particles, increasing resistance to detachment and transport. Root channels also create preferential infiltration pathways: each root represents a pore that moves water vertically through the soil rather than allowing it to flow horizontally across the surface. The root mats of well-established leguminous cover crops create a dense network of these infiltration channels in the topsoil layer that surface runoff would otherwise carry away.

The third mechanism is litter decomposition and aggregate stability. Cover crop residues decomposing on the soil surface produce humic acids and polysaccharides that bind soil aggregates: the crumb structure visible in well-managed soils. Stable aggregates resist detachment under rainfall impact far better than dispersed, structureless soil. Aggregate stability tends to improve the longer a leguminous cover crop is maintained, creating a progressively more erosion-resistant topsoil over time.

Why leguminous cover crops suit erosion control

Grass covers, natural vegetation regrowth, and physical terrace structures all reduce erosion relative to bare soil, but leguminous cover crops also fix nitrogen and suppress weeds, so they do more than hold the soil in place. Mucuna bracteata can reach full interrow ground cover within about 6 months: Samedani et al. (2015) recorded about 100% ground cover at 6 months from 680 seedlings/ha planted at 1.5 m spacing on mineral soil. A full MB cover also suppresses weed regrowth that would otherwise require herbicide application or manual weeding: two operations that disturb soil structure and create bare patches vulnerable to erosion. MB's dense vine canopy provides continuous raindrop interception year-round, including during Malaysia's most intense rainfall months.

Pueraria javanica establishes more slowly than MB but tolerates a wider range of soil conditions, including the strongly acid, nutrient-poor soils found on degraded slopes where establishment of faster-growing species can be difficult. On sites where initial soil conditions are too poor for rapid MB establishment, PJ's lower nutritional requirements and tolerance of acid soils make it a reliable erosion protection option in the early years while soil improvement measures take effect.

Calopogonium mucunoides establishes quickly and gives early ground cover: PROSEA 11 (Chen Chin Peng and Aminah, 1997) reports that it covers the soil 3 to 6 months after sowing. Tropical Forages notes that it is not tolerant of heavy shade and is suppressed as the canopy closes, so it is mainly an early-cover species. For older stands under a closing canopy, Calopogonium caeruleum is the more shade-tolerant option: Tropical Forages reports 1 to 1.5 t DM/ha a year under mature oil palm at 10% light, compared with 3 to 7 t in the open.

In combination, these species cover most of the slope, shade and soil conditions found in Malaysian oil palm plantations. Using Seed Activator at planting can help achieve more uniform germination, reducing the window of vulnerable bare ground in the early post-planting period when erosion risk is highest.

Soil health building alongside erosion control

Erosion control and soil health building are two outcomes of the same cover crop system. While the physical ground cover protects the existing topsoil, the nitrogen fixed by leguminous species builds the organic matter pool that improves aggregate stability long-term. SoilBoost EA, a leonardite humic acid soil conditioner, can be applied to the interrow as part of the same soil programme to support soil structure and organic matter cycling.

Erosion management is not a one-time investment. Cover crops require maintenance: periodic trimming to prevent competition with palms, reseeding in areas damaged by harvesting traffic, and monitoring for patchy establishment on the most challenging slope positions. The return on this maintenance investment is measured not just in the erosion prevented but in the soil organic matter accumulated, the nitrogen supplied through biological fixation, and the long-term productivity preservation of the site.

Sources: Perron, T. et al. (2024), Science of the Total Environment 913: 169335; Samedani, B. et al. (2015), International Journal of Agriculture and Biology 17(2): 251-260; PROSEA 11, Auxiliary plants (Chen Chin Peng and Aminah, 1997); Tropical Forages (CIAT and ILRI, 2020), www.tropicalforages.info.