Living mulch vs bare soil: the soil moisture data Malaysian planters need to see

Living mulch vs bare soil: the soil moisture data Malaysian planters need to see

The Malaysian plantation sector has a complicated relationship with water. Annual rainfall in the major production states is high, which sounds like a planner's ideal. But the distribution is uneven, El Nino years bring extended dry spells, and the interval between useful rain events during dry periods can stretch to several weeks in some production zones. What happens in the root zone during that interval determines whether a mature palm maintains production or enters moisture stress that depresses bunch production in the following season.

The evidence points to one low-cost, scalable response: keeping the soil covered. Field studies agree that mulch reduces soil evaporation. The management question is which species to plant and how to combine them with complementary soil amendments for better root zone moisture retention.

The Malaysian drought risk that most planters underestimate

Mature oil palm uses a large amount of water through evapotranspiration, which leaves little buffer during El Nino dry periods when monthly rainfall can stay low for extended periods. Under bare soil conditions between palm rows, solar radiation drives direct soil evaporation that compounds the moisture deficit, and the topsoil that feeds the palm's surface roots dries out first. Under mulch cover, more of that moisture is retained.

The critical distinction is between soil evaporation and plant transpiration. Cover crops do transpire, which consumes water. But the net soil moisture effect of a dense cover crop canopy is positive under dry conditions because direct soil evaporation, which is high and largely unproductive, is replaced by canopy-controlled transpiration, which is linked to stomatal regulation and photosynthetic productivity.

How soil evaporation works under different cover conditions

Bare soil exposed to direct solar radiation loses water through an uncontrolled evaporation gradient. The drying front moves downward from the surface, creating a dry surface layer that paradoxically insulates against further evaporation for a period, but at the cost of eliminating productive moisture in the feeder root zone. Surface cracking on clay soils accelerates this process by opening pathways for moisture escape from deeper layers.

A mulch layer interrupts this evaporation pathway. The mulch surface absorbs radiation and returns it as sensible heat rather than driving subsurface moisture evaporation, and the soil below stays cooler, which further slows evaporation. A 2024 review of 58 field studies found that mulching reduced soil evaporation in every study, with reductions ranging from 17% to 79%; straw mulches averaged about 30 to 35% and plastic mulches about 38 to 47% (Ramos, Darouich and Pereira, 2024). Those studies measured crop residue and plastic mulches laid on the soil, not living cover crops, so they show the direction and rough scale of the effect rather than a figure for a cover crop canopy. Even so, a reduction of that order is not a marginal agronomic benefit: it changes the water balance of the root zone.

What the evapotranspiration data shows

Total evapotranspiration in a plantation is the sum of palm and cover transpiration plus evaporation from the soil surface. The soil evaporation component is largest under open canopy conditions in young plantings and falls as the palm canopy closes. In the immature phase, when the oil palm canopy does not yet provide significant interrow shading, bare interrow soil evaporation can represent a substantial loss of applied irrigation water and rainfall.

Mucuna bracteata is one of the faster options for closing the interrow canopy: in a UPM trial on mineral soil, MB planted at 680 seedlings/ha reached about 100% ground cover at 6 months (Samedani et al., 2015), although cover develops much more slowly on deep peat (MPOB TT No. 501). Once at full cover, the Mucuna canopy shades the interrow soil and greatly reduces direct soil evaporation, converting much of that water use into biologically productive transpiration within the cover crop system. The leaf litter accumulation beneath the canopy adds a second evaporation barrier at the soil surface, extending moisture retention benefits through dry periods when the above-ground biomass may senesce.

On soils with poor water holding capacity, cover crop mulch alone may be insufficient to bridge extended dry periods. This is where Rootlife provides a complementary function. Rootlife is designed to add a water-retaining organic matrix to the root zone, to help extend the period of available moisture between rain events. The combination of surface mulch cover reducing evaporation and a subsurface moisture reservoir extending retention addresses the soil water balance from both directions.

Cover crop selection for maximum moisture conservation

Ground cover speed determines how quickly the mulch benefit is established after planting. Mucuna bracteata seeds are a strong primary choice when the establishment window aligns with the wet season onset; on mineral soil, MB has reached about 100% cover 6 months after planting (Samedani et al., 2015).

Where Mucuna bracteata has limitations, such as steep terrain or partial shade zones, Pueraria javanica seeds and Calopogonium mucunoides seeds provide alternative ground cover options. Calopogonium mucunoides gives quick early cover but is short-lived and is the first species to be shaded out in mixtures (PROSEA 11). For zones under partial shade or a developing canopy, Calopogonium caeruleum is the better fit: Tropical Forages describes it as one of the most shade-tolerant tropical legumes.

For litter accumulation and long-term mulch depth, a well-established Mucuna bracteata stand builds up a substantial layer of leaf litter over successive seasons, which adds to moisture insulation during extended dry events.

Rootlife as a complementary soil moisture reservoir

Cover crop mulch addresses the above-ground evaporation pathway. Soil water holding capacity addresses the below-ground storage problem. These are separate mechanisms that work together in an integrated moisture management programme.

Sandy soils and degraded mineral soils common in some Malaysian plantation areas have low organic matter content and correspondingly low water retention. Raising soil organic matter generally increases the soil's available water capacity, particularly on sandy soils, which extends the period of available moisture between rainfall events.

Rootlife is designed to add organic matter to the root zone. Applied at planting or as a soil incorporation treatment, it is intended to help build the moisture-retaining matrix that complements the mulch layer above. The combination programme (cover crop mulch at the surface plus Rootlife in the root zone) addresses both evaporation reduction and storage capacity in a single management system.

For planters assessing El Nino risk across their blocks, the cost-benefit calculation is straightforward. Cover crop seeds and soil amendment application costs are fixed inputs. The moisture retention benefit repeats across every dry period for the life of the planting without additional inputs, provided the cover crop stand is maintained. Against the cost of a single stress event that depresses FFB production for months afterwards, the insurance value of the moisture management programme is clear.

Sources

• Ramos, T.B., Darouich, H. & Pereira, L.S. (2024). Mulching effects on soil evaporation, crop evapotranspiration and crop coefficients: a review aimed at improved irrigation management. Irrigation Science 42: 525-539. https://doi.org/10.1007/s00271-024-00924-8
• Samedani, B., Juraimi, A.S., Rafii, M.Y., Sheikh Awadz, S., Anwar, M., & Anuar, A.R. (2015). Effect of cover crops on weed suppression in oil palm plantation. International Journal of Agriculture and Biology, 17(2), 251-260.
• Hasnol Othman, Farawahida Mohamad Darus & Zulkifli Hashim (2012). Best management practices for oil palm cultivation on peat: Mucuna bracteata as ground cover crop. MPOB Information Series 588 / MPOB TT No. 501. Malaysian Palm Oil Board.
• Chen Chin Peng & Aminah, A. (1997). Calopogonium mucunoides Desv. In: Faridah Hanum, I. & van der Maesen, L.J.G. (eds), Plant Resources of South-East Asia No. 11: Auxiliary plants (PROSEA 11), pp. 84-86.
• Tropical Forages: an interactive selection tool (CIAT and ILRI, 2020), Calopogonium caeruleum fact sheet, www.tropicalforages.info