The underground network your oil palm cannot live without

Most agronomic attention in oil palm management focuses on what happens above the soil surface: bunch counts, leaf area index, pest pressure, harvest intervals. Below the surface, a biological service runs continuously that is responsible for a substantial share of the phosphorus that reaches the palm. This service is provided by arbuscular mycorrhizal fungi (AMF), and decades of high-input management in Malaysian and Sumatran plantations has quietly degraded it, often without any single clear symptom pointing to the loss.

Why oil palm roots cannot feed themselves

Oil palm roots are structurally different from most crop species in a way that makes the AMF relationship not a bonus but an architectural necessity. Oil palm roots are thick, coarse, and lack root hairs. Root hairs are the primary surface-area-multiplying structure that most plant species use to access phosphorus in the soil solution. Without them, the effective absorptive surface of an oil palm root system is a fraction of what it would be in crops such as maize or wheat.

AMF compensate for this structural limitation by extending hyphal networks into soil volumes that roots cannot reach. A single gram of AMF-colonised soil can contain metres of fungal hyphae, accessing phosphorus pools several centimetres beyond the depletion zone that forms around actively absorbing roots. Inoculation studies with oil palm seedlings have reported better phosphorus uptake in colonised seedlings than in uninoculated controls. For a nutrient as mobility-limited as phosphorus, this additional reach is not marginal, it is foundational.

The AMF-fertiliser conflict

The relationship between AMF colonisation and conventional fertiliser programmes is antagonistic in ways that most fertiliser recommendations do not account for. When phosphorus is abundant in the soil solution, as it is following heavy P fertiliser applications, the plant has reduced physiological incentive to maintain costly symbiotic relationships. The carbon cost of supporting AMF hyphal networks is substantial, and plants regulate this investment based on P availability signals. High soil P suppresses AMF colonisation intensity.

A molecular study in Jambi, Sumatra (Edy et al., 2022, Frontiers in Plant Science 13: 1004097) found a strong reduction in AMF species richness in oil palm and rubber roots compared with rainforest roots; the authors suggest fertiliser and pesticide use as likely contributors, but the study did not test fertilisation or measure colonisation. A meta-analysis of published AMF inoculation trials (Wu, Chen and Wang, 2024, BMC Plant Biology 24: 960, doi:10.1186/s12870-024-05638-9) found a mean 27 percent increase in plant phosphorus concentration and a 105 percent increase in whole-plant phosphorus uptake across a range of crops, with responses substantially higher in laboratory than in field experiments, and in legumes than in non-legumes; it is not specific to oil palm.

How soil management affects AMF populations

AMF populations are sensitive to several features of conventional plantation management beyond fertiliser application rates. Tillage severs hyphal networks and reduces spore populations in the rooting zone. Fungicide applications with broad-spectrum activity damage AMF alongside pathogenic fungi. Soil pH extremes reduce AMF diversity and colonisation potential. Soil organic matter level is positively correlated with AMF spore abundance because AMF rely on organic carbon compounds both as structural materials and as signals from host plants routed through the rhizosphere.

Cover crops play a critical supporting role in maintaining AMF populations between palms. Leguminous covers including Mucuna bracteata, Pueraria javanica, and Calopogonium mucunoides are AMF host plants. By maintaining live root systems year-round in the inter-row, these covers sustain AMF spore populations, hyphal networks, and host-plant signalling that keeps the fungal community active and ready to colonise palm roots extending into that zone.

Restoring the biological partnership

Restoring AMF function in heavily managed plantations requires reducing suppressive inputs where possible and creating soil conditions conducive to AMF activity. A humic soil conditioner such as SoilBoost EA may contribute by adding organic matter and supporting soil aggregation, conditions associated with stable hyphal networks and spore survival. Humic substances have also been proposed to act as signalling compounds in the rhizosphere, which may influence the chemical dialogue between AMF hyphae and root exudates; this has not been tested for SoilBoost EA in oil palm.

Where soils have been under continuous high-input management for many years, inoculation with AMF at the nursery stage for replanting cycles is increasingly considered in progressive plantation management. Seedling inoculation studies that report better phosphorus uptake in colonised seedlings provide the agronomic reasoning for this investment; confirm the response in your own nursery before scaling up.

What this means for your fertiliser programme

The practical implication of AMF biology for fertiliser management is not to reduce P applications arbitrarily but to distinguish between situations where AMF suppression is the limiting factor and situations where soil P is genuinely inadequate. Estates with low foliar P despite adequate soil P application are candidates for AMF investigation. Periodic reduction in P application rate to test whether AMF activity recovers, combined with leaf analysis monitoring, is a legitimate diagnostic strategy. Cover crop establishment in all inter-row spaces is the lowest-risk starting point for restoring the biological service that oil palm roots depend on structurally.