Mycorrhizae and phosphorus in Philippine acid soils

Mycorrhizal fungi help plants reach phosphorus that is locked up in acid soils, and that matters because most Philippine plantation soils are acidic and fix phosphorus tightly. Arbuscular mycorrhizal fungi (AMF), also called vesicular-arbuscular mycorrhizae (VAM), colonise plant roots and extend fine fungal threads far into the soil, reaching phosphate the root hairs cannot. In low-phosphorus acid soils, this partnership is one of the main routes by which plants stay fed without piling on more fertiliser. Here is what the research shows and what it means for your block.

Why is phosphorus a problem in acid soils?

Phosphorus is hard to access in acid soils because it binds to iron and aluminium and becomes unavailable to roots. When you apply phosphate fertiliser to a strongly acidic soil, much of it does not stay in solution: it reacts with iron and aluminium and turns into forms the plant cannot easily take up. So a soil test can read low available phosphorus even when total phosphorus is not low. The phosphorus is there, it is just chemically tied down. This is the everyday reality on the acid soils common across Mindanao and the uplands.

How do mycorrhizae increase phosphorus uptake?

Mycorrhizae increase phosphorus uptake by extending the root system with fungal hyphae and by mobilising phosphorus that roots cannot reach on their own. In two tropical acid, phosphorus-fixing soils planted with micropropagated oil palm, growth responses to phosphate fertiliser were low unless the plants were inoculated with mycorrhizae, and inoculation made the plants use the applied phosphate much more efficiently (Blal et al. 1990). The fungal hyphae are far thinner than roots and reach into pores and soil volume the roots never touch, then deliver phosphate back to the plant in exchange for carbon.

A mesocosm study points to a second mechanism: the fungi, with their accompanying microbes, released organic acids that mobilised phosphorus bound to iron oxides, freeing phosphate that was otherwise locked to iron (Andrino et al. 2021). In other words, in that greenhouse study the fungi and their accompanying microbes also appeared to release some of the phosphorus bound to iron oxides. Both effects point the same way: more of the soil's existing phosphorus reaching the crop.

Do cover crops support mycorrhizae?

Cover crops support mycorrhizae by keeping living roots in the ground, which is what these fungi need to survive. AMF are obligate symbionts: they cannot live without a host root. Bare, fallowed soil starves the fungal network, while continuous living cover from a legume like Pueraria javanica or Calopogonium keeps host roots present year-round and the fungal population active. A healthy cover-crop stand is therefore not just nitrogen and ground protection, it is also a way to keep the soil's mycorrhizal community alive between or beneath your main crop.

Note the naming point here, because the host species matters: Pueraria javanica is a managed tropical legume, not the invasive kudzu (Pueraria montana). It is grown deliberately as a cover and green manure across Southeast Asian plantations.

What can I do with this on my farm?

Treat mycorrhizae as part of your phosphorus strategy, not a replacement for it. The practical moves are: keep living roots in the soil with cover crops so the fungal network persists; avoid long bare fallows that crash it; and manage soil acidity, since correcting extreme acidity with lime also helps free up phosphorus directly. None of this removes the need for sound fertility management, but it gets you more out of the phosphorus already in the soil and the phosphate you do apply.

FAQ

What is the difference between VAM and AMF?

They are the same group of fungi. Arbuscular mycorrhizal fungi (AMF) is the current term; vesicular-arbuscular mycorrhizae (VAM) is the older name still seen in plantation literature. Both refer to the fungi that colonise roots and extend hyphae into the soil to scavenge phosphorus and other nutrients.

Will mycorrhizae replace my phosphate fertiliser?

No. Mycorrhizae help the plant access phosphorus that is already in the soil, including some that acid-soil chemistry has locked up, so you get more from each application. They reduce inefficiency rather than removing the need for fertility management. On genuinely phosphorus-poor soils you still need to supply phosphorus.

How do cover crops help the soil fungi?

Cover crops keep living roots in the ground year-round, and mycorrhizal fungi cannot survive without a living host root. A continuous legume cover such as Pueraria javanica or Calopogonium maintains the fungal population, whereas long bare fallow starves it. Keeping the network alive means it is ready to work for your next crop.

Compare cover-crop species for your soil

We supply cover-crop legume seed suited to Philippine acid soils, germination- and purity-tested to ISTA and AOSA methods, and can help you match species to your phosphorus and acidity situation. To compare species, request a quote or message us on WhatsApp at +60 17-237 4058.

Sources

• Blal, B., Morel, C., Gianinazzi-Pearson, V., Fardeau, J.C. and Gianinazzi, S. (1990). Influence of vesicular-arbuscular mycorrhizae on phosphate fertilizer efficiency in two tropical acid soils planted with micropropagated oil palm (Elaeis guineensis Jacq.). Biology and Fertility of Soils 9: 43-48. https://doi.org/10.1007/BF00335860
• Andrino, A., Guggenberger, G., Kernchen, S., Mikutta, R., Sauheitl, L. and Boy, J. (2021). Production of organic acids by arbuscular mycorrhizal fungi and their contribution in the mobilization of phosphorus bound to iron oxides. Frontiers in Plant Science 12: 661842. https://doi.org/10.3389/fpls.2021.661842