Mycorrhiza in drought and degraded soil: water, carbon and what inoculants can do
Dry, bare and eroded soils are where crops need help most and where living inoculants struggle most. How mycorrhizal fungi help plants with water, how they help hold soil and carbon, what restoration studies show, and what encapsulation can and cannot add.
In brief
- Mycorrhiza helps plants through dry spells: leaf pores stay more open, growth is better, and in rainfed trials yields rose by 23% on average.
- The fungus brings more phosphorus and some water to the root, changes how roots handle water, and in a pot study even passed water from pigeon pea to finger millet.
- Fungal threads bind soil into crumbs that hold carbon, but the effect is clearer in pots than in fields, and soil carbon builds up slowly in dry land.
- In degraded land, inoculating seedlings in the nursery is among the most effective restoration treatments, and local inoculum often beats commercial products.
- Inoculate in the nursery or root zone, into moist soil, keep the soil covered and phosphorus moderate, and ask for field data before believing big water or carbon claims.
Does mycorrhiza help crops in drought?
On average, yes. Across hundreds of published comparisons, plants with mycorrhiza kept their leaf pores (stomata) more open than plants without it, which helps them keep taking in carbon dioxide, and the difference was larger under drought than with plenty of water.2 A review of experiments under water shortage found that mycorrhizal plants grew and reproduced better, with perennial plants gaining more than annual crops.3 In rainfed trials, mycorrhizal fungi raised crop yields by 23% on average, and legumes gained more than other crops.4
Most of these results come from pots and plots where the fungus was well established. They show what the partnership can do; a product only delivers it if the fungus gets into the roots.
How the fungus helps with water
More nutrients when the soil is dry. In dry soil, nutrients such as phosphorus move even more slowly to the root. The fungal threads reach past the dried-out zone around the root: in studies of plants under drought, mycorrhiza raised phosphorus uptake by 86% on average.5 A well-fed plant copes better with stress.
A little water through the threads. The threads also carry some water to the root. How much varies: one careful measurement in barley put it at about a fifth of the plant's water uptake, at the high end of published values.6
Roots that manage water differently. Mycorrhiza changes how water moves inside the root, turning the root's water channels (aquaporins) up or down depending on how dry the soil is7 and letting more water move between root cells.8
Sharing water lifted by deep roots. At night, deep roots can bring water up from moist subsoil and release it into dry topsoil, a process called hydraulic lift. Mycorrhizal threads take up part of this water, which kept them alive through long droughts.9 In a pot study, a shared mycorrhizal network moved water from deep-rooted pigeon pea to shallow-rooted finger millet during drought,10 and in another, finger millet grown with pigeon pea kept its leaf pores more open in drought than finger millet grown alone.11 This is a promising idea for intercropping in dry regions, still to be proven in farmers' fields.
Holding dry soil together, and the carbon in it
Mycorrhizal threads wrap soil particles into crumbs, and crumbs protect the organic matter inside them from being broken down quickly.12 A review of 35 studies found that mycorrhizal fungi improve soil crumb structure on average, but the effect was strongest in pots and sterilised sandy soils and weaker in fields.13 In drought experiments, soils with mycorrhizal fungi kept more of their functions, including crumb formation, than soils without them.14
The fungus's sticky protein, glomalin, is often given credit for this, but the usual soil test for "glomalin" also picks up other proteins and humus, so it overstates the amount.15 For how mycorrhiza helps build soil organic carbon, see types of mycorrhiza and why they matter in farming.
Keep carbon claims realistic. Soil carbon builds up slowly in dry land. When farmland on China's dry Loess Plateau was converted back to trees and grassland, soil carbon rose by about 0.3 tonnes per hectare per year on average.16 Claims that an inoculant alone adds several tonnes of carbon per hectare a year should be treated with caution until field data are published.
Restoring barren and degraded land
Where vegetation and topsoil have been lost, the native fungi are often lost too, and adding mycorrhiza can help new plants get going. Across restoration projects, inoculation raised mycorrhiza in the roots and helped plants establish, and inoculum taken from nearby healthy land worked better than commercial products.17
A review of 26 field studies found that inoculation increased plant growth most in nitrogen-fixing trees and shrubs, warm-season (C4) grasses and soils low in phosphorus; the benefit grew over the first three years, and inoculated sites ended up with about 30% more plant species.18 Across 88 dryland planting studies, inoculating seedlings with mycorrhiza in the nursery, alone or with organic amendments, was among the most effective treatments for seedling survival and growth.19
| Claim | What the research shows |
|---|---|
| Helps plants through drought | Yes, on average: leaf pores stay more open, growth and yield are higher; most data from pots and plots |
| Carries water to the root | Some: up to about a fifth of uptake in one study, often much less |
| Shares water between plants | Shown in pots (pigeon pea to finger millet); not yet in fields |
| Improves soil crumbs | Yes, on average; clearer in pots than in fields |
| Adds tonnes of soil carbon a year | Not shown; restored dry land typically gains a few hundred kilograms per hectare a year |
| Helps restoration | Yes, especially nursery inoculation and local inoculum |
| Encapsulation fixes establishment in drought | Not shown in field studies yet |
What encapsulation can add
Living inoculants struggle most exactly where they are needed most: in hot, dry, bare soil. Packing the fungus into protective beads is one answer. Beads have kept mycorrhizal fungi infective for six years in cold storage,20 and the beads open as they take up soil moisture (see how mycorrhiza finds the root and our VAM field note).
What has not yet been shown is the field result. Reviews note that many encapsulated formulations that work in controlled conditions fall short in the field,21 and in dryland planting trials the results for water-absorbing gels (hydrogels) were uncertain and possibly overstated by selective publishing.19 We found no published field study showing that encapsulated mycorrhiza improves establishment, soil water or soil carbon in dry land. Such claims need trials with an uninoculated comparison before they can be relied on.
What to do in dry and degraded fields
- Inoculate seedlings in the nursery before planting them out; it is among the best-supported treatments in dryland planting.19
- Place the fungus in moist root-zone soil, not on a hot, dry surface: it is alive and sensitive to heat and drying.
- Keep living roots and cover on the soil. Long bare periods starve the fungi; see what weakens mycorrhiza in your field.
- Go easy on soluble phosphorus. Across field studies, phosphorus fertiliser reduced mycorrhiza by 32% on average.22
- Consider deep-rooted partners. Intercrops such as pigeon pea with finger millet may share water through the fungus; the idea is promising but still being tested in fields.11
- Ask for field data. Before paying for promises of large water or carbon gains, ask for trial results with an untreated comparison.
For another input that helps crops cope with drought, see silicon for crops.
Editor's note: this article is based on a technical note by Dr. Abhishek Mathur. NanoKheti's editors rewrote it in plain language, checked each statement against the published research listed under Sources, and added the summary, table, questions and links.
