In Drought Conditions, What Matters Is Not How Much Water Soil Stores, but How Much Actually Enters It
- Aljaž Novak
- Jul 10
- 8 min read
Summary of a presentation by Dr Gernot Bodner (BOKU University, Vienna) at the Soil Evolution 2026 conference
Drought is becoming one of the greatest constraints on modern agriculture. We often ask how to increase the amount of water stored in soil, but much less often how to reduce water losses. This was the question highlighted by Dr Gernot Bodner of BOKU University, Vienna, at Soil Evolution 2026. His presentation offered a clear, evidence-based perspective on the role of humus, soil structure and conservation agriculture in adapting to increasingly frequent dry periods. These findings are especially relevant at a time when numerous research and development projects are seeking ways to improve agriculture’s resilience to climate change. One of these is the European project TRAILS4SOIL, in which partners and farmers work together in living labs to test practices that improve soil health, use water more efficiently and make farming systems more resilient to drought.
When discussing drought, we usually begin by asking how much water soil can store.
Bodner, who has studied interactions between soil, water and plants for decades, asks a different question: how much water actually enters the soil, and how much is lost along the way? This is the central theme of his presentation and the reason why several widely held assumptions about humus and water deserve closer examination.
Water is becoming a limiting factor
Climate change does not necessarily mean less rainfall. More often, it brings increasingly uneven rainfall distribution during the growing season. At one point there is too much water and at another too little, so many farmers face both extremes within the same season. The problem is often not the total amount of rainfall, but when it occurs. Managing the soil water balance is therefore becoming one of the key challenges of modern agriculture, comparable to nutrient management.
Soil as a living, partly managed system
Bodner compares soil to a reactor: organic matter and roots are fed into it, while tillage represents a major intervention in how the system operates. Biochemical processes occur within soil that cannot be fully controlled. They take place at the micrometre scale, between bacteria and clay particles. The role of farming practice is not to exert complete control, but to identify the points of leverage: where we can genuinely influence the system and where we remain constrained by natural conditions, especially soil texture.
Soil physics addresses two fundamental balances: the water balance (precipitation, surface runoff, infiltration, evaporation and transpiration) and the energy balance (solar radiation, the warming of soil and air, and the energy used for evaporation). The two are closely interconnected.
Soil structure is a question of energy
One of the presentation’s clearest messages was that good soil structure is an ordered state, and maintaining that order requires a continuous supply of energy. This energy enters the soil through roots as a result of photosynthesis. When that input is absent—for example, in winter when soil is bare and there is no living vegetation—soil structure begins to deteriorate and loses some of its organisation. This is not necessarily a disaster. When plants resume growth in spring, soil structure gradually re-establishes itself. It is a natural seasonal cycle, not a one-off or irreversible collapse.
Stable aggregates are formed through the combined activity of roots, fungi, bacteria and their secretions. Bodner uses an instructive analogy for the microbial mucilage that binds soil particles together: like chia seeds soaked in water, it forms a gel-like, sticky substance—a kind of “biological glue” that holds microaggregates together.
The data presented show that stable soil aggregates are closely linked to biological activity in the soil. Key factors include organic carbon, microbial necromass (the remains of dead microorganisms), water-soluble carbon, microbial biomass and fungal hyphae. Bodner emphasised that aggregate stability is not merely the product of physical processes, but the result of continuous activity by living organisms that bind the soil into a functional and stable system.
Bodner also noted that soil temperature has a major influence on the activity of soil organisms. Plant cover not only reduces erosion and evaporation, but also prevents excessive heating of the soil surface. The research examples presented showed that, under summer conditions, bare soil can be more than 15°C warmer than soil covered by vegetation. Such differences have a significant effect on the activity of fungi and bacteria involved in forming and stabilising soil structure.
Why conservation agriculture works
Bodner presented research comparing conventional tillage systems with long-established conservation or regenerative agriculture systems. The results show that mechanical soil disturbance accelerates aggregate breakdown. The more frequently soil is disturbed, the harder it is to maintain a stable structure. No-till therefore generally has an advantage in terms of aggregate stability, whereas differences in humus content between shallow tillage and no-till are often less pronounced.
Permanent soil cover, the presence of as many living roots as possible throughout the year, and reduced mechanical disturbance have a clear effect, particularly on the stability of structural soil aggregates and on reducing surface runoff and erosion. These are, in fact, two expressions of the same mechanism: a protected soil surface prevents raindrops from striking aggregates directly and breaking them apart. At the scale of individual soil particles, raindrops can carry surprisingly high energy; repeated impacts therefore contribute substantially to the breakdown of soil structure and the formation of surface crusts.
Even approximately 30–50% soil cover can substantially reduce erosion and surface runoff. Reducing evaporation losses, however, requires considerably more crop residue or mulch. In dry regions this is often difficult to achieve because cover crops cannot produce enough biomass when water is scarce.
Pores matter more than aggregates—but they are less visible
Structural aggregates are easy to see on a spade, but pores are what actually store water and air, support microbial life, and allow roots to develop and grow. Under no-till, soil may be denser, but this should not be confused with harmful compaction. The main change is a shift from large pores towards medium-sized pores that retain water more effectively, while the pore network remains connected and functional. In intensively tilled soil, ploughing or loosening often creates numerous unstable pores that may rapidly collapse or become clogged after heavy rain. By contrast, pores under no-till remain more stable over the long term.
Tap-rooted plants and earthworms play a crucial role in creating deep, vertical pores. No tillage implement can replace these biological structures. Bodner also stresses the importance of protecting the soil surface. Around 30–50% soil cover already reduces surface runoff substantially, while greater cover further reduces evaporation losses. Crop residues form a protective layer that cushions the impact of raindrops, prevents aggregates from breaking apart and allows a larger share of rainfall to infiltrate rather than run off. Soil cover also limits soil heating and direct evaporation, thereby improving the water balance of agricultural land.
In Bodner’s view, it is not only how much water the soil retains that matters, but also how quickly it can absorb water and how deeply that water penetrates the profile. Water that remains on the surface or in the uppermost centimetres is exposed to rapid evaporation. By contrast, water that moves deeper into the profile through biopores remains available to plants for substantially longer.
Humus matters, but it is not the only answer
One of the presentation’s more striking findings concerns the role of humus in retaining soil water. Although humus is vital to soil function, Bodner cautions that its direct contribution to increasing water storage is often overestimated. His calculations indicate that changes achieved through soil management mainly affect the upper 15–20 centimetres of the soil profile, adding approximately 6–15 millimetres of water storage. This is not insignificant, but it is less than is often assumed. During severe drought, plants increasingly draw water from deeper soil layers, where soil texture becomes more important than humus content.
The “fast versus slow” trade-off
Every benefit has a cost. Soil that retains more water contains less air, warms more slowly and releases nutrients more gradually. This creates a challenge for crops that need large amounts of nitrogen within a short period. Bodner is frank: there is no magic solution, only trade-offs. Options include adapted fertilisation, such as the CULTAN method, or strip-till as an intermediate approach in which only part of the surface is loosened while the rest remains undisturbed.
Bodner confirmed that this is a genuine trade-off faced by growers in dry conditions. Rather than offering a single answer, he stressed that such decisions must be evaluated in the light of local conditions and production objectives.
What we can do today
Although Bodner offers no universal prescription for dealing with drought, the presentation clearly identifies measures that can improve soil resilience and make more efficient use of rainfall. Most are closely aligned with the core principles of conservation agriculture:
Keep the soil covered with crop residues, cover crops or growing plants for as much of the year as possible.
Maintain living roots in the soil for as much of the year as possible, because they supply energy to soil organisms and help preserve soil structure.
Reduce mechanical soil disturbance wherever possible, because intensive tillage accelerates the breakdown of aggregates and the pore network.
Promote the development of biopores and deeper rooting by selecting tap-rooted plants and maintaining favourable conditions for earthworm activity.
Do not expect humus alone to solve drought problems. Its greatest value lies in improving soil structure, water infiltration and biological activity, and in reducing losses through surface runoff and evaporation.
Key messages
Conservation agriculture effectively reduces surface runoff and erosion.
Stable soil structure depends on a continuous supply of energy from plants and roots.
Biopores created by roots and earthworms play a crucial role in water infiltration.
The effect of humus on increasing water storage is often smaller than assumed.
Adapting to drought requires reducing water losses, not only increasing water storage.
Conclusion
Gernot Bodner’s presentation does not offer a single solution; instead, it helps explain which soil processes truly determine drought resilience. The greatest potential lies in reducing water losses, protecting the soil surface, maintaining stable soil structure, and promoting a deep, connected pore network. The practical message is clear: when adapting to drought, the most important question is not how much water the soil can store, but how much water actually enters it and how much is lost along the way.
Article prepared by:
Aljaž Novak, Slovenian Association for Conservation Agriculture (SZOK)
Based on the presentation:
Die Biologie der Bodenphysik: Wie optimiert man den Wasserhaushalt? (The Biology of Soil Physics: How Can the Soil Water Balance Be Optimised?), by Gernot Bodner (BOKU University, Vienna), presented at the SOIL EVOLUTION 2026 conference in Gurten near Bern on 4 June 2026.
Photographs:
Tomaž Škorjanc
Technical review:
Assoc. Prof. Dr Rok Mihelič, Biotechnical Faculty, University of Ljubljana
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