In times of heat and drought stress, what is the best way to keep water in the soil so that it is available for the plant at the right time? Heat in summer or even in spring and fall causes groundwater supplies to dwindle. But there are ways to keep water in the field, store it and even harvest it.
“It's not drought that causes bare soil. It is bare soil that causes drought!"
This statement by a Zimbabwean farmer puts it in a nutshell: our primary goal must be to keep the soil covered at all times. Be it through harvest residues over shorter periods of time until the next main crop or the establishment of catch crops. Open soil loses water through unproductive evaporation and is defenceless against wind, water and sun. Everyone knows this. Nevertheless, it is impossible to remind ourselves of these connections often enough.
Of course, plants also need water to grow. But they form a water cycle. Through evaporation in the leaves, water is released from the soil into the air (and at some point clouds form as a result). This evaporation removes heat from the air and lowers the ambient temperature. However, dew also forms on the leaves, which runs off and is available to the plant. Plants also release carbon into the soil in the form of sugar compounds, which in turn nourishes soil life and improves soil yields. And at the end of their lives, the plants themselves are also available as food for soil life, which at some point serves as food for a new plant.
Intercropping is key to a water-efficient system
Haste is required after the harvest. The catch crop must be in the soil just a few days after the harvest. This gives volunteer cereals and weeds the least chance. This requires a balanced mixture of species. Some catch crops grow very quickly and cover rapidly. These include buckwheat, mustard, oil flax and cress. Other species need more time to develop. Legumes in particular take over later, when the fast-growing species start to decline. They are real powerhouses when it comes to building up humus and fixing nitrogen. Such intercrop mixtures are able to leave between 50 and 100 kg of nitrogen for the following crop, depending on how they develop, as studies by Bioforschung Austria show. That saves money! Trials at the Hollabrunn Technical College in Austria, carried out by the University of Natural Resources and Life Sciences, show that although the soil water content under catch crops as well as under black fallow varies, in the end at least the same content is present in the soil at the start of sowing, often even more!
Cover crops not only collect the little snow in winter. They also collect large amounts of dew during fog. One argument often put forward by farmers as to why they don't want to grow catch crops is that it is always wetter under the catch crop in spring and you can't work so well there. Shouldn't that give us pause for thought? It is important to continue working with suitable measures to keep the moisture in the soil.
Conserving water with no-till
Of course, no-till means more patience until the time is right. It should not be too moist, but not too dry either. And the technical requirements are also more complex and elaborate, and usually more expensive. Of course, many mistakes can happen - significantly more than with conventional sowing. However, our experience shows that a well-executed direct sowing usually emerges better and is better supplied with capillary water than conventional sowing. And it is precisely the start of a new plant's life that causes more and more problems with conventional systems.
In direct sowing, the remains of the old catch crop or previous main crop also protect the young plants from wind and weather. During heavy rainfall events, water can infiltrate better through existing pores and earthworm tubes in the soil, while it can run off conventional areas and cause erosion.
The “Soil Pioneers” project provides interesting figures
The project compares a constructive cultivation method with the conventional method and samples and evaluates a natural reference area (meadow, grassland) in the immediate vicinity. It was found that the proportion of so-called central pores in the soil is significantly higher with constructive cultivation methods than with conventional cultivation. And it is precisely these central pores that are important for water storage in the soil. As a result, soils cultivated in this way can store up to 20 % more water than with conventional cultivation.
In addition, the average humus content was almost 20 % higher, especially in lighter soils. The best 25 % even had up to 65 % more humus. The proportion of so-called “readily available carbon”, which comes from plant roots, was also significantly higher in the pioneer areas at an average of 31 %.
In addition, soils cultivated in this way exhibit significantly higher aggregate stability, on average around 14 %, the best 25 % up to 65 %. This means that the soil holds together better and is much more resistant to erosion of any kind. In addition, the microbial biomass was on average approx. 35 % higher, with the best 25 % even 48 % higher. And, of course, the infiltration rate, i.e. the rate at which water seeps into the soil, is also greatly improved.
How water can be stored
There are a variety of ways to retain more water in our soils. The early sowing of a biodiverse catch crop mixture immediately after the harvest is important, or at least covering the soil with harvest residues for short periods until the next main crop. The associated promotion of soil life in conjunction with direct sowing has many advantages, as the “Soil Pioneers” project at BOKU Vienna shows. By improving soil properties and soil life (fungi, earthworms), the water storage capacity of the soil can be increased by up to 20 %.
A balance of nutrients in the soil is also particularly important so that the plant can make the best possible use of the scarce resource of water. Potassium and magnesium in particular greatly improve water utilization efficiency.
And the planting of hedges also leads to more water in the region and can help to adapt to climate change. The measures described above mitigate the negative effects of heat and drought. They are not a panacea and do not replace rain or irrigation. But they do help plants to better withstand these periods of heat and drought. The process of building a resilient soil structure takes many years - years in which there can and will be setbacks. Nevertheless, we should increasingly take this path.
Earthworms as “water bringers”
A high number of earthworms in constructive systems contributes to better “digestion” of the water. Vertically burrowing species allow it to reach the soil more quickly, and horizontally burrowing species allow it to reach the plant roots easily from the side. There are now numerous scientific studies that prove that plants with earthworms in the soil grow better and produce higher yields than those without. This is not least due to the organic fertilizer that the animals leave behind.
Earthworms are best encouraged by feeding on the soil surface in the form of catch crops and crop residues. And, of course, as little tillage as possible or as gentle as possible. For example, the shallow use of a disc harrow is easier for the worms to cope with than a deep cultivator pass, which destroys the tubes in which they live. The plow is therefore particularly problematic for earthworms. It removes the food on the surface and destroys the animals' habitat.
Soil cultivation causes problems for fungi
Tillage cuts through fungi and prevents them from developing properly. Fungi are also important for supplying plants with water and nutrients. Mycorrhizal fungi supply most crops with water and nutrients and in return receive sugar compounds from the plant. Plants that are well colonized with mycorrhiza increase the root reach of the plants by a factor of two to three. They supply water that the plants could not reach on their own.
The most important prerequisite for promoting mycorrhizal fungi in the soil is green plants. This is how the fungi get their food and can later multiply. To be fair, it has to be said that fungi do not tolerate fertilizers and plant protection well either, but can still live better with them than with too much tillage. However, they are indispensable when it comes to adapting to climate change!
Can you simply “fertilize” water?
Of course not! Unless you irrigate. But you can make sure that the plants are well supplied with all the important nutrients that help the plant to use water efficiently. These include potassium and magnesium in particular. Potassium is involved in many metabolic processes. It improves water use efficiency, frost hardiness and many quality characteristics. If potassium is lacking, this leads to a reduction in assimilation and a poorer water supply to the plants. Magnesium is the central building block of green leaves and is directly involved in photosynthesis. It also activates various enzymes, which in turn enable the formation of proteins, starch and sugar or oils and fats. It is also responsible for the stability of the cell walls and is important for the transport of carbohydrates to the roots.
But phosphorus also helps to increase efficiency. It is important for root growth, the energy balance and helps soil life, which in turn helps the plants. Sulphur improves nitrogen efficiency and therefore has a positive influence on the quality of the crop. If sulphur is lacking, the plants lag behind in growth, they sprout less and form fruit clusters less well, or the plants become more susceptible to disease.
Trace elements such as manganese, zinc, copper and boron should not be forgotten. Balanced fertilization helps the plant to have more vitality and make better use of water. And nitrogen can now be applied in liquid form via depots (the Cultan method) or solid fertilizers can be applied in rows. Both help the plants in dry conditions.
Hedges for more water
We all know that little or nothing harvestable grows right next to a hedge or windbreak for the first few meters. But do we also know that the effects of a hedge lead to more water further away? The wind is slowed down, dew forms earlier in the evening and stays longer in the crop in the morning. According to studies carried out by Bioforschung Austria, this means that although area is lost to the hedge, yields still increase by 7 to 8 %. The positive effect extends to about 20 times the length of the height of the hedge. A hedge with a height of 15 m therefore has a positive effect for around 300 m in the direction of the wind. In addition, the hedge cools the surrounding area through the evaporation of water, thus mitigating the heat and creating a small regional water cycle.
