Water is often the limiting factor in growing food, especially in dry regions and disadvantaged communities. This article explains a simple, practical method for using water far more effectively. By putting water into the soil instead of on the surface, measuring how deep it travels, and matching irrigation to plant root depth, it is possible to achieve better growth with less water. These principles are based on real-world irrigation research and are suitable for gardens, farms, and small food systems alike.
Introduction
Efficient water use is not about applying more water, but about applying it in the right way, at the right depth, and at the right time. Colin Austin, a research consultant on irrigation technology to World Vision, has spent many years helping disadvantaged people grow sustenance food using limited water supplies. The methods described here are simple, low-cost, and based on observing how water actually moves through soil and how plants access it.
Put Water in the Ground, Not on the Surface
The first principle of efficient irrigation is simple: do not spread water across the entire soil surface. Water on the surface is exposed to evaporation and is quickly lost to the atmosphere. Instead, water should be applied into furrows or holes and filled quickly.
When water is placed into a hole or furrow, it moves sideways and downward through the soil, remaining protected from evaporation. This allows moisture to spread through the root zone where plants can access it, rather than being wasted at the surface. The goal is not to make the ground look wet, but to ensure that the soil beneath the surface is properly hydrated.
Irrigate to the Root Depth of the Plant
Plants can only access water within their root zone. Any water that passes beyond the roots is effectively wasted, while water that does not reach the roots cannot be used by the plant. For this reason, irrigation must be matched to the depth of the root system.
After irrigation, it is important to measure how deeply the water has penetrated into the soil. This ensures that the root zone is fully wetted without water passing below it. Measuring irrigation depth removes guesswork and replaces it with clear information about how water behaves in a specific soil.
Using an Irrigation Depth Meter
An irrigation depth meter allows growers to see exactly how far water has moved into the soil. Stainless steel probes are pushed into the ground and readings are taken before and after irrigation. This provides a clear picture of how effective the irrigation has been.
By regularly checking irrigation depth, it becomes possible to adjust water application so that every irrigation reaches the base of the root zone, but no further. This single practice can dramatically improve water efficiency and plant growth.
Understanding Maximum Water Holding Capacity
Every soil has a maximum amount of water that plants can effectively extract. This is known as the maximum water holding capacity of the soil within the root zone. Knowing this value is essential for good irrigation management.
To measure it, allow the soil to dry out while monitoring depth readings to see when moisture has fallen to the base of the root zone. During this period, plants should be observed closely, either with a plant moisture sensor or by visual inspection, to ensure they are not suffering from water stress.
Once the soil has dried to the root depth, irrigate and measure how much water is required to wet the soil back to the base of the roots. This amount represents the maximum water the plant can effectively use in that soil.
Avoid Full Saturation Cycles
It may seem logical to let soil dry out completely and then refill it to full capacity, but this approach does not produce the best growth. Repeated cycles of extreme dryness followed by saturation place stress on plants and reduce nutrient uptake.
Better growth is achieved when soil moisture is kept within a moderate range, never too wet and never too dry. This requires a combination of smaller and larger irrigations rather than relying on a single heavy watering.
Using Small and Large Irrigations Together
Soil does not dry evenly. Often the topsoil becomes dry while deeper layers still contain moisture. This matters because most nutrients are concentrated in the topsoil, where roots are most active.
Even when there is water deeper down, it is often beneficial to irrigate lightly to rewet the upper soil layers. This supports nutrient uptake and encourages healthy root activity near the surface.
A Practical Irrigation Pattern
A practical method is to apply small irrigations equal to half the maximum water holding capacity of the soil. These smaller irrigations should be applied several times—often four or five—until the deeper soil layers have dried out.
Once the deeper soil has dried, a larger irrigation is applied to fully wet the entire root zone. This resets the system and ensures that roots have access to moisture throughout the soil profile.
Each small irrigation should use the same amount of water. Consistency allows the soil to stabilise and makes it easier to predict plant water needs.
Why This Method Works
This approach keeps moisture levels within an optimal range. Roots are not stressed by drought, and nutrients are not leached away by excessive watering. The soil environment remains biologically active, supporting healthier plant growth.
By alternating small and large irrigations, water is used where it does the most good, rather than being lost to evaporation or deep drainage.
Using Software to Improve Accuracy
For those with access to a computer, the WaterRight program can provide additional guidance. By entering data such as irrigation amounts, evaporation, rainfall, and depth readings before and after irrigation, the program learns the water needs of specific plants.
Over time, the software can automatically advise when to irrigate and how much water to apply. This reduces guesswork and helps growers adapt to changing conditions as plants grow and water use increases.
Measuring Evaporation
Evaporation between irrigations is another critical factor. A simple evaporation meter, such as the Magic Mushroom evaporation meter, can be filled when irrigation occurs. An initial estimate of allowable evaporation can be made by dividing the root depth by 20.
For example, if the root depth is 200 mm, an initial evaporation estimate would be 10 mm. A marker is placed at this level, and irrigation is scheduled when evaporation reaches the mark.
Adjusting Evaporation Settings
After irrigation, the depth of water penetration should again be measured. If water has not reached close to the base of the root zone, evaporation between irrigations is too high, and the marker should be raised so irrigation occurs sooner.
If water has passed beyond the root zone, too much water has been applied, and the marker should be lowered to allow more evaporation before the next irrigation.
As plants grow and use more water, evaporation between irrigations may need to be reduced. Regular measurement allows these adjustments to be made accurately.
Why Measurement Matters
The key message is that good irrigation is based on measurement, not appearance or habit. Soil can look dry on the surface while still holding usable water below, or it can look wet while roots remain dry.
By measuring irrigation depth and evaporation, growers gain direct feedback from the soil. This allows them to respond to real conditions rather than assumptions.
Conclusion
Making the most of water does not require complex or expensive systems. By putting water into the ground, matching irrigation to root depth, and combining small and large irrigations, plants can grow better with less water.
These methods are especially valuable in regions where water is scarce, but they are equally useful anywhere efficiency and sustainability matter. With simple tools, careful observation, and consistent practice, water can be used where it counts most—supporting healthy plants and reliable food production.
Download ‘Make the Most of Water: Practical Irrigation Methods for Maximum Growth’ (full PDF)
