How does the soil water balance work?

Plants absorb water and nutrients through their roots, especially in the zone where most fine roots are located. In the garden, these fine roots are usually found at a soil depth of 10–60 cm (depending on the plant). Of course, there are plants that root many meters deep, but this is usually not relevant for gardens and will not be further discussed here.

Why is the root zone important for irrigation?

• Water uptake: Plants absorb water through their fine roots
• Nutrient availability: Nutrients in the soil are only absorbed in dissolved form. Therefore, the root zone must be sufficiently moist so that nutrients can dissolve and be reached by the roots.
• Soil properties: The storage capacity, drainage, and air circulation in the root zone determine how much water is available to the plants before the soil becomes dry or waterlogging occurs.
• Plant growth: Dryness or waterlogging in the root zone can inhibit root growth and make the plant susceptible to stress. Balanced moisture is therefore essential.

At the root of a young Swiss chard plant, you can clearly see how quickly roots grow:

Soil consists predominantly (>90%) of mineral components of different grain sizes and a proportion of organic substances (<10%).

Mineral components and their grain sizes

The mineral components of the soil are classified according to their grain size:

Sand: Grain size: 0.063–2 mm

Silt: Grain size: 0.002–0.063 mm

Clay: Grain size: < 0.002 mm

In practice, almost only mixed forms occur, which are referred to as loam. The classification is done using the soil triangle, which represents the mixing ratios. For example:

Sandy loam: 60% sand, 30% silt, 10% clay

Clayey loam: 35% clay, 40% silt, 25% sand

1. Sandy Soils

Organic Material Content: 1–2 %

Description: Sandy soils have a low capacity to store organic material due to their small surface area and poor nutrient retention. Organic substances decompose and leach out more quickly.

Water Retention:

Low fertility

Low water retention capacity, rapid water loss

Improvement needed by adding humus or compost

Plants often have access to water only for a short time

Suitable Irrigation: Frequent but small water applications to maintain surface moisture.

2. Silty Soils

Organic Material Content: 2–4 %

Description: Silty soils have a medium structure that allows moderate storage of organic material. They promote even decomposition of organic matter.

Properties:

Good water retention capacity

Medium fertility

Susceptible to erosion by wind and water, so mulch or organic cover layers are recommended

Suitable Irrigation: Even irrigation with moderate amounts

3. Loamy Soils

Organic Material Content: 3–6 %

Description: Loamy soils offer the best mix of water retention, nutrient availability, and decomposition rate. They promote the buildup and storage of humus.

Properties:

Suitable Irrigation: Medium-sized water applications at longer intervals. Flexible, generally tolerates both longer dry periods and abundant water well.

4. Clay Soils

Organic Material Content: 2–5 %

Description: Clay soils store organic substances well due to their fine particles, but slow air exchange can inhibit decomposition, especially in waterlogged conditions.

Properties:

Suitable Irrigation: Longer, slow irrigation with small amounts of water to achieve even moistening.

5. Peat Soils (Special Case)

Organic Material Content: 30–90 %

Description: Peat soils consist mainly of organic material because decomposition is greatly slowed under water-saturated conditions.

Properties:

Very high water retention capacity

Low in nutrients as organic substances decompose very little

Extremely low in oxygen, thus usually unsuitable for agriculture

Valuable CO2 storage

Two central concepts in soil water balance are field capacity and the wilting point.

1. Field capacity
   Field capacity describes the maximum amount of water that the soil can hold after a rain without excess water draining away.
2. Wilting point
   The wilting point is the state at which the soil contains so little water that plants can no longer absorb water and begin to wilt.

The range between field capacity and wilting point is available as water for plants and is referred to as plant-available water.

Relationship between soil type and field capacity

The following summary table shows the field capacity, the wilting point, and the plant-available water (PAW) for different soil types. The values are averages as they depend on factors such as organic content, soil structure, and compaction:

Organic substances, such as dead plant material, roots, and microorganisms, are called humus. Humus is constantly decomposed by microorganisms in the soil and thereby converted into plant nutrients.

• Humus improves water retention, soil structure, and nutrient availability.
• Organic substances contribute to the formation of macro- and micropores, which are important for water and air circulation.
• Soils with a high humus content can store more plant-available water.

Evapotranspiration consists of evaporation (water evaporation from the soil) and transpiration (water release by plants). These processes, together with infiltration, determine how quickly water is lost from the root zone.

Factors that influence evapotranspiration

• Climate: High temperatures, direct sunlight, dry air, and wind accelerate water evaporation.
• Soil cover: Dense vegetation reduces direct soil evaporation but increases transpiration.
• Soil: Sandy soils dry out faster, while clay soils retain water longer.

What are typical evapotranspiration values

Evapotranspiration per week in Central Europe in spring and autumn ranges between 4mm and 20mm and rises in summer to 15mm to 50mm/week. 1mm corresponds to one liter of water per m². This water loss must be compensated by rainfall or irrigation to adequately supply the plants. The large fluctuations in evapotranspiration should be taken into account in irrigation.

How MIYO calculates evapotranspiration

MIYO calculates evapotranspiration for each of your irrigation zones based on the weather data of the location, the entered soil type and garden parameters, as well as the solar intensity, temperature, and soil moisture measured by the MIYO soil moisture sensor.

• Consider soil type: Sandy soils require more frequent but smaller amounts of water, while clay soils can tolerate larger, less frequent watering. With MIYO, all irrigation parameters can be set based on specifications and individually adjusted to the garden's needs if necessary.
• Monitor evapotranspiration: MIYO calculates the evapotranspiration precisely for each irrigation zone based on measured and transmitted data. This ensures the irrigation exactly compensates for water losses through evaporation and transpiration.
• Optimize irrigation timing: Watering early in the morning before it gets hot reduces water loss through evaporation. The afternoon is also an option. Avoid watering in the evening or at night, as prolonged moisture on leaves can promote the growth of fungal diseases. In the MIYO app, you can adjust the times for each day with just a few clicks.
• Drip irrigation: This method delivers water directly to the plant roots and minimizes losses through evaporation.
• Mulching: reduces evaporation and ensures that water stays longer in the root zone.
• Avoid overwatering: Waterlogging causes severe and long-term damage to plants and soil and should therefore be avoided. The most convenient way is with a smart MIYO irrigation system. Irrigation controllers with fixed intervals should definitely be avoided, as they inevitably lead to constant over- or underwatering.
• Adjust irrigation intervals to the plant: Shallow-rooted crops benefit from more frequent watering. Deep-rooted plants need water less often but in larger amounts to moisten the deeper layers. These parameters can be easily adjusted to the plant's needs in the MIYO app.
• Measure soil moisture: Soil sensors or simple digging tests can help monitor moisture in the root zone and avoid overwatering or drought stress.
• Increase water retention capacity: Organic matter (e.g., humus) in the root zone improves water storage capacity and increases the amount of water available to plants.

1. What is the difference between field capacity and wilting point?
Field capacity describes the maximum amount of water the soil can store, while the wilting point defines the condition where plants can no longer absorb water.

2. Which soil types retain water best?
Loam soils have the best combination of water absorption and retention, while clay soils hold a lot of water but make it poorly available.

3. How can I calculate the water needs of my plants?

The water requirement depends on evapotranspiration, soil type, and plant species. A rule of thumb is to multiply the daily evapotranspiration (in millimeters) by the cultivation area. MIYO calculates the water requirement based on entered parameters, values measured by the soil moisture sensor, and weather data from the internet.

4. How can I tell if the soil is too dry or too wet?

Too dry: The plants begin to wilt, and the soil feels crumbly and hard.

Too wet: Water accumulates on the surface, or the soil feels muddy (especially in clay soils). The MIYO soil moisture sensor is a precise method to monitor moisture and determine irrigation needs.

The key to a healthy garden is the right soil moisture. Not too dry, but not too wet for extended periods. You should definitely avoid overwatering, as this leads to soil compaction and valuable nutrients being washed out. Timed irrigation must therefore be continuously adjusted to seasonal and weather conditions. We therefore recommend using the MIYO moisture sensor especially in sensitive garden areas.

How to control your smart irrigation with MIYO

Divide your garden into irrigation zones. Areas with similar water needs can usually be grouped into one zone. Later changes and reassignments of sensors and valves are easy to perform with MIYO.

To save water, it is best to plant plants with similar water needs in one area. The sensor should be placed at a representative spot in this area. Targeted water supply, e.g., drip irrigation, is more economical than watering large areas, e.g., with sprinklers. Irrigation should be designed so that the area is evenly supplied.

MIYO can irrigate both purely time-controlled and with soil moisture measurement. Even with irrigation controlled by soil moisture measurement through the sensor, you can enter time windows during which irrigation can take place. In the irrigation settings, you can set the lower and upper moisture limits. To achieve natural fluctuations and save water, make sure to set the moisture limits far enough apart. It usually does not need to be watered every day.

Consideration of rain and rain forecasts can be optionally set for both time-controlled and sensor-controlled zones.

You adjust the irrigation to the water consumption of the plants in the irrigation settings of the zone in the MIYO app. There are no generally valid guidelines; the values must be determined individually for plants and garden. Besides the type of plants, location, sun orientation, wind exposure, type of planting, and soil condition of the garden should also be considered. The more drought-resistant a plant is, the lower the lower limit of soil moisture measured by the MIYO sensor can be set. Due to the numerous adjustment options, the intelligent MIYO irrigation is equally adaptable for flowers and vegetables as well as for lawns and trees.

Use mulch: If you apply a layer of mulch around your plants, it helps retain soil moisture by reducing evaporation. Mulch also helps suppress weed growth, regulate soil temperature, and improve soil structure in the long term.

MIYO is the best guarantee for optimal water supply for your garden and for avoiding overwatering. Adjustments to the growth phases of the plants or observations of plant health are easily done on your phone.

Switch now and let your smart MIYO irrigation system do the work for you.