Irrigation management basically comes down to two decisions: when to irrigate and how much to irrigate? To make these decisions properly, we need to know what the current soil water content is and whether it is right for the crop at that time. Each species and even variety within a species may have different preferences for optimal soil moisture.
Soil is a three-phase system, consisting of the soil skeleton (mineral and organic solids), air and water. In simple terms, the soil skeleton can be compared to a glass filled with bearing balls. There is a lot of free space between the balls, which can be filled with air or water. If we put the roots of a plant into such a hypothetical soil, then if we fill the space between the balls with water, we will have complete saturation with water at zero air content, which is unfavourable for most plants because the roots cannot breathe. In the opposite situation, if we pour out all the water, the plant will wither because it will have no access to water. By ensuring optimal access to water, we create optimal growth conditions for the plant, so that an optimal yield can be achieved. Experiments carried out on potatoes at the IUNG-PIB show a more than twofold increase in yield through the use of irrigation optimisation based on measuring soil moisture with sensors. This method gives absolute certainty about the water content of the soil and allows the necessary irrigation to be easily calculated. The decision of when to irrigate can be supported by a soil moisture observation system via a smartphone app.
Another method, widely used worldwide, is the determination of irrigation rates based on calculations of evapotranspiration, i.e. evaporation of water from the soil surface (evaporation) and plant respiration (transpiration). The amount of daily evapotranspiration can be calculated on the basis of meteorological measurements: sunshine, air temperature, wind speed, air humidity and atmospheric pressure. On a hot day in the conditions of central Poland, the actual evapotranspiration reaches 5 mm per day. This method has a number of advantages – primarily the lack of need to purchase sensors and instrumentation, but it also has significant disadvantages – lack of information on current, actual soil moisture and lack of consideration of water runoff into the soil profile and surface runoff. This is a calculated method, whereas moisture sensors give a measurement of the actual water content of the soil, directly in the root zone.
The most common method used by farmers to determine the need for irrigation, in practice, remains the visual method, based on observation of soil moisture or plant condition. Visually, the farmer is only able to roughly assess the need for irrigation time based on his own experience. The decision on how much to irrigate is usually based on the assumption of soil saturation, i.e. irrigation up to the full water capacity of the soil, the achievement of which is manifested by the appearance of water ponds on the soil surface. For obvious reasons, the visual method is far from perfect. Firstly, the farmer decides on the basis of his own experience not supported by any objective measurement of soil moisture, secondly, often after a dry period, rainfall moistens only the top layer of soil, so an assessment of the surface moisture of the soil may suggest an abundance of water, while below the moistened layer, in the root zone, there is dryness of the soil profile. It is uneconomical and environmentally damaging to water the crop until soil saturation is reached. Water from a saturated soil (all capillaries and pores saturated with water) will usually drain away within one day to a moisture content corresponding to the field water capacity. The farmer loses all this excess water volume together with nutrients in the form of readily soluble ions, such as K2O+ and NO3-, which end up in the groundwater and pollute it unnecessarily.
Due to recurring droughts and increasing climate warming, optimising water use in agriculture and the need to promote small-scale retention in rural areas and beyond will soon become a question of agricultural survival, especially as climate change forecasts for the coming years predict droughts occurring up to 10 times more frequently than in the 20th century.