It is essential to know when to irrigate, how much water to apply and when not to irrigate, according to Robbie Childs, irrigation agronomy expert. “If we do not have this information, we are at risk of over irrigating, wasting water and fertiliser and placing the plant under pressure. There is also a risk that we under irrigate and limit the plant’s production potential.”
Childs shares the following important equation that sums up the goal of irrigation scheduling:
This means that a plant must use its full transpiration potential. If the plant is under pressure, the answer of the equation will be smaller than 1, as actual transpiration will be lower than the plant’s potential transpiration.
To collect the necessary information, we must follow a process that includes several important measurements, tests and calculations. “The goal is to develop a user-friendly scheduling plan that ensures sustainable production and uniform results.” This requires certain tools: profile pits, soil water monitoring equipment, a scheduling programme, weather stations, and most importantly, knowledge. “Do not take shortcuts and omit any tools or steps. If we do not perfectly fit in every piece of the puzzle, optimal irrigation will not be possible, and production will suffer.”
We must never underestimate the value of profile pits. When we dig a profile pit, we must pay attention to soil type and texture, signs of excessively wet soil, the depth and width of the active root zone and soil layers that may impede root development and water distribution. “Our goal is to answer a few questions: What amounts of clay, sand and silt are contained in the soil? Do we need to install a drainage system? What is the size of the root zone? Can we mechanically improve the impeding soil layers?”
This explains the initial purpose of a profile pit during the planning and preparation phase. After irrigation has commenced, the profile pit becomes a tool to recalibrate soil water monitoring equipment and to manually monitor soil water levels. “We ‘monitor’ the soil water levels by taking a sample with your hand, assessing it and ranking it. This measurement is used to calibrate monitoring equipment and to assist in interpreting data.”
It is very important to irrigation scheduling that we distinguish between the active root zone and the deeper, less active roots, or the buffer zone. “This is to prevent the over irrigation of the buffer zone as water uptake is slower in this zone. If we make this distinction, we can schedule shorter maintenance irrigation periods and longer corrective irrigation periods only when the buffer zone requires it.