How much water should be delivered to each plant, and when? Netafim agronomist Jovan Erasmus has advice on calculating the precise water needs of your crops for maximum growth and minimum waste.
By now, the benefits of precision irrigation are undeniable. So is the importance of selecting the correct system and best possible equipment. But setting up the perfect system is only one half of successfully introducing a precision irrigation solution to your farm. The other half is getting the irrigation schedule just right.
Netafim agronomist Jovan Erasmus explains that – like with every other aspect of precision irrigation – irrigation scheduling should never be a guessing game. Knowing how much water to deliver to each plant, and when, should be based on data, calculations, tried and tested general principles, coupled with the unique conditions on your farm.
Air, soil, water
First, Erasmus explains that growing plants need both water and oxygen around their roots, and that the amount of water around the root system impacts the amount of oxygen in the root zone.
“The balance between air, soil and water is dynamic, with the soil particles staying constant. If you increase the water in your soil, you displace the air. The more water you put in the soil, the more you drive out oxygen. As the roots use the water, oxygen returns. But giving too much water, can negatively impact oxygen content and production. Giving too little water, of course, puts plants under stress which will also impact production negatively.”
Rooting depths and soil composition
Finding the balance entails a bit of theory and a bit of practice, Erasmus says. Farmers need to know the crops’ rooting depths as the aim of precision irrigation is always to wet only the rooting zone and not to waste water by sending it beyond that zone.
Then, farmers need to understand how deep a specific amount of water will wet the soil through their specific irrigation system. For this reason, Erasmus advises farmers to get the soil on their farms tested – not only chemically to inform fertiliser and soil improvement but also physically to inform irrigation scheduling.
“For irrigation purposes, a physical analysis is important. It is a basic analysis that determines the percentage sand, silt, and clay and thus the soil texture. This tells you the sizes and distribution of soil particles, which in turn impacts the water-holding capacity, the availability of water and nutrients to the plants, and the balance between soil particles, air, and water.”
Water demand
Then comes the crop’s water demand, which changes with the crop’s growth stage but also with environmental temperature, humidity, wind, and rainfall, to name a few.
Erasmus says that standard calculations exist to take all these variables into consideration. They help farmers use the rate of evapotranspiration (ETo) on their farm and the water demand of their crop – at a specific growth stage and relevant to that ETo (called the crop coefficient) – to calculate how much water must be delivered to each plant.
“These calculations are simple to work with but can be difficult to understand,” says Erasmus. He recommends a combination of expert advice to set a good baseline to work from, and available apps to help farmers from day to day.
The irrigation system
Lastly, farmers should understand the specifications of their specific irrigation system. “It’s important to consider your dripper delivery rate and dripper spacing.
“Water has a specific distribution pattern in the soil, called the width-to-depth ratio, and the deeper the roots, the further apart your emitters can be. The shallower the roots, the closer together you need to bring your emitters.”
Getting your hands dirty
It is valuable for farmers to go out into the field and feel the moisture in the soil with their hands, from as shallow as 10 cm to as deep as 30 cm. “From this, you can know whether the soil is too wet or too dry or just right,” says Erasmus. “Align those measurements with the calculations that you used to plan your irrigation.”
He strongly advises farmers to dig different profile pits throughout the season and to use them as an indication of the soil moisture conditions in their fields. “Get to know your soils, how deep the crop roots are at different growth stages, and what your irrigation system does to your soil.
“A profile pit might disturb one or two plants, but in the long run it is worth a lot more to know that your plants are getting the water that they need. It will give you confidence for the future.”
Monitor, adapt, learn
A final word of advice is to call in the help of an expert to make sure you understand the tried and tested formulas, to monitor the soil moisture in your fields continuously, and to adjust your scheduling guidelines as circumstances and data change.
In the meantime, farmers who want to gain more knowledge and practical tips on irrigation scheduling, can visit Netafim’s Precision Irrigation Academy at www.netafim.co.za.
• To hear a full interview with Erasmus, listen to the latest episode of Farmer’s Inside Track. For more information on Netafim South Africa, click here.
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