The basics of effective irrigation

Crop expert Bill Kerr gives tips on how to ‘read’ a crop and understand its irrigation requirements.

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One of the questions that new farmers often ask when growing their first crop is: “How much water must I apply and how often?” Many farmers – including those who are experienced – tend to think that one has to follow a specific recipe to grow each crop properly.

But we’re not baking a cake, where a mistake with one of the ingredients will spoil the finished product. Instead, farmers have to deal with constant changes that affect their ability to obtain a good yield, and it’s only careful observation and common sense that will ensure success.

Water is one of these variables – and one of the most important to get right. Yes, we can determine how much water to apply to make sure the crop’s root system gets wet, but it doesn’t stop there. There are many factors to consider, including temperature, humidity, wind and the stage of development of the crop. The soil condition – that is, the organic content, structure and amount of clay present – also influences the frequency of irrigation.

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Tools that can be used to check water penetration the day after irrigating.

Clay soils require more water in order to irrigate to the required depth than sandy soils. A general rule is that 1ml of water will wet 1cm of soil. But it’s necessary to check the depth of penetration the following day. Use a sturdy trowel for this when dealing with crops with shallow roots. Alternatively, press a thin steel rod into the soil.

The wet area will allow the rod to go down with ease; the dry soil will be much harder to penetrate. At first, it may be necessary to do this after every irrigation. With experience, it’s possible to look at the the crop and judge whether it requires irrigation.

Begin by checking the colour and condition of the plants at the edge of a land. If there’s a difference between these and the rest of the crop, wind during irrigation may have caused insufficient wetting. Study the condition of the crop just before irrigation and see how it changes afterwards. This is easy to check if the land is irrigated on one side – the irrigated and dry sections can then be compared.

It is also a good idea to check the condition of the soil with a trowel just before and just after irrigation. If there is a big difference, you probably waited too long before irrigating and allowed the soil to dry out. This places stress on the crop and affects the yield. On the other hand, over-irrigating causes nutrients such as nitrogen to leach beyond the reach of the crop’s roots. This wastes money and potential yield.

If the water does not soak down deep enough, the crop will be unable to get nutrients and will dry out quickly. In the case of root crops such as carrots, the roots will be short and stumpy and crack easily. After a while, the irrigation can be fine- tuned to satisfy the crop’s requirements. For instance, if there is a sudden heatwave or warm, dry winds, it makes sense to provide the soil with a higher level of moisture, helping to prevent stress.

It is also a good idea to irrigate sooner than usual after a short period of cool, wet weather, as root development will have slowed down and the plants may become stressed if water is not readily available again.