How much water to apply?: Knowing how much water to apply to turf is often seen as a matter of guesswork. Peter robin suggests that a more analytical approach is taken to irrigation.

Firstly, why do we irrigate turf? This might seem a dumb question but let’s look at it in more detail. Irrigation applies water when rain hasn’t been adequate. The roots suck up the water; this keeps the leaves turgid, and the water mobilises nutrients (your fertilisers) in the soil so they can be sucked up and used. So, no water = no nutrient uptake = an unhealthy plant.

How much water to apply?

Getting back to watering. How much water should you apply to keep your turf happy? You could make the same assumptions you made yesterday and the day before, but if you want to be more accurate then we can use daily evaporation figures from local weather stations. There are also very useful tools like the POGO if you’re lucky enough to own one (go to www.rigbytaylor. com if you want more information); this will accurately measure your soil moisture levels

These figures are useful but we need to manipulate them. Let’s start by understanding water loss by daily evaporation. A plant is not the same as a puddle of water because it has stomata that open and close according to the turgidity of the leaf. As soon as the leaf starts to lose too much water the stomata close; this reduces how much water can escape from the plant.

Therefore, while in the example of the puddle, water is still evaporating, the evaporation rate from the leaf has reduced. This ability of the stomata to control evaporation is called the Crop Coefficient.

A tropical fern dropped into the sunshine will wilt within minutes; that’s because its crop coefficient is very high. Compare that with a cactus. It can sit in the same sunshine for a month and not lose any water. Its crop coefficient is very low.

A temperate turfgrass has a crop coefficient of around 0.7 (coefficients never get above 1). This means that the amount of water lost through the leaf, called evapotranspiration (evaporation and transpiration stuck together), is the evaporation rate from the weather station x the plants’ crop coefficient. So when the weather stations’ evaporation rate is say 5mm, the amount of water the turf is likely to lose is 5mm x 0.7 = 3.5mm. This is the amount of water that you need to re-apply for the turfgrass to grow optimally.

If you apply less than 3.5mm the plant is likely to become stressed, while more than 3.5mm is just a waste of water and kilowatts from your extra pumping. However, if you apply 3.5mm, is it even going to get down into the rootzone or just sit near the surface?

This is a real concern, because the last thing most turf professionals want is a whole bunch of roots growing very close to the surface; made worse of course if you happen to be growing your turf on a sand carpet. If the main feeder roots are down an inch or two in the profile, how do we get 3.5mm of water to travel down deep enough

This brings us to how we irrigate. We need to irrigate in such a way that the water reaches the feeder roots two inches below the surface. To do this we will probably need to apply significantly more than 3.5mm. If for example we apply two days’ worth; 7mm, then that water will wash down into the soil profile much better.

To do this we need to change our irrigation to watering every two days. The problem is that you are growing the turf for reasons other than it just being healthy; it will have a ball kicked across it and players running over it. It therefore needs to take a stud, provide a consistent bounce and help create the optimum playing surface.

This perhaps can be managed by timing what day or days of the week you apply a deep irrigation. One useful process is to use your irrigation controller to apply the water in a better way

If you just dump several millimetres of water onto the surface of your pitch, some will sink in, some will puddle, some might run off, and some will travel all the way through your soil profile and into your drainage. We need to make sure as much of the water you apply stays in the rootzone. In other words we want the soil to reach Field Capacity; this is how much water the soil can actually hold. To allow the soil to reach field capacity it is important to apply the water in a few smaller doses, so each dose has the chance to be absorbed by the soil. If you are applying say 10.5mm of water (3 days’ worth assuming a 5mm evaporation each day and a 0.7 crop coefficient), then by applying 3.5mm, waiting an hour, applying another 3.5mm, waiting another hour, then finally applying the last 3.5mm, there is much more chance for the soil to have absorbed and held onto as much of the water as possible. This can be achieved by changing from having one start time on your irrigation controller to having 3 start times an hour apart; many controllers name this “Cycle & Soak”.

This is made simple because you can just tell the controller how many total minutes you want to irrigate for, and how many cycles (3 in this instance) you want. Then you can set how long you want to wait between applications, which is the “soak” part. This makes life simpler.

It is important that an irrigation change of routine is not made overnight and hope for the best; if the turf has had lots of shallow watering, its roots will be designed for shallow watering. You’ll need to “train” your turf to grow differently, and this could take a few weeks at best. However, the results should be a stronger turf plant with more resilience.

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