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Knowing your soils: Trigger points and over-irrigation


When irrigation is applied to pasture, it’s generally invisible. 

Some of the advantages of New Zealand soils is that they’re very free draining, but as a result, the consequences of over-irrigation take a while for people to observe - it’s hard to know how much you’ve put on and to see where you are.

Now people have learnt that when too much water is put on, water washes through the soil - taking the nitrogen through with it, which ultimately end up in waterways.

Over-irrigation is also a loss to the farmer – that’s nitrogen they’ve paid one way or another to have on their land and that’s been washed away - which has to be replaced with fertiliser.  Balance therefore is again important. By only putting on enough water as you need to optimise pasture growth, you’re not making it too wet. When the grass is too wet, a lot of problems occur. You have too much mud, pivots cause ruts, and the grass doesn’t grow as well as the plant needs a ratio of air around it as well as adequate moisture. Pasture damage also occurs easily when grass is too wet, with  heavy cows and machinery causing soil compaction. 

Soil composition diagram

You’re also limiting the biological activity in the soil when everything gets squashed up, and you get trouble with infiltration rates – where water can’t actually seep through because you’ve made a hard concrete layer on top. These negatives don’t necessarily jump out as a big red flag but over time, they impact on the efficiency of irrigation based on how much extra new grass you can grow because of the extra water you put on. 

Irrigation is wonderful – it enables land to be productive when otherwise it wouldn’t be. It’s about trying to find the way to do it. That’s where technology and innovation really can help. Farmers are out in their paddocks, they’re getting stock in, they’re doing activities and they’re seeing their pasture growth, but what you see on the top is not necessarily what’s happening underneath.  How do you know what normal or good looks like if you have nothing to calibrate that visual against? 

Collecting and having data also involves looking at it and analysing it is a way to calibrate the data and visual information for yourself. What you observe is not redundant – going out and looking is not redundant – it’s another piece of information you put together like a jigsaw puzzle and you’re making the picture. Over time, putting in sensors and seeing the soil moisture changes against what you’re observing in terms of the quality of your pasture, what you might be measuring as well in terms of pasture growth rates, how easy it is to drive across the paddocks, you’re not getting as many pivot ruts – then a farmer can get a better feel to how much water my soil needs.

Soil types

Another factor to consider with irrigation is soil.  Not all soils are the same - some soils are more rocky and stony than soil. This soil type can only take a small amount of water – any more and it washes away  - the soil can’t hold it. Other soils are more like clay, which can hold a lot of water. However, when they’re wet they can get really claggy and horrible and then become extremely hard when they dry out. Then you’ve got the loams in the middle – which are lovely but not everyone has them! 

So understanding what kind of soil you’ve got, and how that differs across your farm, and under different irrigation systems is another thing to think about on how to manage the irrigation that they can do best to manage the soil types they’ve got. 

The types of soil types are optimal in terms of how we apply water to them, how do we know what those gaps are to fill? From a logistical, labour point of view, how do you fit that in with all the other jobs there is to do on the - turning them on or off and moving them around – and then, you need to be able to report how you’re doing those things. What’s your process around keeping records and knowing that you’re operating your system as well as you could be?

So Irrigation 101 quickly has to get to Irrigation 304 in terms of being able to meet the expectations of what it takes to be a good irrigator in 2020. 

Trigger Point

The trigger point is established under a pivot block – whether that’s a pivot or a series of k-lines. It’s configured through understanding the soil type that’s there, knowing the differences of soil moisture across the seasons and where ideally you want your soil moisture to be at. If you keep your soil moisture around an ideal point consistently, pasture growth would be optimised.

In the Regen system, this ideal point is customisable to 365 days in the year. It checks the trigger point against multiple factors, including the weather. As long as there’s not rain in the forecast, it’ll show how much you need to be irrigating so when you irrigate you’re always above or at that trigger point. You can’t control the rain that comes – it might keep you above that trigger point, but our system makes sure you don’t drop below it.

 You’re always wanting to keep a little buffer at the top to stay below field capacity and ensure the soils are not too wet. Field capacity is not a target, like they say on road signs - 100km is not a target!  There’s certain circumstances where you might be at field capacity but that’s not what you’re aiming for. 

If you’ve had a history of slightly over-irrigating too much, does your soil become degraded over time?

AgResearch Canterbury study where they compared farms that have been irrigated and soil next door to it that hasn’t been irrigated and looking at the changes in soil composition over time, it has shown that the soil that hasn’t been irrigated, the benefits from irrigation were balanced out by the impact of over-irrigation, compaction, washing nutrients out of the soil – if done wrong can take away the gains you get from irrigation because you take away the gaps with water. It’s a particularly large pivot that they’re applying – what they call an instantaneous application rate – gets really heavy  and what they call the end gun – the thing that shoots water out the end – that can be 40 or 50 ml of rain just thundering down.

Can you imagine if there was a thunderstorm and it was 40-50 ml in an hour - how heavy and intense that is? When it falls on the ground, it causes physical impact and damage that is magnified with stock and other stuff on the pasture. 

When irrigation is done well, it improves soil health. The pasture grows, amazing biological activity happens with worms and fungi. However, when you do it poorly, you’re making all that stuff hard to happen and it doesn’t happen.  The balance and finding that sweet spot where that’s happening is vital. 

There’s not a great fundamental knowledge about soil and how to manage soil. The focus on irrigation management for a long time was how to use it – how to turn it on, turn it off, making sure things are out of the way, you haven’t got blocked nozzles – the really obvious stuff. When you first start irrigating, the difference is staggering – it’s really hard to see the negatives, you’ve suddenly got a big positive. But over time, some of these gains can be lost if you’re not irrigating well.

Bridgit Hawkins

CEO/Executive Director