26 Sep 2014

Spring irrigation: do I, don’t I?

Spring can be a tricky season to decide when to start irrigating. Some parts of the country are already getting dry and you may be thinking about or have already turned on your irrigators. Too early and you risk putting on too much water causing drainage, losing nutrients, and keeping soil temp down. Leave it too late and the grass may get into moisture stress causing reduced pasture growth rates.

So what are the key variables to think about when assessing whether to put on some water at this stage in the season? Your soil’s total available water, your current soil water deficit, forecast rainfall and evapotranspiration rates are all key variables to know. Once you know your soil’s total available water, a daily soil water balance gives an indication of how dry the soil is, and therefore whether irrigation is needed and how much water the soil can hold.

The capacity of your soil to hold water (total available water) is dependent on your soil type; shallow gravelly or sandy soils hold less water than a deep silt loam. When your soil is full of water this is known as field capacity. Plants extract water most easily from the top half of available water. When your soil drops to 50% of it’s total available water this is known as the refill point. Keeping your soil between just below field capacity (allowing some room for any rain) and refill point is the optimum zone for pasture growth (shown as the green dotted lines in the graph below).

You want to keep your soils within this optimum zone for as much of your growing season as possible. The graph above shows a typical soil water deficit over a year. Over winter you tend to be at or near field capacity. It is important in spring not to irrigate too soon as putting lots of water on cold soils can slow pasture growth. However as the spring progresses evapotranspiration rates start to rise, rainfall is less frequent and your soil starts losing water. It doesn’t take long for the soil water deficit to drop below the optimum zone and it becomes increasingly difficult to catch up.

Regen is working on a solution for tracking your soil water deficit and managing your irrigation more efficiently. Keep an eye out for more details about our new irrigation service in the coming weeks.


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