Irrigating the root zone
To use water efficiently, irrigation should meet the water-holding capacity of the rooting depth of the crop. Excess irrigation leads to water draining past the root zone, leaching nutrients and reducing water and nutrient use efficiency.
Too much water in the root zone reduces the amount of oxygen available plants leading to plant stress.
In most vegetable crops, roots will generally not exceed the depth of cultivation. While some stabilisation roots are found beyond 30cm, most fibrous feeder roots are located in the top 30cm. Scheduling irrigation to a depth of 30cm for mature plants reduces excess drainage and increases water and fertiliser use efficiency. Information on the effective root zone of crops is described in our page on calculating readily available water.
Soil moisture probes
Soil moisture logging probes placed in and below the root zone can be used to assess the effectiveness of irrigation. A good setup is to have at least three probes, one measuring the top 15cm, one at 15–30cm and one below the root zone at 30–60cm.
In a plot of soil moisture over time, the rise and fall and the slope of the graph lines indicates the amount and rate of drainage from one soil layer to the next.
The time between movements of the different lines show the time water takes to drain from one layer to the next. Understanding these graphs lets you tailor your irrigation to minimise drainage, retain nutrients within the root zone or alternatively, to leach salts from the root zone.
An example of one of these graphs is shown in Figure 2.