HortResearch Publication - Irrigating for efficient water use

HortResearch Publication - Irrigating for efficient water use

Brent Clothier and Steve Green - HortResearch, Palmerston North

Water availability varies between the different horticultural regions from being in plentiful supply to being a limited or scarce resource. In many districts, growers now need to monitor their water use with a water meter. However, regardless of whether your water supply is abundant or low, it makes sense to only use the amount of water needed to actually grow a particular crop. By using only what is needed you will :

save water for yourself ;
ensure sufficient water is available, both now and in the future, for other growers and the wider community ;
help prevent the pollution of water-ways by the run-off and leaching of leachates ;
save on electricity through reduced pump use.

Some crops actually benefit from selective irrigation. For example, apples can have better blush colour when there is an irrigation defecit. Reduced irrigation can also lessen stem-end splitting in apples.

This article examines some of the ways in which you can irrigate your crops to allow for efficient water use without affecting the growth of the crop.

The number one rule

In the past, growers have been recommended to wet the soil to "field capacity". In other words, supply sufficient irrigation water to the rootzone to compensate for what is being lost to the air by transpiration.

However, instead of focussing on merely wetting the soil, we should irrigate to supply just sufficient water to active roots to meet the requirements of the plant.

Therefore the number one rule for maximum savings on water and fertilisers is to use high frequency irrigation - that is apply water little and often. It is only necessary to apply enough water to meet the short term demands of the plant rather than saturating the soil with water.

Why irrigate small amounts frequently?

Brent Clothier and his team at HortResearch have shown that it is the roots closest to the soil surface which are most likely to take up water and nutrients. Extra water will drain below the rootzone and can be lost forever to the plant. This excess water can potentially contaminate the ground water.

In an experiment using kiwifruit vines, previously-dry soil was irrigated to a depth of 650 millimetres. Although the soil at 650 millimetres was wet, there was no uptake of water by the roots at this depth. Most of the water was being taken up by the roots nearer the surface (measured by a rapid decline in the tensiometer pressure potential of soil water at 250 millimetres). Therefore, supplying 50 millimetres of water, which wet down to a depth of 650 millimetres, was wasteful. Greater efficiency could have been obtained by applying a smaller amount of irrigation water.

In the same experiment, irrigation water was applied to one half of the vines roots, while other half of the roots were kept dry. The roots which had been wetted had a much higher uptake of water (measured in litres per day) than that of the ‘dry’ roots. And when finally the entire vine’s roots were wetted, the roots that had been ‘previously dry’, bounced back and took up water at a rate higher than they had ever done previously. This may be due to a growth spurt of the near surface roots in the newly wetted soil. This shows the root system is able to quickly adapt in structure and function and adjust to conditions of wetness.

Macropores = uneven wetting of soil

Macropores are gaps in the soil, caused by worm holes, roots or just by the nature of the soil itself. Water moves freely through macropores, so a soil with a large amount of macropores can conduct more water when it is saturated than a soil with few macropores. In a saturated soil with a high number of macropores, rootzone wetting will be less uniform, and there is a greater possibility of losing water by leaching.

Therefore "flooding" the soil to saturation point may not lead to uniform soil wetting and can waste water especially in soils with high numbers of macropores. The rate at which you apply irrigation via a sprinkler must match the conductivity of the soil. If in doubt, apply water at lower rates.

Fertilisers and nutrient loss

Like water, nutrients are taken up by the plant roots closest to the surface. Uptake is governed by various physical, chemical and biological processes. Some of the nutrients are transported by irrigation water into the soil. The rest of the nutrients are drawn by capillary force into the soil’s system of micropores. Once the nutrients are in these micropores, they are less likely to be lost by leaching.

The efficiency of fertilising irrigated soils is greater when the fertiliser is first "washed" into the micropores of the dry soil by a small amount of irrigation. In the dry soil the capillary forces will pull the nutrients into the micropores where the nutrients are available for plant uptake, yet less prone to subsequent leaching by heavy rains.

Summary

Apply irrigation little but often
Do not saturate the soil
In preference, apply fertilisers to dry soils but "wash-in" with a small volume of water.

References

Clothier, B.E. and S.R. Green. 1994. Rootzone processes and the efficient use of irrigation water. Agricultural Water Management. 25 : 1-12.

Clothier, B.E., S.R. Green and G.N. Magesan. 1994. Soil and plant factors that determine efficient use of irrigation water and act to minimise leaching loss. 15th World Congress of Soil Science, Volume 2a : 41-47.

Clothier, B.E. 1989. Research Imperatives for Irrigation Science. Journal of Irrigation and Drainage Engineering. Vol 115. 3.: 421-448.

New for HortNET. Written and submitted by Helen Percy, January, 1997

Copyright © 1997 The Horticulture and Food Research Institute of New Zealand Ltd. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of The Horticulture and Food Research Institute of New Zealand Ltd is prohibited.