HortResearch Publication - Crop Water and Irrigation Studies at HortResearch
See also : Irrigating for efficient water use.
"Sustainability" is a well-used buzz word at the moment. Many growers are now aware of pest and disease control initiatives such as "Kiwigreen" , Integrated Pest Management(IPM) and Integrated Fruit Production (IFP) programmes. However, producing fruit and vegetables in a sustainable manner does not just involve cutting down on pesticides. A sustainable horticultural unit is one where water is considered to be a scarce resource and used sparingly. Leaching of fertilisers and chemicals into the groundwater is also unacceptable in a sustainable production system - nutrient leaching is not only inefficient, but the leachates can also pollute the groundwater.
In order to develop more sustainable horticultural systems, HortResearch’s Environment Group are gaining a better understanding on how water and fertilisers move through the soil and how the tree roots uptake and use this water. By understanding the physical and physiological principles of water and solute movement, the scientists can then make recommendations on water use. The Environment Group have already been contracted by several regional councils to recommend water allocation levels for horticultural crops.
In this article, we discuss some of the research currently being undertaken by the Environment Group and its potential application for growers wanting a more sustainable horticultural operation.
HortResearch scientists are at the forefront of world research on the transfer and uptake of water by plant roots. To enable our researchers to study the roots without removing the tree from the ground, an underground camera attached to a computer has been developed. Miniaturised sensors are used to measure the sap flow within roots, and underground "time lapse" photography provides a picture of how roots grow.
For more information see : A root with a view?
To develop more efficient and environmentally sustainable strategies of rootzone management, we need to understand the physical processes which govern water and chemical entry into the root zone. However, it is also important to understand the biological and physiological processes of how water and water-borne chemicals are taken up by the roots. HortResearch scientists have predicted and then measured root water-uptake of mature apple trees in an orchard in order to quantify this water uptake process.
For apple trees, the fine roots are mostly found in the top 40 - 60 cm of the root zone. This is where most of the uptake of water, and presumably nutrients, is occurring. Any nutrients which are not taken up in this "active part" of the root system will eventually travel downwards, through the soil profile. This could lead to contamination of the ground-water supplies. Further work will lead us to predict the transport, uptake and eventual fate of any surface-applied agrichemicals.
One experiment was set up so that a waterproof plastic sheet prevented any rain from entering the root zone of apple trees. This meant that a controlled amount of water could be applied to the tree. The surrounding trees (which had normal amounts of rainfall), received about 1.5 times more water than the protected tree. The trees exposed to rainfall would be expected to have greater drainage than from the trees with regulated irrigation. The scientists calculated that, by the end of the experiment, 120 millimetres of water would drain out of the root zone. Heavy rain towards the end of summer is especially likely to cause the leaching of mobile chemicals and nutrients beyond the root zone because the soil is more likely to be saturated, and the tree will be transpiring less than in mid-summer.
Understanding how surface-applied fertilisers move through the soil can give us a better understanding of when and why we will get fertiliser loss. Water, and hence solutes, does not flow uniformly through the soil. Water moves much faster through large pores than small ones. However, we don’t yet know how much water is needed to move the solutes (fertilisers) through the soil when different flow rates of water are applied. More information needs to be found out about the movement of cations (such as magnesium and calcium) and anions (such as chlorine) in the soil. HortResearch scientists have been studying solute transport by using large columns of soil in the laboratory to represent soil in the field. The researchers use mathematical equations to calculate the movement of solutes through the soil.
They have found that about 90% of the soil water is associated with the movement of solutes in the soil and about 80% is involved in cation transport. This suggests that cations applied in fertilisers are more likely to be retained near the soil surface.
See : Looking at real world problems.
The ARC was finding that, in general, the current monitored water usage does not justify the volumes of water requested in the resource consent applications. In Auckland, as in other regions around New Zealand, very little information exists on the water use by specific crop types, as well as differences in climate and soil types.
To calculate the water-use of the plants, the HortResearch scientists considered all the variables involved : stored water, crop water use, precipitation, surface runoff and deep percolation, irrigation. The scientists made their calculations based on average soil and growing conditions.
These findings indicated that the annual average irrigation requirement for orchards was 1700 m3ha-1 (close to the current criterion of 1500) ; for market gardens 1200 m3ha-1 (well under the actual allocation of 2100); and for greenhouses some 5100 m3ha-1(which is at the low end of the present range of 4800-12,000). However, for greenhouses, the currently low efficiency of the greenhouse emitters might call for greater water applications.
Therefore, considerable savings can be made to water allocations for market gardens (and other horticultural operations) in the Auckland area. The ARC now have a more accurate guide to base their water allocations on, leading to more efficient water-use in this region. Horticulturalists in the Auckland region can be assured that their water allocation is based on the crop’s requirements. More fine-tuning is still needed to determine soil water storage for each specific combination of soil and crop.
Previous studies have shown that water deficit irrigation can advance maturity in apple trees. In recent HortResearch studies, Dr Mills showed that although water deficit irrigation influences commercial maturity (measured by soluble solids) and colour, it does not affect the physiological maturity of apples (measured by the level of ethylene). Growers want to harvest a flavoursome apple with high blush colour. However, the apple must not be physiologically over-mature (high levels of ethylene) as this leads to poor storage qualities. Water deficit irrigation, therefore, appears to have positive effects for maturity of Braeburn apples by advancing colour and flavour, but not physiological maturity.
For more details see: Water status and apple maturity