Certain stages of tree crop development are more sensitive to water stress than others:
Early season water stress compromises a range of growth processes including bud break, bloom, flowering, fruit set, and shoot growth:
As a result, common practice is to start the season with a near-full soil water profile, and irrigation is used to maintain a good water supply.
As a general rule, plant water stress should be avoided during rapid fruit growth. Thus, common practice is to minimize water deficits at this stage, when the crop is most susceptible to drought.
Nut tree growth is generally less sensitive to water stress than fruit trees at this stage. However, other factors, such as hull split in almonds, shell splitting in pistachios, and kernel sunburn in walnuts are affected by large water deficits.
The post-harvest period is generally when trees can tolerate most water-stress. Irrigation can be safely reduced (not omitted) in most species.
The rule of thumb is that as long as the trees do not defoliate, the stress can be tolerated with little impact on production in the following years. The notable exception to this rule is almonds (see below) where water stress post-harvest reduces fruiting in the following season.
In almonds, full irrigation early in the season is important for good growth of the fruiting spurs and therefore necessary for long-term tree productivity. In early to mid-season cultivars, irrigation can be curtailed between mid-June and harvest (for around a two-month period).
Later-harvested cultivars can also tolerate a two-month period of severe water deprivation before harvest. This reduces kernel weight by around 10% and reduces hull splitting; resuming full irrigation two weeks before harvest will increase hull splitting.
In almonds, on shallow or drip-irrigated soils, post-harvest water management is crucial for the following year’s crop. Research in California shows that postharvest water deprivation reduced fruit set and, to a lesser extent, bloom.
On deep soils that are surface or sprinkler-irrigated close to harvest, post-harvest irrigation may not be as important.
Premature defoliation of trees indicates inadequate irrigation.
Pre-harvest defoliation followed by post-harvest irrigation leads to a stronger leaf canopy in the following spring. This practice is now also thought to benefit the tree by improving tree water status during the critical period of flower bud formation.
Effective fertigation is a key means of producing optimum yield, without adversly affecting the environment. Research by UC Davis, California shows that fanjet irrigated trees can outperform drip irrigated trees.
Average yield increases were between 3-7% but, where potassium was continuously fertigated, fanjet irrigation raised yields by 14% compared to drip irrigation.
The most efficient irrigation systems also ensure better utilization of nutrients e.g. nitrogen.
When managed correctly, continuous fertigation will reduce the risk of deep nitrate leaching by maintaining the nitrogen in the root zone.
It also provides greater flexibility to adjust in season fertilizer rates than less frequent irrigation use.
Research at UC Davis shows that applying the nitrogen in the final two hours of the irrigation schedule results in less leaching. Allow sufficient time for the fertilizer to be flushed out of the lines before the irrigation terminates.
For further information on effective fertigation, consult your Yara specialist and ask for Yara's Fertigation Manual.
Growers need to assess the salinity of irrigation water – particularly levels of chloride and boron - and adjust practices accordingly.
Irrigation water with a total salt status, or ECw (dS/m) of 1.0 will have little effect on almond yield. However, when salinity increases to 2.8dS/m, yields can be halved.
If saline water must be used because of drought, applying more water to increase leaching can lessen the effects of salinity on plant growth.
Consult with your local Yara agronomist for specific recommendations when dealing with saline or sodic soils or irrigation water.
Yara’s nutrient recommendations can help the crop make more productive use of water. With irrigation water supply and quality an increasingly key issue it is important to gain maximum benefit from moisture and nutrients to maximize nut yields. The use of specific nutrients as part of a tailored fertilizer program encourages strong growth and high yields, leading to more transpiration and less drainage and direct evaporation. For example: - Use of phosphorus encourages rooting, ensuring better extraction of water from the soil over the life of the grove. - Potassium (via its osmotic function) and nitrogen (as a result of the internal regulation of hormones and xylem pH) are important for efficient and optimal regulation and control of CO2 uptake and water loss through the stomata. - Adequate calcium also helps increase water use efficiency. By using comprehensive, tailored, fertilizer programs that ensure no nutrient is lacking, yield is increased, producing more fruit for each inch of water.
Legislation is increasingly influencing the way growers are managing their crops and, in particular, their use of fertilizers. In California, for example, water resources are scarce and severe drought an issue. High levels of nitrate in water are being found and nitrogen use is being monitored and recorded on a farm by farm basis. It is expected that impending future Californian legislation will regulate water and nitrogen use more closely to help minimize these issues. Yara recognizes this and is committed to working with its customers to ensure legislative measures such as those in California are addressed. However, information posted here focuses on best practice nutrition to maximize production and is used as a reference worldwide, so cannot provide the individual advice required to satisfy legislative requirement. For local support on these and other environmental or legislative issues, please contact your local Yara agronomist.