As we push forward onto harvest the effects of deficit-irrigation, poor irrigation water quality, and heat are taking a toll on almond orchards throughout the valley. Subtle changes have occurred increasing the adverse effect of salt buildup. Droughts and reduced water deliveries only exacerbate the situation. The problem is a gradual buildup of salt levels in orchards. Whether this problem originated from poor water quality, a reduced water supply, poor irrigation management, or consecutive droughts, the ramifications of this salt buildup, if left unchecked, will negatively impact almond production.
Salinity reduces water availability for plant use. High salt levels hinder water absorption, inducing physiological drought in the plant. The soil may contain adequate water, but plant roots are unable to absorb the water due to unfavorable osmotic pressure. This is referred to as the osmotic or water-deficit effect of salinity. Plants are generally most sensitive to salinity during germination and early growth.
The second effect of salinity is shown when excessive amounts of salt enter the plant in the transpiration stream and injure leaf cells, which further reduces growth. This is called the salt-specific or ion-excess effect of salinity (Greenway and Munns, 1980). Damaging effects of salinity involve disturbances of ion balance, induced by reduced plant uptake of nutritional elements such as K, Ca and P, and excess accumulation to toxic levels of Na or Cl Symptoms include restricted root growth, marginal or leaf tip burning/ browning, inhibited flowering, reduced vigor, and reduced crop yields.
Irrigation water high in soluble salts and sodium adversely does affect almond productivity by increasing the electrical conductivity (ECe) and the exchangeable sodium percentage (ESP) of the soil. Salinity becomes a problem when enough salts accumulate in the root zone to negatively affect plant growth. Excess salts in the root zone hinder plant roots from withdrawing water from surrounding soil. This lowers the amount of water available to the plant, regardless of the amount of water in the root zone.
There are several more reasons we are recently seeing these changes. Obviously, if the source of irrigation water servicing the orchard has had a decline in quality (i.e. higher salt levels), problems will surface. However, even if water quality is marginally good, problems will arise if irrigations are not properly managed.
Damaging effects of salinity involve disturbances of ion balance, induced by reduced plant uptake of nutritional elements such as K, Ca and P, and excess accumulation to toxic levels of Na or Cl (Munns & Tester, 2008;Bernstein, 2013;Parihar et al., 2015). Salinity alters mineral composition of plants via integrative effects on the ratio between ions in the rhizosphere, uptake processes into the root, and transport or partitioning within the plant body. ... the four most salt-tolerant rootstocks (Empyrean 1, Cornerstone, BB 106 and Bright’s Hybrid 5).
These changes tend to gradually increase the levels of chlorides or sodium each successive year. Because the changes occur slowly over the course of several years, the actual levels may seem un-alarming, even though they may be well above the thresholds that are normally considered toxic to the trees. In addition, this slow change rarely produces the “acute toxicity” symptoms normally associated with salt burn. What does occur is a reduction of shoot growth and smaller kernel sizes. Trees may take on the appearance of being dry stressed, even shortly after an irrigation when soil moisture is ample.
The pre-harvest to post-harvest water deficit period also coincides with the flower bud initiation period. The usual practice of ‘holding off’ the water in preparation for, and during harvest and drying, must be managed carefully because pre-harvest water stress affects current season nuts, and post-harvest water stress directly affects bud initiation and development, and therefore the subsequent season’s yield. The benefits – minimized trunk damage from shakers, hull rot, and ground moisture and humidity for drying of pre-harvest deprivation must be balanced alongside the less desirable effects – increased soil salinity, reduced kernel weight, increase in ‘partial splits and/or ‘hull-tight’ nuts, reduction in late season leaf function, and stress presenting as wilt and/or premature leaf drop and biomass reduction.
Stress results in reduced carbohydrate development. With less sugars available to the tree, there is less energy to devote to flower formation. Consequently, “bloom” the following year will be lower in quantity (fewer flowers) and possibly quality. Water stress also affects the tree beyond flower formation. Premature leaf drop will prevent movement of macronutrients (nitrogen, potassium, phosphorous) from the leaves to the spurs for use in the next growing season.
Remember that all the soil amendments in the world will be of little value if the salt they free up is not leached out the bottom of the rootzone. If reduced water allocations prevent irrigating with leaching considerations, or in many cases even to full ET, problems with salts can be expected.
The perfect cure is a steady rain over several days during the dormant period, since that is when soils temperatures are lowest (reduces Phytophthora risk), root activity, shoot activity, and ET are at a minimum, and crop and cultural activities are not compromised. Recently we have not been blessed with such winters. If supplemental water (either surface or well) is available to winter leach, preferably in early January or sooner, it should definitely be implemented.