More and More Salinity Stress?
Are you seeing an increase of salinity stress in your trees over the past few years? One probable cause for this could be the drought we have been facing the last decade, leading to an increased use of groundwater for irrigation. Groundwater in the central valley is naturally higher in salinity than surface water. Over time, this increased use of groundwater results in salt accumulation in the rootzone and the problem becomes compounded year after year without proper treatment.
Why Does This Cause Stress?
To understand the effects of salinity we need to go back to high school chemistry… Osmosis! Osmosis is the process of a solvent (water) moving through a semipermeable membrane (living cell/tree) into a solution of higher solute concentration trying to equalize the concentration of the solute on both sides of the membrane. Think about after you have eaten a highly salty meal or handful of sunflower seeds, your body’s way of equalizing the salt intake is to make you thirsty. The water you drink then neutralizes the salt concentrations in your body’s cells. The same thing happens in the rootzone of your crop, when you irrigate with water high in salinity, your crop pulls that water leaving just the salts. As these salts accumulate at your rootzone, the natural path of water will be to go from your tree and to this high salt concentrated area in the soil. This natural osmotic force makes your crop work harder to pull that water into its cell membranes. The extra energy expended leaves less energy for proper growth and development in your trees. The first visual sign on this is yellowing of leaves.
How to Fight This?
One way to battle this is through leaching, post-harvest leaching to be specific. Leaching is the deep percolation of water in excess of crop ET needs so that some irrigation water passes completely through the soil root zone. You may practice leaching throughout the season as well, however post harvest leaching is found to be most efficient and effective. Naturally, after harvest your trees are under stress and are pulling deeper profile moisture than during the normal irrigation cycle, resulting in a clear path for leaching water to flow. Because salts will dissolve in water, filling this profile with water will essentially allow the salts to flow where the water flows. Again, because your deep moisture is now depleted, you will push those salt particles down the profile as you leach water past your active root zone. Once the profile is full and the “salty” water is starting to leach past your root zone, any clean rain water will continue push this “salty” water even further down your profile leaving clean percolated water available to your plant’s rootzone.
How Much to Leach?
Determining how much water is needed to leach requires a little research. To properly do this you will need to sample your soil to find your salinity levels. From there you can look to a couple resources to find the Leaching Factor your soil requires, almonds.com has a one pager with sample percentages and inches needed based on your soil conditions. On average you will require a 10-20% increase to your normal post harvest irrigation requirements to sufficiently prep your soil for proper winter leaching. Again, this is based on the current state of your soil salinity as well as your soil type. Lighter/sandier soils require less additional water than heavier/loam or clay soils.
Keep the End Goal in Mind!
With all this research, added efforts and resources the goal is to pass salt particles past your active root zone. As we mentioned, you may do this during your normal irrigation season, however utilizing any clean rainwater will increase the effectiveness of leaching and that is why this timing is critical. Additionally, this preps your upcoming season to start off on the right foot with clean water when your tree starts demanding resources.