Salt Leaching in Orchards and Vineyards
Abdelmoneim Z. Mohamed, Ph.D., Irrigation and Soils Advisor, UCCE Stanislaus County
Leaching refers to the process of flushing excess salts from the root zone of plants through the soil, which promotes a more productive growing environment. However, the combination of limited rainfall and extensive irrigation and fertilizer applications in California orchards and vineyards can lead to insufficient leaching, resulting in an accumulation of salts in the root zone. During the transpiration process, crop roots take up water and leave most of the salts behind. Salt accumulation in the root zone creates osmotic stress conditions and plants must extend energy to get water from the soil. The increase in energy use and reduction in transpiration decreases shoot growth and yield. Salt buildup can also result in toxic conditions for plants and decrease water infiltration in the soil. In drip-irrigated orchards, salts tend to accumulate on the edges of the wetted areas along tree rows while in microsprinkler irrigated orchards, they accumulate in the middle of the tree rows that are on the edges of the wetted patterns. Soil texture plays an important role in salt buildup; more salts accumulate in heavier soils compared to sandy soils. Also, irrigation from surface water has a lower salinity level than well water.
An effective leaching strategy requires soil and water salinity analysis. Regular soil testing and monitoring can help farmers detect salt accumulation earlier so they can take corrective actions. For example, leaching is required for almonds if the electrical conductivity (EC) of soil is higher than 1.5 dS/m (Table 1, Page 5), as yield will decline after this salinity threshold point. When salt levels are above 1.5 dS/m, a reduction in growth rate and yield can range from 18-21% for almond trees on peach rootstocks such as Nemaguard or Lovell.
Leaching can be done in-season or at the end of the season (dormant leaching). In-season leaching is important when soil salinity in the root zone at the beginning of the season is near the threshold. In-season leaching can be implemented by increasing the irrigation duration of each event by 15-20%. This practice has some risks if soils conditions are kept too wet and can result in reduced root development, root diseases, nutrient leaching, and delayed fruit development. Leaching salts is more effective in winter (dormant leaching) because of reduced evaporation and plant water demand. Leaching occurs when soil water content is higher than field capacity. By filling the soil profile with irrigation water prior to the rainy season, this ensures that subsequent rainfall will maximize the amount of leaching during the winter.
After salinity analysis, the leaching requirement can be calculated using the following equation:
where LR is the minimum leaching requirement fraction %, ECe is the average soil salinity tolerated by the crop as measured on a soil saturation extract (dS/m), and ECw is the salinity of the applied irrigation water (dS/m).
Example: Almond has a soil salinity threshold of 1.5 dS/m (Table 1, Page 5). What is the leaching requirement if the salinity of irrigation water is 1.5 dS/m?
Answer: LR = (1.5 × 100) ÷ {(5 × 1.5) – 1.5} = 25%
The total irrigation water amount needed to meet crop water demand and leaching can be determined as follows.
where ETc is the crop water requirement (mm) and WR is the water requirement depth (in).
Example: Crop ET is calculated to be 0.8 inches, and the desired leaching requirements (fraction %) is 20%. How much water must be applied?
Answer: WR = 0.8 ÷ {1 – (20 ÷ 100)} = 1 in
The amount of water required for leaching to reduce soil EC to 1.5 dS/m can be estimated from (Table 2, Page 2) adapted from the reclamation curves for saline soils using sprinkler methods or intermittent ponding sprinkling methods (Pritchard et al. 1985).
