Author:

Stephen R. Grattan
Plant-Water Relations Specialist
University of California, Davis
Jim Oster
Soils and Water Specialist
University of California, Riverside

Drought Tip 92-19 is a publication series developed as a cooperative effort by the following organizations:

California Department of Water Resources - Water Conservation Office
Department of Land, Air and Water Resources University of California
USDA Drought Response Office
USDA Soil Conservation Service If you have comments or suggestions, please email lawrweb@ucdavis.edu.

Last reviewed December 19, 2002

Drought Tip 92-19
Water Quality Guidelines for Trees and Vines

Agricultural soils and irrigation water contain varying amounts and types of salts, but a soil is not considered saline unless the concentration of salts in the crop rootzone is high enough to reduce crop growth and yield. Tree and vine crops are generally more sensitive than field crops to salinity, chloride, sodium, and boron.

Salinity affects tree and vine performance in two ways. First, the plants must acclimate themsoelves to a saline environment in order for water to become available. This process requires energy the plant normally uses for growth and production. Second, chloride, sodium, and boron can reduce yields because of speicifi ion toxicity. Sensitive trees and vines can accumulate these elements in leaves, causing leaf burn or in limbs or new shoots affecting the yield potential the next season. The two processes can operate simultaneously and can reduce crop yield.

Salinity Threshold

All tree andvine crops can tolerate some salts in the rootzone without harm to yield or plant quality. The maximum amount of salt the plant can tolerate in the rootzone without reduction in growth or yield is called the "salinity threshold". Beyond this level crop yields are reduced in proportion to the salt concentration in the rootzone.

Irrigation Management

Effective irrigation management is important anytime, regardless of the availability of water, but becomes essential during a drought. After irrigation water and its dissolved salts move into the crop rootzone, the plant extracts "pure water", for the most part leaving the salts behind. The amount of salt in the rootzone will increase over time unless more water than the crop uses is applied. This excess water controls soil salinity levels by leaching some of the salt from the rootzone. The fraction of applied water that moves downward through the rootzone and is not used by the crop is called the "leaching fraction".

Soil salinity is expressed as the electrical conductivity of the saturated soil extract (ECe) (with the units usually expressed as mmhos/cm or dS/m). rootzone salinity (ECe) increases as the leaching fraction decreases for a given irrigation water salinity (ECw). Increasing the leaching fraction when using a more saline irrigation water can result in the same average rootzone salinity as using a less saline irrigation water with a lower leaching fraction. In short, if a more saline water must be used because of drought, applying more water to increase leaching can lessen the effects of salinity on plant growth.

Water Quality Guidelines

Table 1 lists water quality guidelines for the most commonly grown tree and vine crops in California. These guidelines assume that the soil is well-drained -- that is, that adequate soil aeration exists for root respiration and disease control -- and that the leaching fraction is 0.15. Under these conditions the relationship between average rootzone salinity (ECe) and ECw is ECe = 1.5 ECw. It is also assumed that all other factors (such as fertility, irrigation scheduling, and pest control) are managed for optimal crop performance. The ECw values given in the table represent the maximums that can be continuously used to achieve the given yield. For example, the ECw values at 100% yield represent the poorest quality water that, if used continuously, will produce ECe levels equal to the salinity thresholds.

Toxicity to Specific Elements

Unklike most annual crops, tree and vine crops are generally susceptible to boron and chloride toxicity. Tolerances vary among species and rootstocks. Tolerant varieties and rootstocks restrict the uptake and accumulation of boron and chloride in leaf tissue. Boron concentrations in the irrigation water exceeding 0.5 to 0.75 mg/L can reduce plant growth and yield. Climatic effects are also important. In the cool moist coastal climates, irrigation waters with boron concentrations exceeding 1 mg/L are used successrfully on treee and vine crops.

Chloride moves readily with the soil water, is taken up by the plant roots, translocates to the shoot and accumulates in the leaves. Table 2 contains maximum permissible concentrations of chloride in the irrigation water (assuming a 15% leaching fraction) that various rootstocks or cultivars can tolerate without experiencing leaf injury. Injury does not necessarily mean a reduction in yield or vice-versa. Chloride injury usually begins with a chlorosis (yellowing) in the leaf tip and margins and progresses to leaf burn or drying of the tissue as ijury becomes more acut. Chloride injury can also result from direct leaf absorption during overhead sprinkler irrigation.

Short-term Versus Long-term Use of Water

These guidelines are based on the long-term use of the given water quality. Poorer quality water can be tolerated if used on a short-term basis.

If good quality water is used for one-third of the irrigation season, saline water with an ECw that would cause a 25% to 50% yield reduction if used continuously (Table 1) may be used for the remaining season with little or no yield reduction. Caution is advised in using this irrigation strategy since reduced growth and increased levels of chloride and boron in the soil and plant tissue could reduce yields in future years. Sufficient rainfall or good quality water is needed the subsequent year to leach most of the salts from the upper tow or three feet of the rootzone. In some soils, good quality water following saline water could cause reduced soil-water infiltration, creating an ideal environment for root diseases.

Table 1. Estimated crop yield using irrigation water of various qualities over the long-term. Potential yields are based on a 15% leaching fraction.

Table 2. Maximum chloride concentrations that various tree and vine crops can tolerate without developing leaf burn. (Assumes long-term effects and a 15% leaching fraction.)

*These concentrations may exceed the salinity threshold and cause yield reduction in some crops.
**Data from Australia indicates that rough lemon is more sensitive to Cl than sweet orange.
***Data available for one variety of each species only.