How Textural Stratification of Soil Impacts Irrigation

  • Feb 28, 2021

As we approach the bloom season in California avocado groves, it’s important to take action to leach the salts from grove soils. Because this past winter has been low in rain, and California avocado growing regions have experienced high winds and heat the groves will be particularly vulnerable to salt damage.

Salination of the soil can be caused by:

  • Evaporating rainfall or irrigation water — avocado trees do not utilize the salt in the water, thus salt accumulates in the soil
  • Chloride-based fertilizers
  • Some mulches and manures
  • Water logging from irrigation, that brings salt to shallow soil depths

To effectively leach salt from grove soils, it’s important to understand how soil types affect water movement. Water moves fastest through coarse textures (sand) and slowest through finer textures (clays). If grove soils have textural stratification — layers of soil that are substantially different in textural class than the layer above — this can effect the movement of water.

A Walla Walla Community College video illustrates how this works. If you have a layer of fine soil with coarse soil below, as you water the grove the wetting front (where soil beings to move into the soil and also where salts tend to accumulate) moves laterally and downward. It can be pictured as a half circle of water moving downward and outward in the soil. As that wetting front meets a coarse layer of soil below it (which has a greater hydraulic conductivity), one would expect the water to rapidly disperse into the coarse soil. However, that is not what happens. Coarse soils have large particles surrounded by large pockets of trapped air — it is the pressure from this trapped air that prevents the water from moving downward into the coarse soil. Thus, the wetting front expands outward and builds up in the loamy top layer of soil until enough pressure builds up from the excess water in the upper layer to overcome the air resistance of the lower soil layer and push down through the coarser texture.

If grove soils are comprised of loam (medium soil) above clay (fine soil), you also see a slower movement of water. However, this is due to the low hydraulic conductivity of clay, which simply can’t transmit water fast enough.

To ensure salts are leached from the soil, it’s important to remember the top six inches of avocado grove soil are the most important to manage for salinity as avocado trees have a shallow root system.  Salinity management best practices include:

  • Avoid short, frequent irrigation cycles, as salts are not leached
  • Avoid prolonged saturated soils, or standing water, that leads to root rot
  • Optimal irrigation requires uniform water application and mass, in a large, low-EC zone, under trees
  • Drip irrigation is not ideal for managing salinity in hot, dry weather, as it only supports roots in a narrow zone of low EC soil
  • Soil water of about 4 EC, or TDS of 2000, is too salty, as water will leave roots
  • Effective leaching requires monitoring soil water to determine irrigation volume and duration
  • Generally aim to use a 10-20% leaching fraction at each irrigation, to maintain a root-zone salinity of soil water below EC 2
  • Leaching fraction is the amount of additional irrigation water needed to maintain the correct salinity; this, however, depends on salt levels in irrigation water
  • Bumping is the technique whereby an irrigation is stopped and then restarted in order to improve leaching and reduce runoff

If you use two sources of water when bumping, use the poorer quality water first, then use the better quality water when your restart the irrigation

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