Soil Sampling and the Effectiveness of Leaching

Unwanted sodium, chloride, and boron ions can accumulate and cause damage to the almond tree. These ions are introduced into the rooting zone through irrigation, and will remain within the rooting zone until they are either removed by the plant or leached beyond the rooting profile.

Where soil salinity is a problem, periodic soil sampling should be performed. This analysis will provide the information to determine if the salts are accumulating to a toxic level and if the applied leaching fractions are adequate. Samples should be taken from areas of the orchard showing uniformity in reduced growth or toxicity symptoms. At each sampling location, soil should be taken for each foot for the top five feet. Do not pool the soil to create a composite sample; rather, take enough samples to represent the growth differences within the orchard. The sample should also take into account the emitter patterns as differing locations may have differing salinity levels. The samples should be submitted to an analytical lab and tested for the salts of concern.

Once the results from the analysis are received, the concentration of salts at the various depths can determine the effectiveness of the applied leaching fractions. If the soil salinitity levels are the lowest near the soil surface and increase with depth, leaching is occurring. This gradient is due to the relatively low salinity of the irrigation water, the movement of salts with the water as the water infiltrates the soil during an irrigation. In contrast, if the salt levels are the highest near the surface, and decrease with depth, no leaching is occurring. The leaching fraction must be adjusted to help move soils below the active rooting zone.

Keep in mind that larger leaching fractions will result in more uniform salinity as depth increases. Inadequate leaching fractions will result in increases in soil salinity as depth increases. Water containing high amounts of salts will require a larger leaching fraction than irrigation water with low salt concentrations.

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2 thoughts on “Soil Sampling and the Effectiveness of Leaching

  1. David, another good article, some questions that come to my mind from this article as well as the email discussion we had are:
    – What is the best time to water? According to Almond production manual winter time is what they recommended. I am also toying with the idea of watering with well water only at nights, let’s say 6PM to 6AM pretty much every night, since we will not loose lot of water to evaporation, I have not seen this mentioned anywhere what are the pro’s and cons to doing that?
    – How does leeching work with different soil types? Especially with micro-sprinklers

    Another request on a future topic “some kind of data or analysis on how many acres of almonds are planted on what soil types in northern California and what type of production as well as challenges exist based on soil types.

  2. Some great questions. I will try and address some of them here, while some may be deserving for an article themselves.

    1 – Best time to water. I assume by this you mean in regards to leaching. Filling the soil profile in the winter time provides the ability for the rainfall to help in leaching salts. Think of it this way – if it takes 5 inches of water to refill the soil profile, and it only rains 8 inches, only 3 inches of water are available to leach salts beyond the rooting profile. If the soil profile is filled before the rains begin to fall, all 8 inches are available to provide a leaching effect. Caution should be used to avoid soil saturation during cold periods as this may increase Phytophthora root and crown disease.

    Watering at night: Does reduce the salt load because you are increasing irrigation efficiency. This increase in efficiency is due to the reduction of water lost to evaporation. Other than loss of sleep, I do not see any cons to this practice. Does anybody with experience experience things we should be aware of in regards to evening irrigations?

    The biggest challenge – since the irrigation set time is reduced – is to make sure enough water is applied to the orchard.

    Soil types: Yes soil types do matter, mainly due to their difference in cation exchange capacity and water holding capacity. Finer soils will take more water and time to leach salts, while coarse soils will leach salts with less water. Finer soils are able to hold more sodium ions due to the increased exchange capacity of the soil. In coarse soils, salt damage will occur at lower sodium levels than in finer soils – again due to the exchange capacity of the soil.

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