David Doll3 CommentsSeptember 28, 2010Salinity

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

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David Doll1 CommentJune 8, 2010Irrigation

Irrigation Scheduling Part 2 – Determining Water Holding Capacity

Last week we discussed how to calculate almond tree water usage by using evapo-transpiration data provided by CIMIS. Now that tree water use can be determined, we have to determine the amount of water that can be applied to the soil to maximize irrigation efficiency. It is important to understand a few terms when dealing and discussing soil water holding capacity.  Soil saturation is when all of the soil pores are filled with water – this occurs immediately after an irrigation and continues until gravity pulls the water through the pores of the soil (Figure 1). Once the soil has drained, usually 1-3 days after an irrigation/rain event, the soil is at field capacity. This water is held against gravity within small pores by the soil particles, and is the largest amount of water that the soil can hold between irrigations. In contrast, the permanent wilting point is when all the available water is gone. The amount of water between the field capacity and wilting point is termed allowable depletion – or the amount water available to the plant provided by irrigations. It is important to note that at field capacity not all of the pore space is filled with water (about 25% air space), and at permanent wilting point not all of the water is removed from the soil (it is stuck to the soil particles). Figure 1: Differing terms regarding water holding capacity of soils. Basically, when efficiently irrigating, water needs to be applied to refill the soil profile to field capacity but prevent the loss of water to deep percolation. The amount of water to refill the soil profile is related to the texture of the soil; coarser soils hold less water while fine, heavy soils hold more. This is due to the differing size of soil particles and pore space in

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