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Fluid Journal : Summer 2014
6 The Fluid Journal Summer 2014 Figure 2. Improperly spaced subsurface drainage resulted in non-uniform leaching of soil salts leading to extreme stunting in the saltier areas of the field. Note the lack of overt salt toxicity symptoms in the stunted plants. ECe in the non-affected area was 0.5 dS/m and 5-8 dS/m in the affected areas. of entire plants or limbs and scaffolds in tree crops in extreme cases. Sodium further complicates salinity issues due to its role in degrading soil structure. As sodium is added to the soil via irrigation water, it can gradually replace calcium on the exchange complex. As exchangeable sodium rises, soil aggregation and structure decrease. This degradation of soil structure leads to decreases in aeration, water infiltration, and percolation. Irrigation water that contains high levels of both sodium and bicarbonate is particularly troublesome. As soils dry between irrigations, bicarbonate (HCO3) combines with calcium ions forming insoluble limestone (calcium carbonate). Calcium ions displaced from the exchange complex may be replaced with sodium. In this manner, a calcium dominant soil can become a sodium dominant soil. Once the Exchangeable Sodium Percent (ESP) reaches 15, the soil is classified as a Sodic soil. But as with Saline soil, crop damage may occur well below this threshold--as low as an ESP of 5 for some sensitive crops. Key to this discussion is that sodium dominant soils do not drain well and, therefore, are an impediment to leaching of salts. As will be discussed later, a key component of managing water and soil salinity/sodicity in years of low irrigation and rainfall is to minimize the effects of both sodium and bicarbonate. A comprehensive guide to chloride, sodium, and boron tolerances can be found at http://www.fao.org/docrep/003/ T0234E/T0234E05.htm Water Quality for Agriculture. Interactions. It is beyond the scope of this article to adequately address nutrient–salinity interactions due to the extreme complexity of the topic. An excellent review of the subject matter is presented by Grattan and Grief at http:// ag.wilburellis.com/Products/Documents/ salinity%20nutrient%20interactions.pdf Though research on nutrient-salinity interactions often provides contradictory results due to differences in protocol (field vs. pot or solution culture, single vs. mixed salts, plant species, duration of the study, etc), it is clear that plant growth and performance may be negatively affected by salinity-nutrient interactions. It is interesting to note that there is little empirical evidence that the addition of nutrients beyond that considered adequate or optimal for growth has any benefit in a saline environment. Calcium may be the exception to this generalization. For crops known to be sensitive to calcium related disorders, the effects are often exaggerated under saline conditions. As soil salinity increases, crop requirements for calcium also increase and at the same time calcium uptake may be suppressed (competition with sodium, precipitation). Supplemental calcium applied under saline conditions has been shown to lessen specific ion toxicities, reverse some of the negative effects of salinity, and even improve yield and marketability of some crops. Thus, calcium may be an important tool to help producers minimize the deleterious effects of salinity even in a low moisture environment. Gypsum (calcium sulfate dehydrate), limestone (calcium carbonate), dolomitic limestone (calcium magnesium carbonate), and various calcium containing fertilizers are common sources of soil-applied calcium. Gypsum offers the greatest flexibility as it will pro vide soluble calcium at any soil pH whereas the carbonate sources require an acid soil pH to release soluble calcium ions. Elemental sulfur upon conversion to sulfuric acid by soil bacteria can be a very economical source of calcium provided the treated soil contains free limestone. A simple “fizz test” can be used to make this determination in the field by pouring an acid (vinegar will work) on soil and observing for effervescence or bubbling. This is CO2 being evolved as the acid digests limestone. Lack of fizzing means lack of adequate levels of limestone to use elemental sulfur for this purpose. Supplemental calcium may not only act to relieve crops of some of the negative effects of salinity and specific ion toxicity, but also benefits soil structure, which leads to optimal leaching when potential rainfall or leaching irrigations occur. Treating high bicarbonate irrigation water with an acid such as urea sulfuric acid to digest a portion of the bicarbonate is another management strategy when using saline waters, particularly if they also contain appreciable levels of sodium. H+ + HCO3 ------------> H20 + CO2 As mentioned earlier, bicarbonate forms limestone when soils dry between irrigations. This may reduce both soluble and exchangeable calcium levels. As calcium is stripped from the exchange sites by this reaction, a negative charge remains, which will attract other cations such as sodium. Digesting a portion of the bicarbonate slows this process and is another strategy that can be used to maximize soluble and exchangeable calcium levels in the soil. As a general rule of thumb, acidifying the water to a pH of 6.0 consumes roughly 50 percent of the bicarbonate content of the irrigation water. Last word The last word of advice is to avoid miracle cures. There are none. Aside from reverse osmosis, there are no magic elixirs, magnets, ionizers, or other devices that remove salt from irrigation water. The only remedy for continued use of saline irrigation water is leaching.Having your soil properly conditioned to take advantage of winter rains by treating the soil with calcium and high bicarbonate irrigation water with an acid will set the stage for effective leaching when winter rains arrive. For information on deficit irrigation strategies and other drought related topics go to http://ciwr.ucanr.edu/ California Drought Expertise/Drought information/ Carl Bruice is National Nutrition Technical Manager West for Wilbur-Ellis Company in Rio Linda, CA.