Poor Infiltration of Irrigations due to Pure Water

As water issues continue to plague California farms, even the most practical solutions can often evade growers. While most water sources in the state have suffered damaging consequences caused by falling water tables and increasing concentrations of salts and bicarbonates, a significant portion of growers are using water that is too pure. Often, these two issues will have the same ultimate outcome: poor infiltration. Water that is too poor can be defined by an E.C. of < 0.20 dS/m and SAR ratio of < 3. When water is too pure, there is only one problem to resolve. Conversely, water sources more central to the San Joaquin Valley may need a combination of biological control, pH adjustment and salt mitigation.

Poor infiltration can be a headache for field activities.  However, standing water can indicate the mid and lower root zones are not receiving adequate moisture and nutrients, which can impact yield and quality. In this scenario, the upper root zone would be receiving too much water (if irrigations are based on ET), which could result in plant diseases such as phytopthera.

In response to grower and PCA questions regarding cost-effective solutions to lower E.C. in water, a study was conducted to titrate several commonly available products with well water from eastern Merced County that was pulled from an approximately 500 ft. depth (see above). Initial E.C. was 0.14 dS/m, while targeted E.C. was set to approximately 0.50 dS/m. Products used were: Potassium Thiosulfate (KTS), Calcium Thiosulfate (CaTS), Ca-12 (HiCal) and Solution Grade Gypsum x95 (CaSO4). Each product has pros and cons related to cost or agronomic impact. Costs were based on a 2200 gpm well delivering 0.420 inches of water per acre during a 10 hour irrigation set, and product costs were approximate.

Thiosulfate products such as KTS and CaTS work to degrade free lime in soil, adding either potassium or calcium that is already in solution and available to plants quickly.  Thiosulfates do increase water E.C., but the cost to continually or regularly apply them may be prohibitive. Solution grade gypsum delivers the most pounds of calcium to the ground (resolving a common soil problem in California), while increasing the E.C. of water due to its sulfate content. Applying gypsum continuously through the irrigation system is a much more effective remedy to getting calcium into the soil than broadcasting course grade gypsum on berms where water may or may not intercept it and take it into the soil profile. Hi-Cal proved to be the most cost effective method of raising E.C.; however, the product is made up of CaCl and the continuous application of even low concentrations of chloride will likely build up in soils that are known to poorly infiltrate water.

It is unlikely the cost incurred to continuously apply any of these products would be practical in most situations. However, regular applications on a weekly or bi-weekly schedule may be enough to supply water and nutrients to the bulk of the root zone throughout the hottest parts of the growing season and through harvest.

- Ben Cantrell, North California Agronomist