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News: Soil Salinity Increases

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Submitted by PSmall on Tue, 01/19/2010 - 12:07

A ubiquitous issue with irrigation in arid areas is the inevitable build-up of salts in the soil. Applying water in excess of crop use is needed to move salts down into the profile, but that becomes impractical under drought conditions. 

Several pest control advisers (PCAs), farm managers and farmers are starting to bring up the issue of salt build-up in the soil on the West Side of the San Joaquin Valley.

When water is imported into a farming area, salts come with it. Regardless of how “pure” the snowpack water may be, it contains salts. Compounded over many years, this salt can kill the fertility of the soil.

Well water is generally more salty than surface water, exasperating the salt build-up problem.

Rainfall is not high enough in California to leach out salts. Farmers need large amounts of imported, pure water to leach salts from crop root zones. Without leaching, the soil will eventually become sterile and unproductive.

Thousands of acres of California open farmland and orchards and vineyards have been converted to drip or micro irrigation in the past couple of decades. This conversion from furrow or flood irrigation has been primarily to irrigate more efficiently with reduced water supplies. However, another benefit has been that constantly irrigating with small amounts of water in the root zone keeps a rather narrow band around roots salt free. This element can actually be equivalent in importance to reduced water use. However, it only staves off the inevitability of long-term salt build-up without aggressive leaching.


The subject of irrigated agriculture sustainability is endlessly fascinating to me. Salt buildup with irrigation presents an interesting water quality conundrum, in that leached water is high in salts and it has to go somewhere: groundwater or surface water.

As a land classifier trained by USDI-Bureau of Reclamation, I had it pounded into my head that when leaching is required to control soil salinity, the water table must rise. It may take centuries, but it is only a matter of time. No amount of devilish advocacy on the part of us land-classifiers-in-training could get our instuctors to give on this point. When the water table gets within capillary striking distance of the soil surface, salts accumulate, and until a subsurface tile drainage system can be installed, leaching becomes water logging. Without drainage, agricutural productivity drops below the point of being able to contribute to irrigation system operational costs. BuRec irrigation land classification is grounded in ag economics. It is particularly sensitive to the economics of installing drain tile systems.  Furthermore, BuRec land classification assumes tile drainage will be eventually be needed on every acre of irrigated land.

Tile systems deliver salt laden water to open surface drains. This water must either be delivered to a river system (think Federal Clean Water Act jurisdiction) or closed basin and evaporated (think Kesterson crisis ) or infiltrated (think drinking water supply). Irrigation project planning was good at anticipating the cost of drainage, and land classification maps allow the project to exclude lands too costly to drain. Water quality issues were never adequately considered in those days. Irrigation sustainability will remain a newsworthy issue for millenia to come. 

Update 01/20/2010: I mentioned this news item in passing to soil science consultant Joel Kimmelshue, owner of NewFields Agricultural & Environmental Resources, LLC (Sacramento, CA).  Joel responded that currently most of NewFields' California client projects are driven by a need to control soil salinization.

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