Going Beyond the Hype of Soil Health in the Central Valley

For farmers, two crucial elements for growing successful crops are water and a good, healthy soil.

Rain, of course, ensures we have enough water to irrigate the crops. On the other hand, what keeps the soil healthy is subject to debate, according to Jeff Mitchell, associate vegetable crop specialist at the Kearney Ag Center.

During a cotton field day last month in Mendota, Mitchell encouraged growers to go beyond the hype about soil health and pointed out that they could do something about it.

“The reason we have such good levels of productivity is we have water. We have the luxury of developed water through the Central Valley Project. That has enabled people to mask a lot of problems.”

Conventional tillage is widely practiced by Valley growers.

Mitchells says farmers have routinely followed the same practices from year to year to prepare the ground for the next season’s crop. Tillage – preparing the soil using mechanical means such as a tractor to turn over the ground – is the common practice in the Valley. 

“Water and tillage have masked a lot of problems,” he said.

Studies have reported tillage causes soil erosion. In hilly areas, the eroded soil moves downslope. On flatlands, the eroded soil can fill drainage ditches.

A 2003 study by the USDA Agricultural Research Service in Minnesota found the tillage erosion loss of 27 tons per acre per year was almost 5½ times more than the natural 5 tons per acre loss per year.

“What constitutes good soil management?” Mitchell asked growers. “Diversity, organic matter and keeping the ground covered.”

“Mother Nature doesn’t do tillage,” he said.

Mitchell said planting a winter cover crop will boost soil health. “This is not new stuff,” he said, noting discussions about cover crops in the Valley have been around for a century. “This is coming back to basic principles.” 

Here is a field with a legume cover crop.

The benefits of cover crops include: Slowing erosion, improving soil health, enhancing water infiltration, smothering weeds, controlling pests and diseases and increasing biodiversity.

The challenge, he added, is “nobody is going to do cover crops without irrigation.”

During the first half of the 20^th century, farmers planted cover crops extensively. But in the 1950s, growers moved away from this practice because of the development of  pre- and post-emergent herbicides. But this is changing with concerns about chemical inputs into the soil.

A tractor is rolling a vetch no-till cover crop.

From 1999 through 2014, Mitchell and other researchers quantified cover crop biomass production for a variety of mixtures under winter rainfall and limited supplemental irrigation.  The group conducted a separate study to determine changes in soil water storage under three cover crop mixtures compared to fallowed plots during the winter of 2013 and 2014 to investigate tradeoffs associated with water use by cover crops in the region.

“ From this long-term systems research, we conclude that while vigorous growth of winter cover crops in the Central Valley may not be possible in all years due to low and erratic precipitation patterns, there may be benefits in terms of providing ground cover, residue, and photosynthetic energy capture in many years. However, cover crop biomass production may come at a cost of soil water depletion in this semiarid, drought-prone region,’’ the study said.

A Texas NRCS soil demo of no-till and tillage in water.

Mitchell brought pans of  till and no-till soil samples to illustrate his point. He dropped a clogfrom each sample into a separate cylinder filled with water. The convention tillage sample started to disintegrate immediately, representing sediment going downstream. The no-till soil soaked up the water and only small particles gradually dropped into the water.

What’s this mean? “Better water holding capacity (with the no-till soil),” one farmer responded.

To view Jeff’s presentation, you can go to our website and see for yourself just how different the two soil samples were. The link is http://www.sustainablecotton.org/videos/index/115.