Examining the Impact of Tillage on Soil Moisture and Health

KEITH ABELES

Sonoma RCD  |  December 2017

To till or not till, that is the question. The ongoing, vigorous debate on this amplified during the drought years, and as proponents of soil health and climate beneficial farming practices advocate for reduced tillage. In vineyards, alternate tractor row tilling is commonly practiced, while many growers till all the rows, and some do no tilling at all, preferring to establish permanent cover.

There are two primary camps of thought on which tillage approach is best.  The more conventional camp tells us to till all the rows. Vegetation in the tractor rows, including cover crops, compete with vines for water and nutrients. Thus tillage (generally with discs) reduces weeds and puts vines in a better position to utilize available moisture and nutrients. Many growers like the look of a well tilled vineyard. Those preferring no till argue that soil structure improves by avoiding tillage, and soil organic matter (carbon) builds up in the soil.  According to research published in the Journal of Soil and Water Conservation, as soil organic matter content increases from 0.5 to 3%, water holding capacity more than doubles for all soil texture groups.  Water holding capacity continues to increase as soil organic matter content goes above 3%. Additionally, no till avoids the costs associated with labor and diesel to disc, while avoiding greenhouse gas emissions. Growers can spend time on other work in lieu of tilling.

So which tillage approach is best for grape growers? There is no definitive answer, but the Sonoma RCD (SRCD) is diving deeper into this question. The SRCD secured funding from the NRCS Conservation Innovation Grant program to quantify the differences in each tillage approach. We are working with John MacLeod at the Indian Springs Ranch in Kenwood, where his family has grown grapes since 1979. Dr. Josh Beniston, soil scientist and agriculture instructor at the Santa Rosa Junior College, has helped to establish a study design and set up research plots in a zinfandel block to investigate each type of tillage practice: no till, alternate tractor row till, and all tractor row till. There are three replicated plots of each tillage practice, to ensure more accurate results. After the 2017 harvest we established the management regime for each plot using standard, locally available cover crop seed in the tilled areas. This winter, Advanced Viticulture, Inc. will place soil moisture probes in the vine row of each study plot. This will allow us to track moisture levels for each tillage regime from full soil saturation in the rainy season to the drier conditions of the growing season, when irrigation is necessary. Study plots in the vineyard will be managed in a consistent manner for two full years, with regular monitoring of soil moisture conditions. While precipitation will likely vary year to year, we will focus on tracking the differences in moisture conditions of each tillage practice from the time it stops raining through the entire irrigation season, to the start of the next rainy season.

Additionally, we took soil samples in May of 2017, and will do so again at the end of the project in fall 2019. We will compare key soil characteristics, and how they have been impacted by each management approach. This includes looking at soil carbon levels, water holding capacity, and soil bulk density, which reflects on soil structure.

Thanks to the grant from CDFA’s Specialty Crop Block Grant (SCBG) program that supports the North Coast Soil Hub, we will be able to look further into these questions of tillage, and do more expansive soil sampling, and test other variables. We look forward to eventually hosting a field day for growers at the site where they can come see the project, learn about the results of the study, and ask questions of the farmer and researchers.

Some growers have strong feelings about which tillage approach is best, while others see this question as wide open. We currently lack regional, crop specific studies to answer this question. We are very excited to investigate this, and start to determine more definitive answers on how each management technique affects moisture availability and soil health.

Please contact Keith Abeles at the SRCD if you have questions or interest in this project. Kabeles@sonomarcd.org or 707-569-1448 xtn112.