Wood Chipping of Prunings to Reduce Air Pollution and Build Soil Organic Matter

 

Chipping brush

Since the passing of The Federal Clean Air Act amendments of 1990, the San Joaquin Valley (SJV) has not met the national ambient air quality standards for particulate matter of 10 microns or less (PM-10). In response, the SJV Unified Air Pollution Control District restricts the burning of agricultural wastes, and further restrictions are likely. Wood chipping is an alternative to burning that would not contribute to air pollution and would add valuable organic matter to soils. If wood chips could be shown to not interfere with harvest or take nutrients needed by trees, then growers would be more likely to chip their prunings.

 

 
In a UCCE experiment, wood chips were mixed with soil and placed in 35-gallon plastic containers, each with a single almond tree. Equal numbers of trees were placed in soil and in containers without wood chips.

After two years, wood-rotting fungi and beneficial soil-aggregating fungi were found only in the wood-chipped soils. Soil analysis showed significantly higher levels of zinc, copper, organic matter, phosphorus, potassium, ammonium, calcium and magnesium in the chipped soils, while the pH was decreased and electrical conductivity increased. Also, there were fewer parasitic nematodes and more free-living nematodes in chipped soils.
 

 
This research has shown that (1) wood chips do not interfere with harvest if they are small enough and that (2) they increase soil organic matter and nutrient cycling, microbial diversity and the water-holding capacity of soils.

Because of these findings and realizing that more burning restrictions are inevitable because of increased air pollution, many growers already have embraced wood chipping as a sustainable alternative to brush burning. Results of this study are important not only to the almond industry but to all tree fruit growers who burn their prunings.
 

Supporting Unit: Madera County
 
Brent Holtz, PhD, UC Pomology Farm Advisor, Madera County, 328 Madera Ave., Madera, CA 93637
559-675-7879 ext. 209, baholtz@ucdavis.edu