Soil health, also referred to as soil quality, is defined as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans. Soil contains living organisms that, when provided the basic necessities of life – food, shelter, and water – perform functions required to produce food and fiber, including the following:
Living organisms in the soil contribute to soil health. Diagram by Laura Crothers of the Agricultural Sustainability Institute at UC Davis.
How does whole orchard recycling impact soil health?
UC research on whole orchard recycling (WOR) shows that, nine years after an almond orchard was recycled and incorporated into topsoil, certain indicators of soil health show significant improvements in comparison to the typical practice of “pushing and burning” orchard trees. This research was conducted as a long-term trial at the UC Kearney Research and Extension Center in Parlier and two short-term trials in Manteca and Bakersfield, California.
Over the long term of nine years, results show WOR increases...
Total soil carbon (+58%)
Total soil nitrogen (+17%)
Organic matter (+42%)
Soil aggregation (+19%)
Water holding capacity (+32%)
Soil microbial activity, as measured by:
Soil microbial biomass carbon (+47%)
Soil microbial biomass nitrogen (+13%)
Soil enzymes for carbon and nitrogen cycling (+38% and 46%, respectively)
...compared with the conventional “push and burn” orchard removal practice
Short-term effects of WOR
Over the short term, results show WOR improves...
Total soil carbon (+22-30%)
Compaction in top 6 inches of soil (-10%)
Soil enzymes for nitrogen cycling (+50-70%)
Gravimetric soil water content (top 6 inches) (+62%)
... compared to removing the entire tree biomass
This study found no significant effect of WOR on:
Soil pH
Soil electrical conductivity
Soil Health Results: Kearney Research and Extension Center (Parlier, Fresno County)
These results are from a long-term trial assessing effects of whole orchard recycling over nine years.
Nitrogen and Carbon Storage
Total carbon content was significantly higher (p ≤ 0.05) in large macroaggregates in the WOR plot (Figure a).
Figure A. Whole orchard recycling increased total carbon in large soil macroaggregates, and total nitrogen in silt and clay soil fractions. Asterisk (*) indicates a statistically significant finding (p ≤ 0.05).
Additionally, an increase in carbon stock of 58% was observed at soil depths up to 30 centimeters in the WOR orchard compared to the conventional burn treatment (not illustrated). No significant differences in soil carbon stock were found at deeper layers.
Total nitrogen content, by contrast, was statistically significantly higher in the silt and clay fractions (Figure a).
Soil Enzymes
Cellulase (CB) and β -glucosidase (BG) are enzymes produced by soil organisms to break down cellulose and cycle carbon. β-N-acetylglucosaminidase (NAG) and Leucine Aminopeptidase (LAP) are enzymes that break down other organic compounds and help to cycle nitrogen in soils. Their presence indicates healthy biological activity supported by organic matter available in the soil.
Research at Kearney found significantly greater activity (p ≤0.05) of CB, BG, and NAG in the WOR plots as compared to the control treatment. An increase in LAP was observed but is not statistically significant.
Soil Microbial Biomass Carbon and Nitrogen
Soil microbial biomass carbon and nitrogen refers to the amounts of these elements embodied in soil microbes and signal an increase in soil microbial activity to break down organic matter.
The WOR trial at Kearney had 46% greater microbial biomass carbon than the control (p ≤0.05). Microbial biomass nitrogen had no significant increase. This result indicates that soil biology continues to be stimulated even 9 years after the incorporation of ground wood.
Soil Structure and Water Regulation and Storage
Wet aggregate stability refers to the ability of soil aggregates to resist disintegration under raindrop impact or water erosion. WOR plots at Kearney showed an increase of wet aggregate stability of 19% over the control treatment. Stable aggregates also help soil to resist compaction and maintain pore space. WOR plots at Kearney reduced soil penetrometer readings (a measure of resistance to root growth) by 14% and bulk density by 4%. With better soil structure, WOR increased water infiltration rates by 200%, and increased soil moisture retention by 32%.
Soil Health Results: Manteca, San Joaquin County
These results are from a short-term trial assessing effects of whole orchard recycling after two years.
Soil Carbon
An increase in soil total carbon content of 22% was measured over the short term, two years after WOR, at this site compared with the control of removing the trees from the orchard entirely.
Soil Enzymes
Cellulase (CB) and β -glucosidase (BG) are enzymes produced by soil organisms to break down cellulose and cycle carbon. β-N-acetylglucosaminidase (NAG) and Leucine Aminopeptidase (LAP) are enzymes that break down other organic compounds and help to cycle nitrogen in soils. Their presence indicates healthy biological activity supported by organic matter available in the soil.
Research at Manteca found an increase in activity of NAG of 70% (p ≤0.05) in the WOR plots compared to the control treatment. Changes observed in the other enzymes were not statistically significant.
Soil Structure
Soil compaction can be understood in at least two ways: soil resistance to the penetration of roots, measured with a penetrometer, and soil bulk density, which is the weight of a given volume of soil. WOR reduced soil resistance by 10% (figure b) and reduced bulk density by 12% (figure c) in the top six inches of soil.
Figure B. Whole orchard recycling reduced soil resistance, a measure of soil compaction. Asterisk (*) indicates a statistically significant finding (p ≤ 0.05). Figure C. Whole orchard recycling reduced soil bulk density, a measure of soil compaction. Asterisk (*) indicates a statistically significant finding (p ≤ 0.05).
Water Content
Water content was measured using the gravimetric method, in which soil is weighed, dried to remove all moisture, and then weighed again; the difference in the weights indicates the amount of water in the soil. WOR increased gravimetric soil water content by 62% compared to the control (figure d).
Figure D. Whole orchard recycling increased gravimetric soil water content. Asterisk (*) indicates a statistically significant finding (p ≤ 0.05).
Results: Bakersfield, CA (Kern County)
These results are from a short-term trial assessing effects of whole orchard recycling after two years.
Soil Carbon and Nitrogen
An increase in soil total carbon content of 30% was measured in WOR plots at this site, along with an increase in total nitrogen content of 50%, compared with the control treatment.
Soil Enzymes
Cellulase (CB) and β -glucosidase (BG) are enzymes produced by soil organisms to break down cellulose and cycle carbon. β-N-acetylglucosaminidase (NAG) and Leucine Aminopeptidase (LAP) are enzymes that break down other organic compounds and help to cycle nitrogen in soils. Their presence indicates healthy biological activity supported by organic matter available in the soil.
Research at site 3381 in Bakersfield found an increase in activity of CB and BG of 80% (p ≤0.05) in the WOR plots compared to the control treatment, and an increase in NAG of 50%. An increase in LAP was also observed but is not statistically significant. These findings suggest an increase in soil biological activity for cycling carbon and nitrogen over the short term (two years) following WOR.
Soil Compaction
Soil compaction can be understood in at least two ways: soil resistance to the penetration of roots, measured with a penetrometer, and soil bulk density, which is the weight of a given volume of soil. At site 3381 in Bakersfield, two years following WOR, the treatment reduced bulk density by 30% (figure E) in the top six inches of soil. A change observed in soil resistance was not statistically significant.
Figure E. Whole orchard recycling reduced bulk density, a measure of soil compaction. Asterisk (*) indicates a statistically significant finding (p ≤ 0.05).
