Enhancing soil quality:
It is wildly acknowledged that
conventional vegetable production practices tend to reduce soil organic
matter content, reduce soil microbial community diversity and activity,
and adversely affect soil tilth.
Conversely, the use of cover cropping, or the application of organic
amendments, can reverse these trends. Increasing the amount of organic
matter returned to the soil can significantly influence soil microbial
community structure and function. Soil aggregate stability and water
infiltration rate can be improved by increasing soil organic matter.
Soilborne diseases may be suppressed in production systems that add
large amounts or organic matter to soil.
There are several reasons why, despite these demonstrable benefits,
western vegetable growers have been slow to adopt alternative soil
management practices. While current practices have historically reduced
soil organic matter, in regions that have been farmed for decades
an equilibrium tends to be established; assuming that high productivity
has been maintained, this equilibrium level could be viewed as sustainable.
Using the SAFS project to illustrate this point, after 11 years of
an intensive rotation (wheat/tomato) soil organic matter was unchanged.
Long-term studies in other locations have found similar results.
Secondly, soil building practices can have significant cost, and it
has been difficult to consistently show that those costs are offset
by increased productivity or reduction of other inputs. In the SAFS
project, the organic (utilizing cover cropping and composted manure
application) and low-input (utilizing cover cropping) farming systems
averaged 13% and 3% lower tomato yield, respectively, than the comparable
conventional system despite having significantly increased a number
of soil attributes (SOC, microbial biomass C and N, water infiltration
rate) purportedly linked to soil quality. Over the final 6 years of
the project (after the “transition” phase from prior conventional
management) the organic system still averaged 6% lower yield, while
the low input system was equivalent to the conventional system. In
the BIFS project there were 14 site/year comparisons of tomato production
with and without the use of cover cropping or compost application.
These alternative practices averaged 3% higher tomato yields, not
enough to offset the additional cost. Interestingly, all the yield
advantage occurred in the first year of the study, with no benefit
seen in the second and third years of alternative practices. Others
found that compost application and cover crop use marginally improved
lettuce yield, but not enough to offset additional costs.
Finally, alternative soil management practices can complicate other
practices, and increase production risk. Cover crop incorporation
in the spring may significantly delay planting and subsequent harvest
date, particularly in wet years. Furthermore, cover crop residue can
complicate seed bed preparation and planting of the crop, affecting
stand establishment. Conservation tillage may require radical changes
in weed control and irrigation practices, and may complicate harvest
activity.
Despite these limitations, expanded use of cover cropping and conservation
tillage is likely to increase in the future, and should be encouraged.
Conventional tillage practices are expensive, constituting as significant
portion of preharvest production costs; reducing the number of trips
over the field, or the power requirement of the tractor, could offer
substantial savings, provided yields can be maintained. Conservation
tillage can also have significant environmental benefits, including
erosion control and minimizing the loss of soil carbon to the atmosphere.
The potential environmental benefits of cover cropping are also significant,
and it is these benefits that are likely to drive expanded use of
this practice. Fall-planted cover crops can sequester large amounts
of soil NO3-N that would otherwise be at risk of leaching. Cover crops
also reduce winter runoff and associated erosion and nutrient loss.
The pest suppression activity of cover crops, in particular various
Brassica species, can reduce the need for soil-applied pesticides
or fumigants. The cost/benefit gap for cover cropping is reasonably
narrow, and additional research and grower education, combined with
grower incentives (recognition as an appropriate BMP to meet water
quality goals, incentives from USDA conservation programs, etc.) will
hasten and expand adoption.
