Agriculture Reference
In-Depth Information
soil's water holding capacity is significant only in sandy soils, which, paradoxi-
cally, are the soils in which increasing SOM is most difficult. Although mulches
have some benefit in increasing SOM, their main purpose is to provide a pro-
tective cover for the soil, and as such they should not decompose too quickly.
Regular applications of compost and manure can build up SOM, although the
former is the more effective because, in the composting process, much of the easily
decomposable organic matter is eliminated and more resistant material remains.
Permanent grass cover crops can build up SOM and improve soil structure.
However, the main effect of a cover crop with legumes is as a green manure, rather
than as a contributor to SOM.
The benefits to soil health and vine performance achieved through cover
crops, composts, manures, and reduced tillage derive primarily from the regu-
lar addition of decomposable organic materials and their transformation (i.e., C
turnover), rather than from SOM accumulation per se.
Testing for Soil Biological Health
In the first instance, simple observations of a soil's biological health can be made
in the vineyard, as outlined in box 5.3. However, in response to a growing interest
in soil biology in recent years, some commercial laboratories now offer a limited
array of tests, at a considerable price, for microbial activity, microbial biomass,
fungi, actinomycetes, bacterial-to-fungal ratio, and identification of specific
groups such as the photosynthetic and lactic acid bacteria.
From an extensive literature review, Riches et al. (2013) recommended several
biological tests that should be part of a minimum data set (together with selected
chemical and physical tests, reviewed by Oliver et al., 2013) to assess the overall
quality of Australian vineyard soils. As shown in table 5.7, soil microbial biomass
(SMB), soil organic carbon (SOC), and potentially mineralizable N (PMN) are
the ones for which the most reliable benchmarking values are available, and of
these SMB and PMN are highly correlated so that either could be used as an indi-
cator. Although more sophisticated tests of biological composition and activity,
such as physiological profiling (e.g., BIOLOG microplates), phospholipid fatty
acid analysis, enzyme assays, and DNA-RNA sequencing, have been developed
in research laboratories, very few are commercially available and none have estab-
lished benchmarks for vineyard soils.
Nevertheless, as suggested for monitoring vine nutrition (see “Interpreting Plant
Analyses,” chapter 3), biological tests carried out from year to year can indicate how a
soil's biological health is trending. Because soil biological properties are very variable
in space and time (less so for SOC), when samples are collected in different years, it is
important that soil moisture and temperature conditions are comparable and that the
same sites are revisited. Also, because analytical methods can differ among laborato-
ries even for the same test, the same laboratory should always be used.
