This time, a little bit more juice from my research. Soil
natural capital and soil health may seem like unnecessary concepts, names that
you already know the meaning of without having to learn them. However I will
outline them briefly and how they relate to my findings so that you are, in
turn, able to relate to them if you come across them in environmental plans,
legislation or elsewhere in the future.
Soils are referred to as a stock of properties or natural
capital which yield a flow of valuable ecosystem goods or services into the
future. Both soil health/ quality and natural capital are similar in that they
use soil indicators and parameters to determine the state or function of a soil
system. However soil natural capital provides a more holistic analysis of the
resource as it takes into account not only the state of the soil itself
(through soil indicators) but also the effect of this state on the products and
services that soils provide and the human needs that are catered for by soils.
In the soil natural capital framework macroporosity is
identified as the key physical attribute. This is because macroporosity
determines: water flow, solute transport and drainage through soil. As a result
macroporosity influences ecosystem services such as flood mitigation and
filtering of nutrients. Macroporosity and associated soil physical properties
provide important services and it is important for land managers to be aware of
the potential to change these properties and the ecosystem services they
provide.
Research has been carried out to determine the effect of
land use practices on other soil physical properties such as bulk density,
aggregate stability, soil carbon and water holding capacity however
macroporosity remains the main indicator of soil physical natural capital and
health because of its sensitivity to intensification.
My research found that on average for the 0-30 cm increment macroporosity
was significantly lower on the Dairy site (9 ± 1%) than both the Sheep farm (19
± 1%) and the Control site (15 ± 1%). This suggests that intensification is
having a significant effect on the Dairy site. Furthermore on the Dairy site
the 0-10 cm and 10-20 cm depth increments both have values for macroporosity
< 10%. Other researchers have proposed that macroporosity values of > 10%
are needed to maintain pasture production near optimum.
Target ranges for macroporosity are given in Table 1 as part
of the National Soil Quality Indicator Programme. Here, for soils under pasture,
macroporosity values < 8% are considered low and could restrict pasture
growth. Macroporosity for the 10-20 cm depth increment on the Dairy site was 7
± 1%, a level where less than optimum production could be expected. Results
from an AgResearch trial found similar values for and changes of macroporosity
with stocking intensity.
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| Table 1 – target values for macroporosity for pasture, cropping & horticulture and forestry |
I did not find any changes in water holding capacity within
the plant available range with increasing land use intensification. This result
in itself was interesting as it shows that intensifying land use practices did
not have a measureable impact on the readily available water (RAW, that
available to plants) of the soil. In comparison other studies have found that
there is a significant decrease in RAW with irrigation and increased
compaction. Finally my study did find that there was an increase in small
micropores holding water at suctions too great for the plant to overcome. These
findings all highlight the importance of on farm soil testing to determine the
RAW of the specific soil textures and under different land uses to increase
management efficiency.
Bulk density values were found to be significantly higher on
the Dairy site (1.40 ± 0.02 g cm-3) than both the Sheep farm (1.26
gcm-3± 0.02) and the Control site (1.31 ± 0.02 g cm-3), indicating
increased compaction on the DF in agreement with macroporosity values. Bulk
density is not as sensitive an indicator of compaction as macroporosity and
this can be seen by the large target range 0.7–1.4 gcm-3 that has
been identified for Pallic soils (Table 2). Therefore it is not recommended as
an indicator for determining the effect of land use intensification on soils.
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| Table 2 – target ranges for bulk density are large indicating that this is not as sensitive an indicator as macroporosity for determining the effect of land use intensification on soils. |
Landcare Research has developed a tool which can be used by
everyone to determine the quality of their soil based on a number of
indicators.
The tool allows you to measure your soil against current
understanding of optimal values for:
Macroporosity, bulk density, Total N, Total C, Mineraliseable
N, pH and Olsen P
It will tell you about the effect each indicator has on soil
quality alongside some general management practices that can be used to improve
your soil. In addition to thinking about the effect of these indicators on your
soil quality I encourage you to take a step back and also think about the long
term effect of the state of these indicators/ properties on your farm’s
functions and the importance of each of these functions to your profitability.
Blog post by Nicole Mesman - BSc (Hons) Soil Science