Promote • Educate
46 September/October 2019
8
nderstanding the soil sometimes
requires understanding the plants
that are interacting with that soil. A
soil test may not always show what will be
truly available to the crop.
Differences between results for an
element in a soil test and a plant test can
include:- the depth from which plants roots are
getting the element - the different abilities of crops to take
up certain elements - the interfering effects of other
elements or soil pH and soil moisture
conditions.
And what is available from the soil also
varies with the time of year.
Strategic plant testing
Most soil testing laboratories also accept
plant ‘tissue’ tests. The challenge is to judgewhat is truly high and what is truly low. The
lab will usually show a normal range for
each of the elements that are being tested,
but it is crucial to follow the lab instructions
on taking a sample. The time of year the
sample is taken and what portions of the
plant are sampled are crucial to help make
the range relevant to your sample.
If you are trying to understand why
a crop is not performing well, a good
approach is to take two samples. One
sample from plants that are not doing
well to represent the problem, and the
other sample from some of the plants that
actually appear quite good. The result is
a powerful comparison of desired and
undesired. Check for one or a few elements
that test lower in the poor performing crops
as an indication of how fertiliser or soil
management could be changed to improve
the overall crop.
The good plant sample can come from
the same field in the case of differing soil
conditions, or otherwise from different
fields. Still use the laboratory guide of
normal ranges to help decide if a level is
high or low, but your own comparison has
the advantage of being taken at the same
time from plants growing in your local
conditions.
For clover-based pastures, the powerleaf analysis for soil assessment
Plant testing
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IURPWKHVRLOAbove: Some labs offer a tests for grape
leaves and, to assess mobile nutrients, the leaf
stalks (petioles). Photo: iStock/karandaevPhoto: iStock/MLiberraof a white-clover-only herbage test is huge.
Clover is what is driving the pasture through
natural nitrogen nutrition and is itself very
nutritious for grazing animals. Timing
in spring to early summer after the clover
has started growing but before flowering is
crucial. Don’t pick the lush clover that may