To build on
the previous blogs discussing and identifying soil variability, it is essential
you determine what causes the variability observed or mapped; for example those
drier areas in the field or differences identified on historic Google Earth
maps or variations identified on an EM survey.
The variability illustrated by these “surface” observations can be
caused by one or more soil characteristics buried beneath the surface. You need to find out if it is variability
resulting from soil moisture, soil chemistry (e.g. pH), residual chemicals
(e.g. previous herbicide use), compaction (e.g. an old gate way or heavy
traffic when the area was wet), depth to gravels (or sand) and other
characteristics.
Nothing beats
a hole in the ground or measurements that will depth profile your soil. Start with a spade – it’s the least expensive
option. Alternatively, take the tractor or
little digger out and dig a hole in the areas of interest when the paddock is
about to be cultivated, get “down and dirty” and observe the differences. The two examples (while a bit extreme in
terms of excavated depth) are on the same property about 350m apart and no
matter how you identified the variability at the surface, the holes tell you why. Make some notes about the soils – you don’t
have to be a soil scientist to gather useful information. Measure the depth, feel the soil, dampen it
and run through your fingers (what does it feel like?), describe the colour,
note the depth of roots – nothing complicated or mind bending.
Notes: 200-250mm sandy silt loam (smooth to
feel but a little gritty as well), then gravels and sandy gravels, dark brown
topsoil, roots 250mm deep, hardly any roots in the gravels.
Notes: 150-200mm red brown silt loam (very
silky to feel when dampened), grey sandy silt with orange mottles and streaks,
not too many roots in top soil and none in the sandy silt.
You are now
well on the way to sorting out the cause of what you’ve seen at the surface. The soils are different (clearly). Time to apply some science to the
differences. The gravelly soil will have
low water holding capacity – inherent because stones have less surface area to
hold water than the same volume of sand or silt. The sandy silt soil has lots of orange
mottling, a sign that drainage is poorer here than the gravelly soil.
How can you measure the differences in soil
moisture characteristics? I’ll delve
into this in a later blog.
Posted by Dr Anthony Davoren from HydroServices Ltd
