This time of year, if not slightly earlier before it gets
too dry, it is very easy to see on dryland properties the different water
holding characteristics of soils from the state of the crops growing in them.
Marking these is crude but still useful way of noting differences in soils,
even better if a simple hand held GPS device is used.
Above: very apparent differences in soil characteristics. A
white clover paddock in early January, in the fore ground the crop has burnt
off but in the background still in mid flower.
The use of yield maps is a great way of identifying a crops
reaction to different soil characters in a specific climatic year, but the crop
type and weather patterns that season should also be recorded as different
crops react in different ways to different climatic conditions. The more years
of harvest data you have the better the bigger picture of variability you will
get.
Another very useful tool to
identify variation if you can’t hop up and have a fly over your place is to
utilise Google Earth and its ‘Historical Imagery’ toolbar to go back through
previous aerial images of your farm and visually identify different areas of
interest. While quite a few recent satellite images on Google Earth may show
you irrigated and non-irrigated areas, particularly in Canterbury, looking back
through historical ones will give you more of a picture of natural soil
variation and how it comes through into what’s grown on the surface. In most
cases significant variation can be seen. The ‘Historical Imagery’ icon is a
little clock like icon and can be found at the top of the map window.
Here is a comparison between
what Google Earth shows is the variation in a paddock at different times and
the EM Map. You can clearly see different soil & crop patterns running
through the paddock that line up well both on Google Earth and in the
subsequent EM Survey completed 2 years later.
To accurately capture the in-field and farm scale
variability in soils an Electro-magnetic
(EM) Survey of your property can be carried
out. This measures the soils conductivity, which is influenced by the
composition of clay, silts and sands in the soil at a given point; were clay
gives a higher EM reading than silt which in turn has a higher EM than sand.
These readings are logged across a paddock with sub-metre
GPS accuracy which also gives you a
3D surface map of the area. This not only helps define the soil characteristics
beneath but is also a very useful management tool in its own right, showing
where water will move in a rain or irrigation even. After identifying different
EM zones you can use them as they are or ground truth the zones to put actual
water-holding capacity figures to the variability; either way you can then
start to manage your different soil characteristics.
Characterising your soil precisely is really important to
enable greater efficiency in irrigation management, pasture management,
fertiliser management, effluent management and a whole bunch of things that are
done on farm in relation to your soils. By a more precise understanding and
management of your soils, this can further enhance the efficiency of your farm.
Additionally, characterising your soils is important for understanding and
managing your soil moisture which Tony Davoren is
going to talk about in the next blog.
We’ll go into more detail on the characteristics of different
soils and what this relates to in terms of EM Surveying and vice versa in
future posts.
