Monday 19 January 2015

Characterising Your Soils

Identifying the variability of your soils on not only a farm but also paddock scale is important if you wish to manage your inputs to meet the soils requirements in a sustainable and efficient manner.
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.