Friday 23 January 2015

Ground Truthing Your Soil Variability

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