101 Questions answered
Understanding Soil Compaction and Root Growth
As a general rule of thumb, crop roots struggle to grow when soil bulk density (the weight of the soilin a given volume) exceeds:
- 1.7 g/cm in sands
- 1.6 g/cm in loamy sands, loams, sandy clay loam
- 1.4 g/cm in clay loams and clays
i.e. the sandier the soil, the harder it needs to be to restrict crop growth, but sands can still becom every compacted.
These numbers are best used as guidelines onlyand the only way to know the bulk density of the soil is to test, which requires collecting soil cores in a steel core and sending it to the lab. This method doesn’t work well in gravelly soils. Soils with less than 10% pore space are a problem for crop roots,but again, this requires specialist testing.
A penetrometer is easier to use as you just push it into the ground. It will give readings in kPa, and readings of 1500 kPa or above are probably starting to impede root growth. Penetrometers are useful if you are trying to find the depth of a compacted layer and map out where compaction is in the paddock.
A rough and ready way to check is to push a sharp stick or pole into wet ground. If it’s a challenge to push it in, it’s probably a challenge for crop roots. What really matters is if compaction is limiting root growth. How can you tell? That’s revealed in the next question
