Too often we dig down in to soils that are planted with a tap-rooted crop such as canola and find the roots are going sideways not far below the surface. A root going sideways is often associated with hard pans in medium to heavy soils, but even in sands the roots can be seen to go sideways.
Sandy soils shouldn’t be underestimated in how they can compact and they can be very unforgiving once they are compacted as they don’t have capacity to shrink and swell like some heavier soils.
In sandy soils it can also be a chemical constraint that is limiting root development down the soil profile. Chemical issues such as low fertility below the topsoil can reduce exploration of the subsoil. Simple things to look at are the field tests for pH and Electrical Conductivity (EC) in the layers going down the profile. It can often be seen that the pH or EC difference can be very significant which can impact on nutrient availability and potentially toxicities in some cases.
Overall, porosity and oxygen movement is the key when looking at a soil profile. Plants don’t create pores, they take advantage of smaller pores, grow through them and make them larger. When a soil is porous with a distribution of larger and smaller pores (macropores and micropores) the soil has capacity to move water and nutrients down and back up the profile.