Claims checked › Claim
Keyline turns subsoil into topsoil in two to three years
“Keyline ploughing converts subsoil into topsoil within two to three years, deepening the profile by 10 to 15 cm.”
The single most-quoted keyline figure online traces back to one on-farm report by the man who invented the method, in 1950s New South Wales. No independent measurement of it was found.
Where it comes from
P.A. Yeomans developed keyline design in 1940s and 1950s New South Wales, using chisel ploughs on contour-derived lines to break compaction and spread water from valleys toward ridges (overview). The claim that subsoil becomes topsoil within two to three years, deepening the profile 10 to 15 cm, is his own on-farm report. It appears in his books and has been repeated ever since, usually without attribution.
Who benefits
Nobody sells keyline as a product, so this is not a vendor claim. It is a founder claim: the method's inventor reporting his own results on his own land, with no control and no independent soil sampling. Enthusiasm for the method has kept the number in circulation.
What the evidence says
Our research found one modelling study, one comparative study and the 1950s on-farm reports. That is a weak base for any figure, and especially for one that describes soil forming at a rate geomorphology would call remarkable. What does have replicated trial data, from arid and semi-humid sites outside Europe, is the broader family of water-harvesting structures: stone terraces and semi-circular bunds cut runoff by 65 to 85% and contour ridges by 73%, with soil moisture up at every site (Environmental Management). That is evidence for slowing runoff, not for keyline's subsoil claim.
What to say instead
Subsoiling on contour can improve infiltration on compacted land; the topsoil-in-three-years figure has no independent support and should not be repeated. The swales and terraces page grades the whole family of contour earthworks and concludes that restoring existing terraces is the better-evidenced move.