Thick Black Soil: original topsoil is 12 inches deep Inches of topsoil removed
Grey Wooded Soil: original topsoil is 6 inches deep
Inches of topsoil removed
The “treatments” are two inches of topsoil added back, 30 tonnes per acre of manure applied once, and 90 pounds of nitrogen per acre plus 40 pounds of phosphate per acre applied every year. Researchers were Izaurralde, R. C., Malhi, S. S., Nyborg, M, Solberg, E. D. and Quiroga Jakas, M. C.
The importance of topsoil in determining soil quality has been known since mankind first stirred the land to place some seeds and grow a crop. Soil destruction by wind in the Dirty ‘30s is legendary and the overuse of diesel fuel and steel in the 1970s was almost as bad.
Does anyone remember the days of incorporating herbicide by double cultivation at right angles at four-inch depth at high speed? At Melfort, Sask., some farmers in the ‘70s talked about seven trips over the field between combine and seeder. The dirt in the air on windy days in May was sickening.
At a recent Saskatchewan Soil Conservation Association convention, Doyle Wiebe of Langham, Sask., recalled a windy day back then when he shut down seeding, retreated to the house and vowed to find a different way to farm or find something else to do. Zero till farming has come along and saved the day — and the soil.
Dozens of experiments have dealt with topsoil loss and soil quality over the past two decades. I recently encountered a very good research paper that I think sums it all up. The work was done on two soils near Edmonton. One was a thick black soil in a canola, wheat, barley rotation with annual additions of around 55 pounds of nitrogen per acre and 30 pounds of phosphate per acre. The other soil was grey wooded and grew intercropped oats and field pea rotated with barley. The grey wooded soil got about the same annual fertilizer as the thick black but it also had two tonnes per acre of manure each year.
The experiment stripped various depths of topsoil off at these two sites. Then it tested three treatments to see how they could make up for the loss of topsoil. The treatments were two inches of topsoil added back, 30 tonnes per acre of manure applied once and 90 pounds of nitrogen per acre plus 40 pounds of phosphate per acre applied every year. The stripping was done in 1990 and CWRS wheat was planted at the two locations on all treatments in 1991. See the table for results.
The matrix of yields that resulted from these tests is truly amazing and speaks for itself. On the thick black soil, yields varied from 15 to 71 bushels per acre, and on the grey wooded soil, yields were from six to 67 bushels. Examine the numbers carefully and you will draw your own conclusions.
What stands out for me is what the soils did all by themselves. The grey wooded soil was only two bushels behind the thick black with no fertilizer and only four bushels behind with a big dose of fertilizer. Regular management of the grey wooded soil included legumes and a generous dose of manure. That data alone drives home my point about management versus inherent soil productivity. There is more in the farmer than there is in the land.
But the numbers also show how soil conservation is important for all soils, but especially for land where the topsoil layer is thin to begin with. Stripping away eight inches from the thick black cut the yield by more than half but completely destroyed the productivity of the grey wooded soil. With eight inches removed the poor old grey wooded soil had nothing left.
This basically confirms what common sense would tell us. A soil with a large depth of topsoil can stand to lose a little without drastic results but a shallow soil will be devastated with almost any loss.
The experiment carried on for an additional four years and manure and topsoil replacement became more important in subsequent years and out-yielded the fertilizer treatment.
Many other experiments have been done along the same line. They all tend to agree that on a completely eroded soil (no topsoil left), the big bang in reclamation comes from getting a few inches of topsoil back. Diminishing returns set in quickly and two to three inches of topsoil can make a huge difference in crop yields.
Having reviewed the dozen or so research papers, and thinking about my bald old knolls at Blackstrap farm, I am about ready to get an earthmover to rearrange some of my real estate. An investment in my land will likely pay bigger dividends than trying to guess what other kind of investment might work in the current economic environment.
So there you have it. Soil quality is all about topsoil, and maintaining and improving the topsoil is all about management.
J. L. (Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms near Dundurn, Sask.