New Zealand soils are young soils and do not have a great depth of topsoil. Any erosion of topsoil from pastoral land has a long-lasting effect on productivity. Measurements of pasture productivity made on old hill country slips have shown that recovery of productivity takes around 80 years, and then recovery is only to ~80 % that of uneroded slopes. It takes many generations for topsoil to reform.
Topsoil is important as a store of plant nutrients and water. Topsoil is easier than subsoil for plant roots to penetrate so plants establish easier, grow more quickly and are more productive where the topsoil remains intact.
Erosion is a natural process, which has been accelerated in our pastoral hill country through the removal of forest cover and the loss of the root network provided by these trees. Replacement of tree cover using wide-spaced mature poplars and willows is effective in reducing the risk of erosion to very low levels compared with slopes not protected by trees. However, it takes time for trees to grow to the maturity required, up to 15 years depending on the site.
Erosion type and severity is predominantly determined by rock type, vegetation cover and rainfall volume and intensity. Due to the low cost hill country farming systems used in New Zealand, hard engineeering solutions to control erosion have not generally been used. This is true for river systems too, since low population means that tax money is not available for hard engineering except where off site values warrant high cost solutions, e.g. protection of main highways and structures such as buildings in urban situations
For further information see Erosion Control in the Reports section.