The effect of erosion on soil productivity and related soil properties
Verity, Garth Edward
It is generally agreed that erosion reduces soil productivity by changing various soil properties that are important to plant productivity. The purpose of this study was to estimate the reduction in yields and changes in soil properties that relate to plant productivity, with increasing levels of erosion. Experiments were located near Viscount Saskatchewan, an area dominated by crop-fallow cereal grain rotations. Dark Brown soils developed on glacial till were used in this study. Various soil properties and dry matter yields were measured along and between catenas with moderate and severe erosion. Grain yields were lowest on upper slopes, and generally increased downslope, often to yields that were double the upper slope yields. The relationship between yields and relative distance downslope was described best by a third order polynomial equation, in that yields on the shoulder were slightly less than on the crest, increased steadily through mid-slopes to maximum values on the lower or foot slope, then decreased again as sampling moved from the slope to the generally lower part of the fields. The moderately eroded catena outyielded the severely eroded catena at the crest position. The impact of varying degrees of erosion on productivity was estimated by adding back incremental depths of topsoil to eroded knolls. Grain yields were increased by 45 to 58 % by adding 5 cm of topsoil. Further topsoil additions increased yields, but generally at a decreasing rate. Fertilizing eroded soil to soil test recommendations produced yields similiar to the 5 cm treatment, but never greater than the 10 and 15 treatments. Changes in soil quality with increasing erosion were measured on otherwise similar soils of increasing length of cultivation (0 to 75 years). Much reduced 137Cs contents on eroded compared to native soil indicated that erosion increased with time of cultivation. Losses in silt plus fine sand fractions suggested that wind erosion had dominated. Organic matter and total N (NT) were lost at the rate of 0.62 and 0.02 Mg ha-1year-1, respectively, over a 75 year period. Potentially mineralizable N (N0) decreased rapidly over the first 40 year period of cultivation, beyond which losses occurred at a slower rate. The relationship between potentially mineralizable S and years of cultivation was linear, indicating that the former was lost at a constant rate. Changes in the N0/NT ratio suggest that potentially mineralizable N was being lost faster than total N. Changes in the S0/ST ratio indicates that total S is being lost faster than potentially mineralizable S. Organic phosphorus decreased with increasing years of cultivation, while inorganic phosphorus showed no significant trends with cultivation. Field capacity and permanent wilting point decreased with increased erosion due mainly to losses in organic matter, resulting in a slight decrease in available water capacity. Cloddiness, as indicated by an increase in the mean diameter of dry aggregates, increased with erosion, but these aggregates were not stable in water.