Page 273 - Nutrient Management Planning Guide

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Chapter 8.1

The proportion of water available for plant use when a soil is moist but not saturated (i.e., at field capacity) will vary according to organic matter content, pore space and soil texture.

s i d e b a r While soil texture cannot be controlled, management and cropping practices can have an important impact on soil properties. Practices that retain or build organic matter and improve soil structure will improve the infiltration and water-holding potential of the soil.

Drawn from data presented in Miller and Donoghue, 1990

Figure 8.1.2 Effect of Soil Texture on Soil Water Holding Capacity

Drawn from data presented in Miller and Donoghue, 1990

Figure 8.1.3 Water Holding Capacities of Three Soils at Field Capacity

Slope

Soil slope has an important effect on runoff since a higher level of erosive energy is generated by water moving over steep slopes than by water moving over shallow slopes. Although precipitation and soil properties cannot be managed, there are management practices designed to reduce water erosion potential by interrupting the uniformity of the slope or by breaking slopes up into shorter segments. These act to slow down the runoff reducing the energy that can be used for erosion and allowing the soil particles suspended in runoff to be deposited back onto the soil.

Length and the grade of a slope influence potential soil (and therefore nutrient) loss. Simulations using the Water Erosion Prediction Project (WEPP) software with Alberta data demonstrate that the volume of runoff (measured as depth of runoff) increases as a product of slope length and grade (Figure 8.1.4).

WEPP

The WEPP model was developed in the United States through a collaborative effort between the Agricultural Research Service of the United States Department of Agriculture (USDA-ARS), the National (US) Soil Erosion Laboratory (NSERL) and Purdue University. It is designed to predict soil erosion losses at a field scale. The model incorporates soil, slope and climatic information to allow the user to see how management factors such as filter strips impact on soil losses.

To download and/or learn more about the WEPP model and its applications, you can visit the USDA-ARS website www.ars.usda.gov/main (search keyword: WEPP or NSERL).