
An important function of soil is to absorb water at the land surface, and either store it for use by plants or slowly release it to groundwater through gravitational flow (figure 5.8). When rainfall hits the ground, most water will infiltrate the soil; but some may run off the surface, and some may stand in ruts or depressions before infiltrating or evaporating. The maximum amount of rainwater that can enter a soil in a given time, called infiltration capacity, is influenced by the soil type, structure, and moisture content at the start of the rain.
Early in a storm, water usually enters a soil readily, as it is literally sucked into the dry ground. As the soil wets up during a continuing intense storm, water entry into the soil is reduced and a portion of rainfall begins to run downhill over the surface to a nearby stream or wetland. The ability of a soil to maintain high infiltration rates, even when saturated, is related to the sizes of its pores. Since sandy and gravelly soils have more large pores than do fine loams and clays, they maintain better infiltration during a storm. But soil texture is also important in governing the number of pores and their sizes: When finer-textured soils have strong aggregates due to good management, they can also maintain high infiltration rates.
When rainfall exceeds a soil’s infiltration capacity, runoff is produced. Rainfall or snowmelt on frozen ground generally poses even greater runoff concerns, as pores are blocked with ice. Runoff happens more readily with poorly managed soils, because they lack strong aggregates that hold together against the force of raindrops and moving water and, therefore, have few large pores open to the surface to quickly conduct water downward. Such runoff can initiate erosion, with losses of nutrients and agrochemicals as well as sediment.
Table of Contents
- About the Authors
- Preface
- Introduction
- Healthy Soils
- Organic Matter: What It Is and Why It's So Important
- Amount of Organic Matter in Soils
- The Living Soil
- Soil Particles, Water, and Air
- Soil Degradation: Erosion, Compaction, and Contamination
- Nutrient Cycles and Flows
- Soil Health, Plant Health, and Pests
- Managing for High Quality Soils: Organic Matter, Soil Physical Condition, Nutrient Availability
- Cover Crops
- Crop Rotations
- Animal Manures for Increasing Organic Matter and Supplying Nutrients
- Making and Using Composts
- Reducing Erosion and Runoff
- Preventing and Lessening Compaction
- Reducing Tillage
- Managing Water: Irrigation and Drainage
- Nutrient Management: An Introduction
- Management of Nitrogen and Phosphorus
- Other Fertility Issues: Nutrients, CEC, Acidity, and Alkalinity
- Getting the Most From Routine Soil Tests
- Taking Soil Samples
- Accuracy of Recommendations Based on Soil Tests
- Sources of Confusion About Soil Tests
- Soil Testing for Nitrogen
- Soil Testing for P
- Testing Soils for Organic Matter
- Interpreting Soil Test Results
- Adjusting a Soil Test Recommendation
- Making Adjustments to Fertilizer Application Rates
- Managing Field Nutrient Variability
- The Basic Cation Saturation Ratio System
- Summary and Sources
- How Good Are Your Soils? Field and Laboratory Evaluation of Soil Health
- Putting It All Together
- Glossary
- Resources