9 Soil and Water Relationships
Recent major droughts around the U.S. demonstrate the importance of the efficient use of water. Accurately measuring the amount of water available to plants is essential in determining irrigation rates and reducing the amount of wasted water. Soil water content is typically measured in the field using soil moisture sensors, which will be discussed in more detail in the Soil Water Measurement and Movement lab. Results from any soil moisture sensor must be interpreted using a “soil water relationship”. Soil water relationships are graphs depicting a measured value versus soil water content, and can be considered a calibration specific to a particular soil. In this lab, soil water relationships will be developed for a clayey soil and a sandy soil.
Learning Objectives
- Understand the effect of texture on pore size and water holding capacity.
- Know relationships among moisture potential, movement, and availability.
- Determine soil water content by weight and volume.
- Measure saturated hydraulic conductivity and understand factors influencing rates of water movement.
- Explain the operation of moisture monitoring devices.
Materials
- Two soils, a sandy soil and a clayey soil, both air-dried, ground, and passed through a 2 mm sieve
- Shallow trays capable of holding water to a depth of 5 cm
- Plastic cups
- A tool (such as a thumbtack) to punch holes in the bottom of the plastic cups
- Filter paper or paper towels
- Scissors
- Permanent markers and label tape
- Laboratory balances accurate to the nearest 0.01 g
- Weigh boats
- Soil scoops
- Graduated cylinders
- Oven
- Soil drying tins
Recommended Reading
- Module 5 – Plant Water Relations (Allison and Jones, 2005)
- Soil, Water, and Plant Relationships (Rogers et al., 2014)
- Available Water Capacity (USDA NRCS, 2008b)
Prelab Assignment
Using the recommended reading resources and the introduction to this lab, consider the questions listed below. These definitions/questions will provide a concise summary of the major concepts to be addressed in the lab. They are useful study notes for exams.
- Define soil water potential in your own words.
- Define and compare gravitational water, plant available water, and unavailable water. Relate each to the different pore sizes (macro-, meso-, and micro-).
- Identify the matric potential values (soil water potential) for saturation, field capacity, wilting point, air dry, and oven dry soil.
- Compare and contrast soil water content on a weight (mass) basis and a volume basis. Show how to convert between these weight and volume.
- Common units used to quantify moisture potential are cm (of water), bars, and kPa. How many cm of water in one bar? How many kPa in one bar? How many cm of water in one kPa?
Introduction
An understanding of soil-water relationships is essential to proper soil management for agronomic or other land use purposes. The soil-water-air continuum involves principles of physics, chemistry, and biology. Water, whic