Key concepts
Geology
Water-holding capacity
Soil types

Introduction
In the summer you might see colorful flowers, fresh tomatoes and yummy berries in the garden. But who is watering them? Maybe you wonder if they get enough—or too much water. Should they get water every day, twice a day or once a week? Should they be soaked or is frequently providing small amounts of water better? Unfortunately there is no general rule because much depends on the type of soil. In this science activity you will see how much water soil can hold by practicing with readily available kitchen substitutes for dirt. Once you understand some general principles, you should be better equipped to tackle the real problem of watering wisely.

Background
Some experts claim U.S. residents use billions of gallons of water in outdoor use each day. Watering gardens is one contributor. Why is it so difficult to water the garden wisely to protect plants and avoid waste?

Ideal watering schedules (how much and how frequently you should water) depend largely on how much water the soil can hold. If the soil cannot hold onto water, a frequently added and smaller amount of water is needed. For soil that can hold onto water, a less frequent substantial watering schedule is better.

Two factors play a major role in determining the water-holding capacity of soil: its structure—or how big or small the particles in the soil are—and the amount of organic material present. Organic material refers to broken-down plant and animal material. Water loves to cling to organic material, so more of it mixed into the soil allows more water retention. In terms of soil structure, smaller particles have more surface area for the same volume of soil, meaning more area where water can stick. This activity will try out this idea with insoluble dry food. Will grinding up the food allow the same volume of substance to hold more water? Once you have put your “soil” to the test, read the conclusion, and check out the links in the "More to explore" section to see how you can apply this to the soil in your garden.

Materials

  • Paper towels or kitchen towels to protect your workspace in case of spills
  • Four identical glasses (preferably with rather narrow openings so the cone-shaped coffee filters can rest in them)
  • Three clean cone-shaped paper coffee filters
  • One third cup measuring cup
  • Two thirds cup dried rice, rice meal (or coarsely ground rice) and rice flour (or finely ground rice) (Alternatively, you can use dried corn kernels, cornmeal and corn flour; or grain berries, bulgur and wheat flour.)
  • A helper
  • Water
  • Workspace that can tolerate some liquid splashes
  • Paper and pen (or pencil)
  • Plastic coffee filter cone(s), which can hold your paper filters and help prevent spills (optional)
  • Kitchen scale (optional)

Preparation

  • Protect your workspace with some paper towels or a kitchen towel
  • Open a coffee filter and let it rest in the opening of a glass. Repeat with two other filters and glasses. You will have one glass left.
  • If you have coffee filter holders available, use them, especially for the dried rice. They will help prevent spills.
  • Add two thirds cup of dried rice in the first filter.
  • Add two thirds cup of rice meal in the second filter.
  • Add two thirds cup of rice flour in the last filter.

Procedure

  • If you have a kitchen scale available, weigh the filters with the dry substances, one at a time. Write down your measurements.
  • Look at the three substances: dried rice, rice meal and rice flour. How are they similar and how are they different?
  • In a moment you will pour water over these substances. Which one do you think can hold more water? How could you measure how much water it can hold?
  • Pour one third cup of water over the dried rice. Before you do so, ask a helper to hold the filter up so it does not flip over and spill. If you have a coffee filter holder, this will do the job as well.
  • Repeat the previous step, now pouring one third cup of water over the rice meal and then over the rice flour. Observe what happens. Does water seep through the substances? Why does this happen?
  • Water gets pulled down by gravity. In the three setups why does the water not seep through in the same way in the three cases?
  • Pour another one third cup of water over each substance. Be careful, the filter might flip. Do you think more or less water will seep through this time?
  • Fill the last glass with two thirds cup of water and place it next to the other glasses. How could this glass help you discover which substance can hold the most water?
  • After about five minutes, compare how much water ran through the substances in the glasses. Your fourth glass shows you how much water you poured over the substances. Which glass has the most water? If there is less water in some glasses, where has the water gone?
  • If you have a kitchen scale, weigh each filter with its wet substance. Are the measurements different from the mass of the dry materials measured in the first step? Which samples gained the most mass? Where is this extra mass coming from?
  • Extra: If you have compost available, you can measure how adding compost influences the water-holding capacity of soil. For example, you could create a 1:2 compost-rice meal mixture by adding one scoop of compost for each two scoops of rice meal and compare the water-holding capacity of this mixture to that of pure rice meal using the method described in this activity.
  • Extra: Can you measure how much water the soil in your garden or flowerpot can hold? Take a sample of dry soil from the garden or the flowerpot. If the soil is wet, you will need to let it dry before you do the test. Crumble the soil so it is loose and grainy and perform the test from this activity. How much water can two thirds cup of soil hold? What would happen if, during a watering cycle, you add more water to this soil? Should you let the soil dry out completely before re-watering? How could that influence your watering schedule?
  • Extra: Geologists classify soils into three main types: sand, silt and clay. Clay has the smallest particles; you might need a microscope to see them. Next comes silt and then sand. Which type of soil do you expect to hold water best? If you can find small samples of these types of soil, repeat the experiment with them to test your prediction.

Observations and results
Was the dried rice barely able to hold water whereas the smallest grind retained the most water?

Water runs downward because it is being pulled by gravity. Water needs a surface to cling to in order to overcome gravity and stay in the sample. The more you grind a substance, the smaller the particles in the substances are—and the more total surface there is for water to hold onto.

Soils differ, not only in particle size but also in consistency (or what materials comprise them). Still, particle size plays an important role in determining how much water the soil can hold. Sandy soil has a larger particle size; it cannot hold much water and needs frequent smaller doses of water. Silt has smaller particles; its water-holding capacity is ideal for most plants. Clay has tiny particles, so it might retain too much water, causing plants to rot.

Adding organic content (broken-down plant and animal material) to soil not only increases its nutrient content but also improves the its water-holding capacity.

Cleanup
Your samples and filters can go into the compost bin. Water your plants with the leftover water.

More to explore
Outdoor Water Use in the United States, by WaterSense
How Dirt Cleans Water, from Scientific American
Soil Science: How Moist Is That Mud?, from Scientific American
Water Retention of Soil, from Teaching Biology Project

This activity brought to you in partnership with Science Buddies

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