Water-wise: Keep Soil Wet Without Waste
It is almost mid-summer! You might have colorful flowers, fresh tomatoes and yummy berries in the garden. But wait, did you remember to water them? Maybe you wonder if they get enough, too much or too little water. Should you water every day, twice a day or once a week? Should you soak the ground, 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 project, you will see how much water soil can hold, by practicing not with dirt, but with readily available kitchen substitutes. Once you understand some general principles, you should be better equipped to tackle the real problem of watering wisely.
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
Some experts claim U.S. residents waste 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?
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 frequent, smaller amount of water is needed. For a 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: the structure of the soil – or how big or small the particles in the soil are – and the amount of organic material present in the soil. Organic material refers to broken-down plant and animal material. Water loves to cling to organic material, so more of it mixed in the soil allows the soil to hold onto more water. In terms of soil structure, smaller particles have more surface area for the same volume of soil, meaning more area for water to stick to. This activity will verify this 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” to see how you can apply this to the soil in your garden.
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 dirt in your garden or flower pot can hold? Take a sample of dry dirt from the garden or the flower pot. If the dirt is wet, you will need to let it dry out before you do the test. Crumble the dirt so it is loose and grainy and perform the test from this activity. How much water can 2/3 cups of dirt hold? What would happen if, during a watering cycle, you add more water to this dirt? Should you let the dirt dry out completely before re-watering? How could that influence your watering schedule?
Extra: Geologists classify soils in 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, while the smallest grind retained the most water? This is what is expected.
Water runs down because it is being pulled on 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 they are made up of). Still, particle size plays an important role in determining how much water the soil can hold. Sandy soil has 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 hold too much water, causing plants to rot.
Adding organic material (broken-down plant and animal material) to soil does not only increase the nutrient content of soil, but it also improves the water-holding capacity of the soil.
More to Explore
Sabine De Brabandere, PhD, Science Buddies
Science Buddies |
Water-holding capacity, soil types
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