ing fine, gritty particles, and clay having extremely fine, powdery particles when dry. The difference in particle size results in a difference in the available pore space between particles. The smaller the pore spaces, the more strongly water will cling inside them. Most soils are various mixtures of sand, silt, and clay, and a given field may contain different types of soil.
- 1-2 Hours
- Estimated Teaching Time:
- Three 1-hour sessions
- Opportunities for up to five 1-hour session follow-ups
Questions for discussion:
For one class demonstration or one participation activity for 3-5 students:
- What are the physical differences between sand, silt and clay particles?
- Which soil will pass water through to the jar the fastest? Why?
- Which soil type will absorb the most water? Why?
- Three transparent wide-mouth jars, each 1 to 3 liters in capacity
- Three small clay flowerpots with holes in the bottom, each 1-2 cups in capacity, of a shape and size that will fit inside the jars
- 1 cup of each of the following types of soils: sandy, silty and clay (get these from a local nursery)
- 1 water measuring device (graduated cylinder or flask).
- 1 stop watch
- small magnifying lens
- optional: weight scale capable of measuring in fractions of an ounce or gram.
Water will saturate the sand rapidly and excess water will quickly flow through the pot into the jar. Silt will saturate a little more slowly. Clay will take the longest.
Conclusions: Sand wets rapidly, but holds the least water, allowing most of the application to flow quickly into the jar. Silt wets slightly slower, but holds more water. Clay soils wet slowest, but hold the most water.
Activity 2: Comparing the efficiencies and irrigation time of gravity flow, sprinkler and micro-irrigation systems
In addition to soil types there are several other factors which affect the choice of irrigation method. Labor and equipment cost, water costs, crop selection and topography all must be considered before a farmer implements an irrigation system. The following activity introduces students to the three main types of irrigation systems and the advantages and disadvantages of each.
Questions for discussion:
- Have the students take a pinch of each of the soil samples with dry fingers, feeling the particles, smelling them, then wetting them slightly, feeling and smelling the change.
- Ask them to examine the pinches of soil with the magnifying lens.
- Explain the differences between sand, silt and clay.
- Place individual soil samples into each of the clay pots.
- Place each clay pot into its own jar.
- Pour equal amounts of water (100 ml) into each of the three pots in succession (Start with the sand, then the silt, then the clay).
- Time the period from water application until the flowerpots stop leaking. (About 15 minutes)
- Measure the amount of water collected in each of the jars under the pots. Optionally, measure the before and after weight of each of the soil-filled flowerpots, to find the weight of water retained.
- Have the students complete a simple lab report on this activity. Be sure to have them include a description of the procedure, observations during the experiment and an explanation of results.
- What are the three main types of irrigation systems?
- List one advantage and disadvantage of each.
- What are the goals of irrigating?
- How does crop selection affect irrigation techniques?
Conduct the demonstration, or group activity models as follows:
- Three 24"x24" nursery flats with solid bases (or a cardboard insert to cover drainage holes)
- Three strips of rag cloth two feet long, one for the top end of each flat
- A metal or plastic gutter for collecting runoff at the bottom of each flat (can be reused)
- 3 liters water - minimum
- 1 water dropper
- 1 garden watering can or hose nozzle with spray diffuser
- 1 garden trowel;
- 1 graduated cylinder or measuring cup (should hold at least 1 liter)
- 1 "six-pack" flower or vegetable seedlings
- 1 flat of grass or ground cover material from local nursery (can be purchased or planted from seeds 3-4 weeks prior to l