Have you ever noticed how much work it is to dig a hole in really hard soil? It's much easier to dig a hole in soft, loose soil. But why is that? Soil that is hard and dry is often compacted, which means that it has been packed down, making it denser and thereby difficult to penetrate. Soil that has become compacted is not only harder for you to dig a hole in, but it can also be much harder for a lot of other organisms, such as helpful earthworms, to survive in.
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Just as it is difficult for you to dig in compacted soil, it is also difficult for soil-dwelling organisms, like bugs and worms, to tunnel in compacted soil. You won't usually find many organisms living in compacted soils because they cannot get the air, space and nutrients they need to survive. Also, compacted soil makes it difficult for plants with delicate root systems to thrive. Very compacted soil tends to support only the growth of weeds, which have thick tap roots that can penetrate deeply into compacted soil and out-compete other plants.
Some areas are more susceptible to soil compaction than others. For example, the number of people and other animals that walk in an area—in other words, the amount of "foot traffic"—can affect how compacted the soil is there. The quantity of sunlight and moisture can also affect how susceptible an area is to compaction.
Extra: Investigate the effect of walking on soil compaction by digging up some soil so that it is loose. First measure its compaction with your spool-and-needle apparatus, then walk over the site and measure the compaction again. How did the soil's density change? If you walk over the site more, will the compaction continue to change?
Extra: Test whether wet or dry soils become more compacted by adding different amounts of water to dry soil and compacting it with a tamper or roller. Compact a sample of dry soil in the same way. Measuring each sample with your spool-and-needle apparatus, which soils are the most compacted? What happens if you let them dry out and measure them again afterward?
Observations and Results
Were some soils you tested more compacted than others? Did you find that the areas that received a lot of foot traffic were the more compacted? Were dry areas more compacted than moist areas? Did more compacted areas have fewer plants?
As soil is pressed down, such as by humans and other animals walking over it, it becomes compacted, or denser. This is why areas that receive a lot of foot traffic or areas that are driven over by vehicles are more susceptible to becoming compacted. When soil becomes too dense many organisms such as bugs, worms and some plants will be unable to live there. This is in part because it is harder for oxygen and water to penetrate into highly compacted soil. The lack of moisture can cause compacted soil to become dry, and you may have found that dry areas were denser than moist ones. What other qualities did you notice about soils that had different levels of compaction?
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