Are you looking for experiences that are rather short-term, that could be completed in one or two class periods?
How about testing water absorption with different materials:
Students receive buckets with drainage holes containing: sand, clay, gravel,
compost, and a mixture of the above. Using a known amount of water, students saturate their soil material and see how much water drains out of each bucket. If you can extend the experience into a multi-day project, perhaps you could test again the next day and see if the results change when the soil material has been pre-moistened the day before. Or a "hugel-pot" could be constructed, with old wood/sticks in the bottom, to see how much water it holds. Buckets could also be weighed before and after being saturated, or right after saturation, and then 24 hours later, to see how much moisture retention each material has. Students can chart their results and speculate about the reasons for different results. I could see this being applicable for Environmental Sciences classes (to talk about run-off, watersheds, etc.), Physics, Geology, Geography (how does soil in different world areas react to rain?) Chemistry, etc., or just to teach basic scientific method skills.
If students are able to do long-term experiments (long enough to grow something!), they could begin experimenting with soil conditions. A control plant could be grown with average top soil, and compared to a plant grown with extra high-nitrogen compost, or one with too much carbon. Color, size, flowering and fruiting, and resistance to insects/disease could all be compared.
Is this the kind of thing you're looking for?
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