Study the long-term effects of fertilisers and pesticides using as examples the eutrophication of the Norfolk Broads, the bio-magnification of DDT in the fish - osprey food chain and the effect methyl mercury had on the human food chain (Minimata disease), which has recently been reported among remote communities in the Amazon.
Legumes are used in some crop rotations. Look at the fresh roots of clover, bean or pea plants and find the nodules which contain the nitrogen fixing Rhizobium bacteria. Research the role of these bacteria in the nitrogen cycle and crop rotation.
Look at Rhizobium in detail. Immerse a washed piece of clover root with nodules in alcohol for two minutes, and then macerate to form a milky liquid. Use this to inoculate prepared Petri dishes of mannitol - yeast extract agar (+nitrogen) and mannitol agar ( - nitrogen).
Follow sterile and safe procedures. Compare the appearance of the bacterial colonies on the two types of medium.