Students sometimes believe that plants get their "food" from the soil. Scientists, meanwhile, understand that plants manufacture their own food -- simple sugars -- using energy from sunlight, water, and carbon dioxide from the air.
Plants also require certain mineral nutrients from the soil for growth, repair, and proper functioning. Nutrients dissolve in water and are absorbed by plant roots. Many factors affect whether plants can take up nutrients in the soil, the most important of which is pH, a measure of the acidity or alkalinity of the soil. Even if your soil has plenty of nutrients, a pH above or below the ideal range (6.0 to 6.8 for most garden plants) can prevent your plants from getting the nutrients they need.
If you have a school garden, you can test your soil's pH and the presence of important nutrients such as nitrogen, phosphorus, and potassium. You can either do this yourselves (so students can think and act like scientists) or you can collect soil to send to a soil-testing lab.
Simple soil-test kits are available through garden centers, seed catalogs, and science suppliers. Since it's difficult to accurately test how much of these nutrients are actually available to your plants, most home garden kits give only broad readings of quantities of nutrients. If you want a more accurate account, you'll need to send your soil to a testing lab. Contact your local Cooperative Extension Service office to find out where to get soils tested locally. If your test shows that the soil pH is too high or low for the crops you're growing, or that it's deficient in certain nutrients, students can research and experiment with different methods of improving the garden soil, then carefully observe how their plants respond.