The benefits of natural soil

The roots of pot grown pea plants, Pisum sativum, illustrating a drawback in the use of sterilised composts. Seeds were planted in a pot with drainage holes, using sterile compost. The pot was left in the open, standing on natural garden soil. The roots of the peas (white mass, top) have encircled the outer edges of the compost in the pot. Some have emerged from the drainage holes and grown into the natural soil (centre, below). These roots have become infected with the soil micro-organism, Rhizobium leguminosarum. This is beneficial to the plant. The bacterium causes nodules to form on the roots (pink, picture centre). The pink hue is due to the presence of leghaemoglobin, and indicates that the nodules are fixing atmospheric nitrogen. This nitrogen is an available fertiliser for the plant. Roots growing in sterilised composts do not fix nitrogen