Doug Lanham, age 11, of Rockford, Illinois, for his question:

How does clover enrich the soil?

Early farmers learned by accident that clovers and other legume plants enriched their fields. They did not know why this happened, but they made practical use of it. The mystery had to remain unsolved until the microscope was invented and revealed the existence of bitsy bacteria. Biologists of the Age of Science learned that certain bacteria thrive in clover roots and repay their hosts with gifts of plant food chemicals. This give and take is symbiosis, a beauteous word that means living together.

This story dates back to the Feudal System of 1,000 years ago. Most Europeans lived in small communities and parceled their acreage in strips. They knew from experience that soil grows tired, that a strip that yielded a good wheat harvest cannot repeat the same bounty the. next year. In those days, the last word was crop rotation. The former wheat field was planted with cabbages or some other crop and every few years it was left unplanted and fallow to rest and replenish itself. No one understood the secrets of plant nutrition and fertilizers. But eventually they noticed something strange about their peas, beans, clover and other legume crops.

These fields did not have to waste a fallow year, because after the legume harvest, the soil was healthy enough to feed other crops. Since the 1880s we have known that essential nitrogen plant foods are added to the soil by bacteria that thrive inside the roots of clovers and other legumes.

If you carefully loosen a clover root from the soil, chances are you will see bumpy little nodules attached to the tangled threads. Each nodule contains a thriving community of busy bacteria. They need carbohydrates, especially sugars, and take these ingredients from the root tissues. But, they do not qualify as true parasites that practice all take and no give. Their life processes put nitrogen compounds into the soil    arid these chemicals are essential nutrients on the menu of all plants.

Nitrogen is a chemical element and goodness knows there is plenty of it in the air and the soil. However, plants cannot absorb it as an element and its atoms must be combined with other elements. This chemical trick is called nitrogen fixation  ¬and nitrogen tends to resist it. The job is done rather spasmodically by a team of raindrops and lightning. But this does not yield enough solid nitrogen compounds for the plant world. Far more are produced by those special strains of soil bacteria that set up housekeeping in the roots of clovers and other legumes. The plants and the bacteria exchange menus, living together in a practical system of symbiosis.

The miniature world of bacteria is populated with a vast variety of organisms and biologists classify them as they do the larger creatures. It seems that different species tend to favor certain legumes. A bacterium that favors alfalfa resembles a smaller footprint. Bean roots are very popular with bacteria and might fool you because, under the microscope, they look somewhat like bitsy bugs.