Tamra Brown, age 13, of Las Vegas, Nev., for her question:

WHAT IS HUMUS?

Humus is a dark brown substance that is often found in soil. It is formed when roots, plants and the dead bodies of small animals decay.

The decay of animal and vegetable material is caused by the action of microscopic living beings called bacteria and fungi. The amount of humus in soil affects the soil's texture and its ability to hold moisture.

A good mixture of humus generally helps to increase the size and quality of a crop.

Humus is spongy and soft. You'll often find it filling the spaces between the mineral grains of the soil: it enables the plant roots to send out tiny hairs through which they absorb food and water.

Humus also holds water and reduces the problem of water running off and wearing away or eroding the soil.