Organic Matter In Soils
For example, there is humus, an important end product of organic matter in soil. In 1935, Dr. Selman A. Waksman, of Rutgers Univer­sity, the noted microbiologist who developed some of our most effec­tive antibiotics, wrote a book entirely on humus and its relation to soil. It is one of the most exhaustive studies of a soil fraction in existence, yet in the end it leaves unanswered as many questions as it answers. (Anyone who thinks soil organic matter is a simple subject should read Dr. Waksman's book, published by Williams and Wilkins Co., Baltimore, Md.)

The recycling of organic matter, using and reusing basic elements over and over to keep unbroken the chain of existence on earth, is vital to all life. Were it not for death and decay, all the carbon and nitrogen in the world would soon be locked up in permanent form in bodies of dead plants, animals and men.

Let us follow a plant in the garden as it dies and falls to the ground in late fall. In spring, the gardener digs it into the soil; the dead plant (what's left of it) then begins the process of changing to organic matter and humus. The words "organic matter" are used here in a special sense-the dead remains of plants and animals. Tissues of the plant that has just been plowed under are largely water-between 75 and 85 per cent in most garden vegetables. Of minerals other than water, about 10 per cent will be carbon and another 10 per cent oxygen (in compounds other than water), plus about 2 per cent hydrogen and 2 per cent ash.

What is particularly striking about the mineral makeup of organic matter is that only 2 per cent of the total of the fresh plant-the ash -is derived from soil. The rest of the plant is made up of elements from air and water. Even the nitrogen, a vital part of the total, small as it is percentagewise, came originally from the air. This emphasizes the importance of air and water relationships (see Chapter Eleven).

By the time our dead plant is plowed under, it has lost much of its water but because winter temperatures are low, bacteria and fungi have not yet been able to attack the solid matter it contains. As soon as soil temperatures go above 60 degrees F. for several days, bacteria and fungi will become active. Their first attacks will be on the starches and sugars in the plant tissues-energy foods they need for continued activity. They will also go to work on proteins, which they need for cell growth.