716 IOWA DEPARTMENT OF AGRICULTURE 
On the other hand, a soil in good physical condition, has much the 
structure of a sponge; it is filled with decaying vegetable matter or humus, 
the soil particles adhere together in small masses, forming what is 
called a crumb-structure. The decaying vegetable matter forms parting 
planes separating these crumbs and rendering the soil friable and loose. 
The air freely penetrates among the particles of the soil assisting the 
bacteria to fit the crude elements for the use of growing plants. Every 
soil particle is surrounded by a thin film of water held in place by the 
force of gravitation. The soil is not wet, it is moist. This soil water 
holds the plant food in solution. Other water rises from below, deeper 
down in the earth, to take the place of that absorbed by the growing 
plants or lost in evaporation, for the capilliary attraction of the soil is 
good. The loose crumb-structure of the soil makes it easy for the 
tender rootlets of the growing plants to push their way among the soil 
particles and lying close to the small soil masses the rootlets drink up 
the soil water with its liquid plant food. 
The soil is warm for the air circulates among the soil particles and 
seed planted in such a soil quickly germinates and like that that fell 
upon good ground, brings forth fruit a hundred-fold. • 
It is a well known fact that a soil containing an excessive amount of 
moisture can not be put into good physical condition. An attempt to 
cultivate such a soil is time and labor wasted. The water in the soil 
prevents the air from entering, prevents the growth of the beneficial 
forms of bacteria and aids in the formation of acids injurious to growing 
crops. Therefore, if a soil is wet, the first step toward putting it into a 
good physical condition, is to drain it. The next important thing in 
maintaining the soil in good physical condition is to keep up the supply 
of humus or decaying vegetable matter. Nature has, when left to herself, 
always provided for the maintenance of the humus supply. She will not 
tolerate bare ground. Let man destroy the vegetation of a plot of 
ground and Nature at once commences to cover it with a mantle of green. 
She covers the ground with a carpet of grasses; their leaves prevent the 
rays of the sun from burning up the organic matter of the soil, prevent 
the winds from scattering its particles and the fioods from sweeping it 
away. The roots bind the soil together and the plant decaying leaves a 
supply of humus in the soJ so that two blades of grass may grow where 
but one grew before. We should learn a lesson from Nature and never 
leave the soil bare to the burning rays of the summer sun. We should 
avoid all unnecessary waste of the humus already in the soil and add 
to the supply by an intelligent system of crop rotation. Clovers or other 
legumes should be frequently grown. Green manure should be plowed 
under. Corn stalks and straw should not be burned but their organic 
matter should be returned to the soil to furnish a humus supply and a 
store of plant food for future years. In this connection it is not generally 
known that the farmer may add indirectly to the supply of nitrogen in 
the soil by plowing under material of this kind. There are several forms 
of nitrogen fixing bacteria that use the carbonaceous matter in the soil in 
the process of fixing atmospheric nitrogen. Speaking of this matter. Prof. 
A. F. Woods, says: "In warm, well areated soils containing sugars, 
starches, and cellulose from decaying grasses and other vegetation, and 
