Plowing is a mechanical operation that deals with physics, chemistry, bacteriology and entomology. The soil is the farmer's laboratory; his soil working implements are his mechanical laboratory appliances. A high order of intelligence is required to merge one operation into the next to take full advantage of the assistance offered by nature. The object of plowing and cultivation is to improve the mechanical condition of the soil, to retain moisture, to kill insects and to provide a suitable home for the different kinds of soil bacteria.
There are aerobic and anaerobic bacteria, also nitrogen-gathering bacteria and nitrifying bacteria which are often loosely referred to as azotobacter species. Few of us are on intimate terms with any of them, but some of us have had formal introductions through experiments and observation.
[Illustration: Figure 133.--Sulky Plow. This is a popular type of riding plow. It is fitted with a rolling coulter.]
_Walking Plow._--The draft of a walking plow may be increased or diminished by the manner of hitch. It is necessary to find the direct line of draft between the work performed and the propelling force. The clevis in the two-horse doubletree, or the three-horse evener and the adjusting clevis in the end of the plow-beam with the connecting link will permit a limited adjustment. The exact direction that this line takes will prove out in question. The walking plow should not have a tendency to run either in or out, neither too deep nor too shallow. For the proper adjustment as to width and depth of furrow, the plow should follow the line of draft in strict obedience to the pull so that it will keep to the furrow on level ground a distance of several feet without guidance from the handles. In making the adjustment it is first necessary to see that the plow itself is in good working order. All cutting edges such as share, coulter or jointer must be reasonably sharp and the land slip in condition as the makers intended.
[Illustration: Figure 134.--Disk Plow. Less power is required to plow with a disk, but it is a sort of cut and cover process. The disk digs trenches narrow at the bottom. There are ridges between the little trenches that are not worked.]