The process of leaving the residue on the surface while the land is being farmed is generally called stubble-mulch farming, or stubble mulching. It involves tillage beneath the residues for soil pulverization and weed control, final preparation of a suitable seedbed without unduly destroying the residue, planting through the mulch, and performing subsequent tillage or cultivation under the mulch. It also involves the use of such crops in the rotation as to supply an adequate amount of residue to protect the land if properly used.
Equipment has been devised for running beneath the surface of the soil and tilling the land thoroughly without inverting the soil or burying the residue. This type of work is called subsurface tillage. Some types of implements performing this kind of tillage are called subsurface tillers, or the name may be shortened to subtiller.
Three main types of equipment have been used for subsurface tillage. One of the first to be used was the V-shaped sweep. The duckfoot or field cultivator with shovels up to 14 inches in width has long been used in the Great Plains, but it is not satisfactory with heavy residues because of clogging and has not been called a subsurface tiller. Later developments have been along the line of wider sweeps with stronger shanks. The use of longer and fewer shanks results in less clogging in heavy stubble, and the wider sweeps cause less disturbance of the surface cover. These sweeps have differed greatly in width, varying from about 22 inches to as much as 7 feet. Most of those used so far have ranged from 22 to 31 inches, although wider sweeps are being used to some extent.
The sweeps are drawn beneath the surface of the soil at depths varying from 2 to 6 inches. As the blade moves forward, the soil over it is first crumpled and elevated, and then dropped as the blade passes. This rise and fall loosens and breaks up the soil thoroughly if it has the proper moisture content. A rolling coulter in front of the sweep cuts through the residue and prevents it from collecting on the shank. If the soil is too wet when subtilled, it tends to hang together rather than break up into small lumps. Cultivation of wet soil does not kill weeds so readily as when the soil is relatively dry. Of course, it is bad practice to plow land when it is too wet, but it seems that land can be plowed effectively with a somewhat higher moisture content than it can be subtilled, because, in plowing, weeds are turned completely under.
If land must be subsurface-tilled to check weed growth when it is too moist for effective work, it should be followed by some implement that further breaks up the soil.
Other types of equipment used for subsurface tillage include the straight-blade tiller and the modified rod weeder. The action on the soil of these implements is similar in principle to that of the V-sweep. The soil is broken as it is raised up onto the blade or rod and again as it falls off behind. The straight blade, when it has considerable rise on the forward side, lifts the soil higher than does the V-sweep and causes more crushing and breaking when the soil is lifted. The slightly greater fall gives more breaking to the rear of the blade. There is a tendency, when the soil is compact or weedy, for the soil to break off in long slabs at the back of a straight blade. Consequently, the tiller must be followed by some other implement to break up the soil further and kill weeds.
The rod weeder can be fitted with a subsurface attachment, which consists of a bar beneath the revolving rod. On this bar are mounted small points or shovels. These enable it to penetrate rather compact Soil. The points on the attachment help break up the soil, and the revolving rod tends to keep the bar free of trash. The rod weeder is an efficient implement for killing weeds. When used without the attachment it is best adapted for use on land that has been cultivated previously With some blade or sweep implement or with a one-way. Besides killing weeds, it brings clods and partly buried residues to the surface. It is frequently used for the last cultural operation before seeding. When operated at a shallow depth, it has a packing effect on the soil below the rod and helps prepare a firm seedbed.