It is also the tool generally used to cover seeds sown broadcast. Harrowing is generally done just before planting, and with some crops just after, to cover seeds or to smooth the ground. Harrowing is also done in the first stages of growth of some crops to kill weeds and make a soil mulch. The harrow should always follow the plow within a few hours unless it is desired to leave the land in a bare fall or winter fallow. At other times of the year the lumps of earth are apt to dry out and become hard and difficult to break. If there is but one work team on the farm it is a good plan during the plowing season to stop the plow in time to harrow the day's plowing before the day's work ends. HARROWS There are several types of harrows in use. They may be classified according to the style of their teeth or cutting parts; they are as follows: Rolling cutter harrows. Spring-toothed harrows. Spike-toothed harrows. Coulter-toothed harrows. Chain harrows. Brush harrows. Plank or drag harrows. These types vary in the depth to which they cut, and the degree to which they pulverize the soil. _Rolling cutter harrows._ Harrows of this type (see Fig. 54) consist of one or more revolving shafts on which are arranged a number of concave disks. These disks are either entire, notched, or made of several pieces fastened together. Examples of these are the disk, cutaway and spading harrows. These harrows cut and move the soil deeper than the other types. They are especially adapted to work on heavy clay soils. The value of this type of harrow as moisture preservers depends on the manner in which they are used. If the disks are so set that they cover but a portion of the surface with a mulch of fine earth they leave a ridge exposed to the action of the wind and sun and the rate of evaporation is greatly increased. The disks should be set at such an angle that the whole surface shall be stirred or covered. Soils which need the disk harrow should generally be gone over again with some shallower working tool to smooth the surface. An objection to the rolling cutters is that unless great care is taken they will leave the land in ridges and valleys. The two gangs of disks throw the earth in opposite directions. They are generally set to throw it from the centre and the result is a shallow double furrow the width of the machine. By lapping each time the furrow is partially filled, but to get the land smooth a smoothing harrow should be used after the rolling cutter.