The objection often made to this last rule is that, aside from the increase in fertility, there is no direct return for the time, labor and seed, and the land brings no crop for a year. It is not necessary to use the entire crop for green manuring--a part of it may be used for hay or for pasture with little loss of the manurial value of the crop, provided the manure from that part of the crop taken off is returned and the part of the crop not removed is turned under. LENGTH OF THE ROTATION The length of the rotation may vary from a two-course or two crop rotation to one of several courses. Crimson clover may be alternated with corn, both crops being grown within a year. A three-course rotation, popular in some parts of the country, is wheat, clover, and potatoes; potatoes being the money crop and cleansing crop, wheat a secondary money crop or feeding crop, and clover the manurial and feeding crop. A popular four-course rotation is corn, potatoes or truck, small grain, clover; the potatoes being the chief money crop, corn the feeding crop, the small grain the secondary money or feeding crop, and clover the manurial and feeding crop. On many New England farms near towns, hay and straw are the chief money crops. Here the rotation is grass two or more years, then a cleansing crop and a grain crop. A Canadian rotation is wheat, hay, pasture, oats, peas. A rotation for the South might be corn, crimson clover, cotton, crimson clover; this rotation covering a period of two years. A South Carolina rotation is oats, peas, cotton, corn--a three-year rotation. It might be improved as follows: Oats, peas, crimson clover, cotton, crimson clover, corn. CHAPTER XXV FARM DRAINAGE Some farm lands contain so much water that the conditions of fertility are interfered with and therefore the crop producing power of these lands is lowered. HOW SURPLUS WATER AFFECTS FERTILITY This surplus water diminishes fertility by reducing the area of film water in the soil. It checks soil ventilation. It tends to keep the soil cold. It dilutes plant food in the soil. It interferes with proper tillage. INDICATIONS OF A NEED OF DRAINAGE The above-mentioned state of affairs occurs sometimes in fields at the foot of hills, or on sloping uplands which receive spring water or seepage water from higher lands. Some fields are underlaid by a close, compact subsoil which so checks percolation that the surface soil is too wet for tillage operations the greater part of the year. In such cases: A need of drainage is generally indicated by the presence of more or less free water standing on the surface. In some lands the surface water does not appear as free water standing on the surface. In such cases: