SOIL EROSION 369 



or wide bottomed ditch, which has but slight fall, so there will be 

 little or no erosion. The Mangum terrace can be used to good ad- 

 vantage on heavy soils which absorb water very slowly. This form 

 provides very effective protection against erosion and eliminates 

 waste land. 



8. Reforesting. As already pointed out, the soil of virgin for- 

 ests is protected by leaves and twigs. On cleared areas where the 

 surface soil has been removed to such an extent that it does not 

 produce profitable crops and especially where gullied it may be 

 advisable to imitate nature by planting trees. The black locust is 

 excellent for this purpose. Being a legume it is capable of good 

 growth on soils very low in organic matter. The leaves and twigs 

 protect the soil and, through the aid of nitrogen fixed by the legume, 

 grasses soon start among the trees (Fig. 181 ). By this time there is 

 little movement of soil material. When abandoned, such areas are 

 reforested naturally, but the process is very slow and much ad- 

 ditional erosion may take place before there has been sufficient 

 growth to hold the soil. The natural growth in most caws will be 

 of far less value than the black locust or other trees which might be 

 selected for this purpose. 



J). Tiling. In rolling sections, " seepy " or "springy" spots 

 are common. On these, crops do poorly, wheat often " heaves " and 

 may be killed completely. In wet seasons these spots are much 

 larger than normal, so the damage is much greater. In many of 

 these places and on much rolling land which does not have an 

 especially pervious subsoil, tile will produce all its ordinary bene- 

 fits, including warmer, drier surface soil in (he spring when early 

 tillage and planting are desirable. The most beneficial effect of 

 tiling is the increase in perviousness of the soil, so that the rains 

 are absorbed more readily, thus decreasing the run-off. This is a 

 very effective method of preventing erosion, but the expense is al- 

 most prohibitive when that is the only purpose to be accomplished. 



II. Gullying 



In any depression extending up and down a slope water col- 

 lects. Its velocity is increased with its volume, as is also it< trans- 

 porting and eroding power. For this reason depressions extend- 

 ing down the slope, such as a furrow, wagon or planter track, a 

 sheep or cow path, or even a mole tunnel, may soon result in a small 

 gully. These should be filled with some coarse organic matter or 

 24 



