SOILS AND FERTILIZERS 363 



legume crop such as clover is introduced, the supply of humus is 

 much better maintained. 



Improper methods of tillage add very greatly to the evil effects 

 that result from lack of humus. In many parts of the country the 

 land is plowed only 3 or 4 inches deep. Below the plowed stratum 

 the soil becomes sour, densely packed, and unfit for plant roots. 

 When such soils are plowed deep and this sour packed subsoil is 

 mixed with the upper portion, the growth of many crops is greatly 

 retarded This has led many farmers to believe that deep plowing 

 is ruinous. Some farmers have tried to remedy the difficulty by 

 subsoiling. The subsoil plow breaks up the packed layer but does 

 not throw it out on top. But while subsoiling does break up the 

 hard layer into chunks it does not pulverize it or put humus into it. 

 In most cases work done in subsoiling is practically wasted, and it 

 is doubtful if it ever pays. A much better method is to plow a little 

 deeper each year until a depth of 8 or 10 inches is reached. This 

 gives a deep layer of good soil, particularly if the supply of humus 

 is kept up. 



When new soil, or that which has lain undisturbed for several 

 years, is broken up, it is always best to plow deep from the be- 

 ginning, for the deeper layers will be about as fertile as any, except 

 the top inch or two. It is wise never to plow the same depth twice 

 in succession. In general, fall plowing should be from 7 to 9 or 10 

 inches and spring plowing from 5 to 7 inches deep. There are spe- 

 cial cases in which these rules do not apply. 



We plow the soil in order to loosen up its texture and get air into 

 it; also to turn under stubble, manure, etc., to make humus. Killing 

 weeds is another object accomplished by plowing. After a soil has 

 been thoroughly pulverized to great depths, so that there is no dan- 

 ger of turning up packed clay, the deeper the plowing the better the 

 crops. But the cost also increases with depth, so that ordinarily it 

 does not pay to plow more than about 10 inches deep. 



Some crops prefer rather a loose seed bed. Millet is such a crop. 

 Farmers sometimes plow a second time in order to sow millet on 

 freshly plowed land. Other crops, such as wheat and alfalfa, prefer 

 a fairly compact seed bed; hence, frequent harrowing and rolling 

 after plowing is good practice before seeding to these crops. Never- 

 theless, it pays to plow the land for them, even if we have to com- 

 pact it again before seeding. The plowing aerates the soil and helps 

 to set plant food free. 



The poverty of the soil may be due to poor texture, unfavor- 

 able structure, lack of humus, deficiencies in the amount, form, or 

 proportion of plant food, and to the presence of harmful mineral 

 and organic compounds. With the exception of nitrogen, most 

 soils, even those that are very poor, usually contain an abundant 

 supply of plant food, though sometimes other elements are lacking, 

 or are present only in those forms that plants cannot use. To in- 

 crease fertility the texture must be improved, and plant food and 

 humus added. Tillage will do much to improve the texture, but will 

 not suffice alone. It will not add humus. 



