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rendered harmless in a variety of ways. Barn-yard manure or de- 

 caying organic matter, sucli as a green crop of cow-peas, turned 

 under has a very marked effect in freeing the soil from them. 

 Thorough and complete airing of the soil will often destroy or 

 overcome these poisonous substances. The beneficial effects of 

 ploughing and of thorough surface tillage are thus explained, in 

 part at least, on the basis of the thorough aeration secured. When 

 the same crop is not grown oftener than every three or four years 

 on the same land the injurious substances the crop throws off seem 

 to have time to disappear before the same crop is grown again ; 

 hence the benefit from crop rotation. When the soil is well sup- 

 plied with humus there is seldom any trouble from this source, and 

 the same crop may be grown year after year with good yields, 

 though continuous cultivation of the same crop may invite injury 

 from certain insects and fungous diseases which live over in the 

 soil or in the remains of the crop. 



EFFECTS OF TILLAGE. 



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 ploughed only 3 or 4 inches deep. Below the ploughed 

 stratum the soil becomes sour, densely packed, and unfit for plant 

 roots. When such soils are ploughed deeply 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 ploughing is ruinous. Some farmers have tried to remedy 

 the difficulty by subsoiling. The subsoil plough breaks up the 

 packed layer but does not throw it out on top. But while subsoil- 

 ing 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 plough 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 plough 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 too, never to plough the same depth 

 twice in succession. In general, autumn ploughing should be from 

 7 to 9 or 10 inches and spring ploughing from 5 to 7 inches deep. 

 There are special cases in which these rules do not apply, but their 

 discussion would take us too far from the purpose of this paper. 



We plough the soil in order to loosen its texture and get air 

 into it ; also to turn under stubble, manure, &c., to make humus. 

 Killing weeds is another object accomplished by ploughing. After 

 a soil has been thoroughly pulverised to great depths, so that there 

 is no danger of turning up packed clay, the deeper the ploughing 

 the better the crops. But the cost also increases with depth so 

 that ordinarily it does not pay to plough more than about 10 inches 

 deep. 



