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agriculture it is usual to practice a rotation of crops 

 whereby plants with different requirements as regards 

 mineral salts succeed each other, and with the periodic 

 introduction of a leguminous crop prevent the exhaustion 

 of the soil. Though this is not so necessary in horticul- 

 tural practice where the soil is generally richer, yet it is 

 equally useful here to vary the crops in different portions 

 of the plot, firstly because as shown above peas and beans 

 enrich the soil in nitrogen, and secondly because such 

 alternation often puts a stop to certain plant diseases 

 which can only propagate themselves when the same plant 

 is grown year after year on the same plot. This is the 

 case with such diseases as " club root," of turnips and 

 cabbages, and the " wart disease " of potatoes. 



Another feature of some importance must be touched 

 upon. Probably every gardener has noticed that the 

 deeper soil of his garden differs very considerably from 

 the surface soil. On heavy land the subsoil, as it is 

 called, consists partly of solid clay, and is not so porous 

 and crumbly as the surface soil. It is also of less nutritive 

 value for it contains little or no decaying organic matter. 

 Both physically and chemically, therefore, it is less suited 

 to the nutrition of plants than the surface soil. It is 

 important, therefore, when cultivating deep-rooted plants 

 to improve the subsoil. This is done by trenching, a 

 process which may be regarded as the cultivation of the 

 subsoil. The surface soil is lifted off by removing one 

 spit of soil, and then the subsoil may be improved both 

 chemically and physically by digging in farmyard 

 manure, or other forms of decaying organic matter. In 

 the case of clayey subsoil, the addition of lime should 

 not be forgotten. If the clay of the subsoil is too solid 

 it may be necessary to replace it by loam, or it may be 

 improved by burning it, whereby it is rendered more 

 friable. By thus improving the subsoil, a much better 

 nutrition of deep-rooted crops will be secured. In the case 

 of lighter soils, where there is less difference, at any rate 

 physically, between the surface soil and the subsoil, it is 

 a common and beneficial practice when trenching to put 

 the surface soil at the bottom of the trench and cover it 

 with the subsoil, to which manure has been added. 



' Another useful process for the amelioration of soil is 

 that of ridging, this is particularly beneficial in the case 

 of heavy and tenacious soils. By digging up the soil and 



