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" liming" heavy soils depends, as the admixture of lime 

 causing the ultimate particles of the clay to become col- 

 lected together, the ground becomes more coarsely grained 

 and therefore more porous. It is then easier to work, being 

 lighter and more open, and is also warmer. This physical 

 change which is effected by lime is not the only benefit 

 derived from its use. Near our large towns the soil is 

 usually rendered acid by the fumes produced by the com- 

 bustion of coal. In foggy weather this acidity of the 

 atmosphere becomes very noticeable. It has been shown 

 by experiment that such acid soil, and even ordinary soil, 

 when watered with acid rain-water collected in towns, is 

 very detrimental to plant growth. This acidity can be 

 removed by adding lime to the soil, a practice therefore of 

 the utmost importance to those who cultivate gardens or 

 plots near our industrial towns. 



Heavy soils can also be lightened by the admixture 

 of sand to the soil. Good natural loams, which are easily 

 worked soils, consist of fine sand with some clay and a 

 little lime. 



' Lastly, farmyard manure, quite apart from its im- 

 portant chemical effect upon the soil in enriching it with 

 many valuable food substances, is of great value in 

 improving the texture of all soils. To sands it gives great 

 water-retaining power, while it renders clay soils more 

 porous and friable. 



Let us now examine some of the chemical needs of 

 the roots of plants. By a chemical analysis of the ashes 

 left after burning plants it has been ascertained that they 

 are mainly built up of four chemical elements, namely, 

 hydrogen, oxygen, carbon, and nitrogen, to which must be 

 added smaller quantities of lime (calcium), magnesium, 

 potash, sulphur, phosphorus, and iron. Of the four 

 essential substances, the first two, hydrogen and oxygen, 

 combined in the form of water are present in all soils in 

 that form. Carbon, though present in the soil largely in 

 the form of lime or calcium carbonate, is mainly obtained' 

 by the plant through its leaves from the atmosphere, 

 where it occurs as carbonic acid. Nitrogen, on the other 

 hand, of which there is a large supply in the air, cannot 

 be made use of by the plant in this free form, but is 

 mainly absorbed in the combined form as nitrates by the 

 roots. In comparison to the other substances dissolved 

 in the water, the nitrates are the most important salts in 



