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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 
