CHAPTER V. 

 PLANT FOOD. 



We have already seen that by far the larger part of the bulk of the plant 

 comes from the air, through the assimilation of carbon by the green leaves. 

 By burning the plant we drive this off into the air again, and with it also the 

 nitrogen, which came to the plant from the soil, but originally was derived 

 by the soil from the air. 



What we have left in our ashes shows the mineral matters that were 

 derived from the soil. Chemical analysis shows us what these were. We 

 find that the ash consists of various combinations of what are known as 

 elements. An element is matter reduced to its final form, or something in 

 which we can find nothing else of a different nature. These elements are 

 either metallic or non-metallic. The element nitrogen, for instance, is 

 a gas existing, as we have seen, in all air. Iron is a metallic element. None 

 of the elements are used by plants as pure elements. Nitrogen must be 

 gotten into the soil in combination with something else to hold it there and 

 render it soluble in the soil-water so that plants can take it up, for the ordi- 

 nary green plants cannot use the free nitrogen gas. Nor can they use a metal 

 like iron, until it is acted upon by the acids and made into an oxide or a 

 sulphate, and even then they use very little of it. It has been found by care- 

 fully conducted experiments that plants cannot grow without a supply in the 

 soil of some combination of the following elements: Nitrogen, potassium, 

 magnesium, calcium, iron, phosphorus and sulphur. As we have already 

 said, while iron is essential, it is used in very small quantities, and all soils 

 in which plants make green leaves will be found to contain an abundance of 

 iron. Magnesium and calcium (the element from which lime is formed) are 

 also generally in abundance for all the needs of them as plant food direct. 

 Of the further use of lime we will speak more fully hereafter. 



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