HOW PLANTS GET FOOD FROM SOIL AND AIR 33 



by the leaves largely from one form of carbon, which occurs 

 as a gas in the air. However, there will be no abundant 

 growth of leaves to make this starch, sugar, or oil unless 

 the roots provide the small but necessary amounts of cer- 

 tain other substances. 



Food taken from the soil. There are at least ten 

 elements that plants draw from the soil alone. All but 

 four of these are nearly always present in the soil in 

 quantities sufficient to supply our crops for hundreds of 

 years. The only plant-foods that are taken from the soil 

 and that we need talk about in this book are those four 

 that are sometimes so scarce that the farmer may have 

 to add them to the soil in order to get a good crop. 



Precious forms of plant-food. The four plant-foods or 

 elements that are often not sufficiently abundant in the 

 soil are nl' tro gen, phos'pho rus, po taVsi um, and caT- 

 51 urn. We call these the precious elements, for they are 

 more important to the plant and to the farmer than the 

 precious metals, silver and gold. Besides, the farmer often 

 has to buy them, paying silver and gold for them. 



One cause of poor crops. These four elements exist in 

 the soil in combinations ; we speak of the potassium com- 

 binations as potash, and of the calcium combinations as 

 lime. In some soils only one of these four may be in- 

 sufficient; in another soil there may be a lack of two of 

 them ; in a third three of them may be wanting ; and in 

 yet another soil all four of them may need to be supplied. 

 If a single one is lacking or insufficient in quantity, there 

 will be a failure of the crop, no matter how abundant the 

 other three may be. So it happens that a farmer may buy 



