L>S EGYl'TIAX AGRICULTURE. 



THE CHEMICAL PROPERTIES or THE SOIL. 



The composition of the soil chiefly depends on the 

 minerals of which it is composed and, ultimately, on the 

 rock from which those minerals have been derived. From 

 the rocks of Egypt only a poor, sandy, calcareous soil 

 could be derived, but from the granite and sandstones to 

 the south, the rich Nile mud is obtained. The plants 

 absorb from the soil all matters which exist dissolved in 

 the soil water ; some of these substances are plant foods, 

 and others, although entering the plant, are of no use. 

 If dissolved in the soil water, poisons may enter and 

 destroy the plant's life. From the atmosphere the plant 

 absorbs its gaseous foods, oxygen and carbon dioxide; all 

 other food material is taken from the soil. The mineral 

 food for the plant can only be taken up by the plant root 

 when it is dissolved in the soil water. Apart from water, 

 the substances obtained from the soil which are absolutely 

 essential to plant life, are compounds of nitrogen, phos- 

 phorus, sulphur, potassium, calcium, magnesium and 

 iron. The plant foods are probably absorbed in the 

 form of nitrates, phosphates, sulphates, etc., and it is 

 convenient to speak of them by those names. Of the 

 total plant food in any soil necessarily only a very small 

 part is soluble and available for the plant. Another part, 

 the surface of the soil particles, is in process of weathering 

 and is slowly but constantly becoming available, while the 

 bulk of the total food, in the inside of the soil particles, 

 is still inert, un weathered and unavailable. All the plant 

 food may at last become available, but at any particular 



