manuring, but it has been demonstrated that the vast 

 number of bacteria existing in the superficial layers of 

 earth are perpetually acting on the atmospheric nitrogen 

 and converting it to the purposes of the soil. 



Sulphur is present as sulphates, like gypsum, which is 

 sulphate of lime ; and to a varying extent this element ap- 

 pears in animal manure. 



Phosphorus occurs in bone-earth, which is phosphate of 

 lime, and in guano, In the majority of our Cape soils it 

 forms a mere trace, and its practical absence is one of the 

 severest drawbacks we have to contend against both in 

 cultures and the raising of stock. 



Potassium is proportionately abundant in such Cape soils 

 as are derived from the weathering down of granite ; iu 

 others the amount is much less. It occurs as potassium 

 nitrate and chloride in many manures, and may con- 

 veniently be applied artificially. 



Calcium, the base of lime, acts in plant tissues chiefly 

 as a carrier of sulphur-elements. When this work has 

 been effected, it is comparatively useless to the plant, and 

 is stowed away as a refuse material in the form of minute 

 crystals of oxalate of lime in the older cells. We shall 

 presently see that other important uses of lime relate to 

 the mechanical constitution of the soil, upon which it has 

 a signal effect. 



Magnesium and iron both assist in stimulating the func- 

 tion of the chlorophyll or green matter of the leaf -eel Is. 

 Plants grown without those elements become pale, sickly 

 and yellowish. The addition of a trace of iron and mag- 

 nesium salts immediately fetches up the normal healthy 

 green appearance. 



These statements are not theories or fancy views. They 

 are drawn from rigorously conducted experiments iu what 

 are termed water-cultures, and it is impossible to traverse 

 the conclusions to which they lead. 



Respiration of Active Roots dependent on the 

 Mechanical Condition of the Soil. 



12. It cannot be too strongly impressed upon the culti- 

 vator that the roots of trees are not passive occupiers of the 



