8 AGRICULTURE 



Evaporation, or changing from a liquid state to a vapor 



Transpiration, or giving out, as vapor, from leaves 

 Lack supplied by irrigation 

 Excess removed by drainage 

 Soil Ventilation: 

 Uses of air : 



Supplies oxygen to plant roots 



Supplies carbon dioxide for chemical processes 

 Evils of under-ventilation 

 Evils of over- ventilation 

 Soil Temperature : 

 Warmth needed for: 



Germination of plants 



Growth of plants 



Chemical processes and work of bacteria 

 Soil warmth affected by: 



Slope of land 



Nature of soil 



Moisture 

 Soil warmed by: 



Drainage 



Tillage 



Manures 



Composition of Soil: 

 Some metallic elements: 



Iron, calcium, magnesium, potassium, sodium, aluminium 

 Some non-metallic elements : 



Silicon, sulphur, phosphorus, chlorine, carbon, oxygen, hydro- 

 gen, nitrogen 



