51 



the permeability of the soil by flocculating the fine clay 

 particles, but alkaline salts like sodium carbonate increase 

 the tenacity and impermeability. Too porous sandy soils 

 can scarcely be kept sufficiently moist for the growth of 

 most crops, and excessive watering of such soils is apt to 

 cause a great loss of plant food through drainage. In 

 impervious soils, the circulation of water is bad and the 

 ventilation is insufficient, and generally plants grow very 

 badly. Thus the Facus soil is quite incapable of produc- 

 ing a crop. With such a soil draining can have little 

 effect and the only method of satisfactorily improving the 

 physical properties would be by mixing with it a large 

 volume of sand, an expensive, but fairly permanent im- 

 provement. The greater part of the irrigated alluvial 

 soil of Egypt suffers from excessive tenacity, shrinkage 

 and impermeability, and poor crops are due far more to 

 those defects than to any deficiency of plant food. 



The movements of water in the soil are dependent on 

 the general soil properties which have already been consi- 

 dered. The plant roots in the soil are constantly absorbing 

 water from the soil in their immediate surroundings, and 

 this part of the soil is drawing on the water further from 

 the plant. Hence, practically at all times, there is a slow 

 but constant movement of the soil water towards the plant 

 root. This will be more clearly shown a little later. In 

 the soil there are other and greater movements of water, 

 movements which are active equally in cultivated and 

 uncultivated soil. For convenience, these may be classed 

 as upward, downward and horizontal and are referred 

 to under the terms capillarity, drainage and infiltration 



