36 EGYPTIAN AGRICULTURE. 



When soluble manures are added to sandy soils there is 

 little fixation, and the crops respond quickly to the increased 

 supply of available plant food. Again, on sandy soils the 

 drainage water will carry away relatively large quantities 

 of plant food. The power of fixation of any soil is 

 limited, and after a certain quantity of material is fixed 

 the addition of more soluble plant food results in its loss 

 in the drainage water, in its going to the plant in excess, 

 or in its setting up soil saltness. The important results 

 of the fixation of plant food by the soil are: (1) the plant 

 food, as phosphates, potash and ammonia salts, in the 

 soluble manures applied to the soil, and those foods 

 weathered in the soil, are fixed in the soil and so prevented 

 from being lost in the drainage water; (2) the plant obtains 

 a more regular supply of food during the whole period of 

 its growth, and not simply an over abundance when 

 soluble manure is applied. 



In connection with this conversion of soluble into 

 insoluble plant food, it might appear as if there were 

 no advantage in applying soluble phosphoric acid and 

 potash to the soil as they are rendered insoluble very 

 soon after being dissolved in the soil water. It must be 

 remembered however, that those bodies are precipitated, 

 or fixed, on the surface of the soil particles, and therefore 

 the insoluble compounds thus produced are widely distri- 

 buted and finely divided, and are directly exposed to the 

 redissolving action of the soil water and, more especially, 

 to the solvent action of the juices excreted by the plant 

 rootlets. A piece of phosphate of lime lying in the soil, 

 is of much less use to the plant than the same food 



