SOILS AND MANURES 



.347 



297. Chemical Elements of the Soil Required by Plants. — CHAP. 

 All of the above-mentioned chemical substances of the soil VIU - 

 occur likewise in plants, with the exception of alumina (oxide 



of aluminium), and this is always present in good soils, so that 

 it may be said that all are requisite to plant growth. Some 

 are always present in such abundance that there is no danger 

 of their becoming exhausted. The supply of others, how- 

 ever, especially phosphoric acid, potash, lime, and nitrogen, 

 is frequently insufficient for the production of good crops, and 

 the deficiency has to be supplied by the farmer. Of these, 

 nitrogen is the one which is most expensive to replace. 



All of the mineral substances found in the ash of plants must 

 come from the soil ; plants cannot get them in any other way. 

 The carbon used in the structure of plant tissue is taken from 

 the air. The hydrogen and oxygen come from the water in 

 the soil. 



298. Soil Moisture. — The chemical substances in the soil 

 can only be made use of by the plant when they are dissolved 

 in water, and water is capable of dissolving from the soil all 

 the substances that it contains which enter into the food of 

 plants ; this explains why moisture is necessary to plant life. 

 Dilute solutions of these substances are drawn in through the 

 minute hairs which clothe the ends of the youngest rootlets, 

 and are carried up into the plant, where they undergo a 

 chemical change into the various compounds on which the 

 plant feeds. 



Soil has the power of absorbing and retaining water that 

 passes through it, and also of drawing up water from below ; 

 the latter is known as capillary action, and is similar to the 

 action by which the oil is "drawn " up into a lamp-wick. Good 

 soils will frequently absorb and hold one-half or more of their 

 own weight of water ; some, much more, and those containing 

 most humus will hold most water. Soils also absorb a small 

 amount of moisture from the air. Even when a soil seems 

 perfectly dry it still contains considerable moisture. Soils ex- 

 posed to the direct rays of the sun or to drying wind give up 

 much of their moisture by evaporation, and as the surface dries 

 water begins to ascend from the lower strata by capillary 

 action. When the soil is protected by a " mulch," evaporation 

 is checked. 



