30 



TROPICAL AGRICULTURE 



CHAP. 



Power of 

 soils to hold 

 water. 



Chemistry 

 of soils. 



Chemical 

 elements. 



the paper for some distance. The same things occurs when 

 the end of a wick of a lamp is dipped into oil. This action 

 is known as capillary attraction, and all soils are capable of 

 exerting such an action, which is one of the greatest im- 

 portance from an agricultural point of view. Sand possesses 

 this power to the least extent and clay to the greatest, and 

 this accounts for the well-known fact that a clay soil is 

 always wetter than a sandy one placed under the same 

 influences. In order to find out the power of any soil to 

 hold water, it is only necessary to place a known quantity of 

 the earth, well pounded and dried, into a glass vessel and to 

 pour water upon it until it is more than covered. It is to be 

 allowed to soak for a day and then the water above the 

 earth is to be carefully poured off. The weight of the 

 remainder compared with the weight of the dry earth will 

 give an idea of the capability of the soil for holding moisture. 

 Another ready way is to take a flower pot, to fill it nearly to 

 the top with dry soil, and then to weigh it. (The weight of 

 the pot, both wet and dry, must, of course, be known.) 

 Then gradually pour water on the soil until it begins to drop 

 from the bottom. As soon as the dropping has ceased 

 weigh the whole, and in this manner the proportion of water 

 held by the soil is approximately arrived at. 



CHEMICAL COMPOSITION OF SOILS. It has been shown 

 that the greater part of all soils has been formed by the 

 wasting or decomposition of hard rocks, and naturally the 

 same substances that occur in the rocks, will be found in 

 the soils also. Chemists have found out that every thing in 

 the world is composed of elementary substances by them- 

 selves, or so combined together as to make up compound or 

 complex bodies. These elementary or simple substances are 

 termed elements, and they are of two kinds, namely, metals 

 and non-metals. There are a good many elements, but it is 

 unnecessary to enumerate them all here, as some of them 

 whilst useful in the arts and manufactures are not at all 



