46 AGRICULTURE. 



I. Sand. 



Sand is "light and open" — that is, easy to 

 work. It absorbs very little moisture from the 

 air. It has little power of chemically holding 

 plant-food. Sandy soils are usually poor in 

 phosphoric acid and potash, two important plant- 

 foods. 



II. Clay. 



If a feldspar — which consists of silica, alu- 

 mina, and one or more of the alkalies, potash, 

 soda, or lime — has been disintegrated, clay will 

 result. The term "clay," however, is very 

 loosely applied to almost any kind of finely pul- 

 verized rock, or mud. 



Clay soils are hard to work; they absorb mois- 

 ture from the air readily. They contain, chem- 

 ically, much plant-food, being often rich in 

 potash and poor in lime and phosphoric acid. 



Shale is a rock consisting of very thin layers. 

 Its composition varies greatly, sometimes grad- 

 ing into limestone or finely grained sandstone. 

 Shales form mud or clay. 



III. Calcareous Soils. 



Some soils are largely composed of carbon- 

 ate of lime from the disintegration of limestone, 

 which is a soft rock and one easily dissolved. 

 Soils containing a large per cent, of limestone 

 are called calcareous soils. Lime makes clay 

 soils more easily worked and sandy soils more 



