t 52 ] 



manures applied to it. The (presence of nitrates and am- 

 monia in the soil is therefore of great importance. In fact 

 the amount of N. present in a soil mainly determines its 

 value. Besides water, carbon, and nitrogen, there are also 

 certain other constituents of plants which are essential, 

 though usually occurring in minuter proportions. Plants 

 depend entirely on soils for these minute but essential con- 

 stituents. When a plant is burnt into ashes, its carbon, water 

 and nitrogen pass away, and the ash left always contains the 

 following : Phosphoric acid, Sulphuric acid, potash, lime, 

 magnesia and iron as protoxide (FeO) and Sesquioxide 

 (Fe 2 O 3 ). Soda, Alumina, Silica and Chlorine are also nearly 

 always present, but some plants can do without these food 

 constituents. 



59. According to the chemical requirements of plants, soils 

 can be divided into: (i) Aqueous or moist soils, (2) Nitrogen- 

 ous soils ; (3) Phosphatic soils ; (4) Potassic soils ; (5) Cal- 

 careous soils; (6) Ferruginous soils; (7) Siliceous soils; 

 (8) Alkali soils (containing an abundance of CaO, MgO, NasO 

 and K 2 O) and (9) Sulphurous soils. Water is of the highest 

 value, then nitrogen, then phosphorus, then potash, then 

 lime, then sulphur, then iron and lastly silica, chlorine and 

 soda. The physical importance of Silica or Sand, as making 

 the soil freer and lighter to work and for roots to penetrate, is 

 very great, but not its chemical importance. The importance 

 of Chlorine and Soda (i.e. of common salt) for certain crops 

 such as cocoanut, mangoes, beet, radishes, potatoes, cabbages, 

 cashewriuts, date, breadfruit tree, asparagus, is undoubted, 

 but the presence of these is not essential in the soil for every 

 crop. K 2 O can replace NasO in some plants, and the pre- 

 sence of K 2 O is therefore doubly important. The absence 

 of any of the essential constituents of plants just enumerated 

 makes a soil quite sterile. But it is rare to meet with a soil 

 wanting altogether in moisture, or nitrogen, of phosphoric 

 acid, or potash, or lime, or magnesia, or iron, or sulphuric 



