671 



the application of manures. The amount of those plant foods that 

 arc soluble in water is very small. Potash is more soluble than phos- 

 phoric acid, the amount of potash that is soluble in a soil varying 

 from 'oor to '010 per cent. Suppose the soil to be cropped con- 

 tains *ooi per cent of potash, soluble and immediately available, 

 and the total potash amounted to 'I per cent., or 3,500 Ibs. per acre 

 of 12 inch soil, then the available potash is only 35lbs. per acre for 

 the same depth. If no potash were added, and the soil cropped 

 with mangels, you could not get a full crop, as that would require 

 the soil to give up about 180 Ibs. of potash. Although the soil con- 

 tained 3,^00 Ibs. of potash, and a full crop required only 180 Ibs. per 

 acre, there would be either no crop or a very poor one, as the 35 

 Ibs. of potash would be so diluted, being spread over so much soil, 

 that the plants would not obtain enough nourishment to maintain 

 their growth. Or if a crop of potatoes were planted in that soil 

 the same thing would happen. A full crop of 6 tons would take 

 76 IDS. of potash from the soil. 



The potash in the soil is brought into an available state by 

 various agencies, such as the action of water and carbonic acid. 

 Gypsum also acts on the insoluble potash compounds and frees the 

 potash. Tillage has a great influence in liberating the constituents 

 of the soil by breaking it up and exposing new surfaces to the 

 action of water and carbonic acid, as well as to the action of the 

 oxygen of the atmosphere. There are other influences at work 

 which increase the fertility of a soil, such as the earth worms, but 

 these will be treated more fully in dealing with the biology of the 

 soil. 



Phosphoric acid, like the potash, must be in an available state 

 and in sufficient quantities for the requirements of the plants. 

 Phosphates are very insoluble in water, and plants cannot assimi- 

 late solid mineral matter. It must be dissolved in water before the 

 plant can take it up into its system. The plants have a power in 

 their roots of dissolving phosphates by the action of the acids they 

 contain acting on the phosphates and dissolving the phosphoric 

 acid. 



For this to take place the phosphates must be within the reach 

 of the roots. As the acids that are exuded from the roots are not 

 very strong in their action, for them to have full power the phos- 

 phates should be in a fine state in the soil. Inorganic salts, sul- 

 phates and nitrates, especially the nitrates, have the power, to 

 some extent, of decomposing natural phosphates, but natural humic 

 compounds, in the state they occur in arable soils or peaty earth, 

 do not decompose natural phosphates. The humoid products of 

 the decomposition of farmyard manure have scarcely any effect on 

 natural phosphates ; nor has carbonic a,cid from the decomposition 

 of organic compounds any effect in accelerating the disintegration 

 of the natural phosphates, at least not to any practical degree. 



