54 THE PHYSIOLOGY OF PLANTS 



will be best to add, in the first place, all the three most impor- 

 tant food substances, and then to judge from the nature of the 

 crops which manures should in future be given in increased, 

 and which in lesser quantities. 



According to the poorness of the soil, potassium and phos- 

 phates should be used to the extent of 25 to 40 lbs. per acre. 

 The nitrogen in the form of soluble nitrates should be added 

 in half the above amount. 



If you notice a tendency in the plants to run too much to 

 leaf, and to remain green for too long a time, decrease the 

 amount of potassium and nitrogen, or increase the amount of 

 phosphoric acid. 



The chief forms of mineral manures are ammonium sulphate 

 and Chili saltpetre (sodium nitrate). The former, to be in con- 

 dition for absorption by the plant, must be transformed in the 

 soil into a nitrate, and should therefore be in the soil some 

 considerable time before the sowing takes place. The Chili 

 saltpetre, which is readily soluble, but is scarcely at all absorbed 

 by the soil, and is therefore liable to be washed into the deeper 

 layers by heavy rain, must be placed on the ground very shortly 

 before the sowing or planting of the field. As it is immediately 

 effective, it may be recommended for occasions where plants 

 which have suffered from external disturbances (cold, hail, or 

 drought) are just beginning to recover, and require some 

 stimulant to accelerate their growth. 



Potassium is commercially obtainable in form of raw potas- 

 sium sulphate, or as a purer and more concentrated salt. In 

 whatever form it is used, it must be mixed sufficiently deeply 

 in the soil, not placed on the top, and must be introduced into 

 the soil at a time when a long damp period is certain to 

 follow (autumn or winter). If the soil is not rich in lime, it 

 is advisable, when manui'ing with the raw potassium sulphate 

 (which contains a large amount of magnesium chloride) to add 

 a considerable amount of powdered quicklime. 



Which of the commercial potassium salts should be used in 

 any special case will depend partly on the distance of transport. 

 If the distance between the market and the estate is great, the 

 more concentrated forms are of course more advisable. 



Phosphoric acid, too, can be obtained in various forms. 



