68o 



but unfortunately they may not be in a condition for the 

 plants to absorb them. More especially does this apply to 

 the phosphoric acid and potash. Of the constituents of plant 

 food, nitrogen, phosphoric acid, and potash are the most useful, and 

 are generally in smaller quantities in the soil than any of the others 

 enumerated. Plants require more of these three substances than 

 any of the others, therefore the soil is more easily depleted of them, 

 and we shall in consequence have to deal mostly with them. These 

 three substances must not only be in the soil, but they must be 

 made easily soluble, so that the plant can absorb them as they are 

 required. The soil may contain more than twenty times the 

 amount of potash required by any crop, and yet be barren to that 

 crop, simply because the plant and the other agencies that assist 

 the plant had not the power to cause the disintegration of the 

 insoluble compound, that the potash was in combination with, 

 quickly enough for the requirements of the plant. No crop is likely 

 to take more potash out of the soil than 150 rbs. per acre, and very 

 few as much as that. A soil may contain as much as 'i per cent, of 

 potash, that is equal to 3,500 rbs. of potash per acre in a soil 12 

 inches deep. Yet, with all that amount in the soil, the crop sickens 

 and refuses to grow, or is of a stunted growth, simply because the 

 potash was not then available for its requirements. The same 

 remarks apply to nitrogen and phosphoric acid. If either of these 

 were not present, or in a state not immediately available to the 

 crop, that soil would be in a state from barren to that of 

 being able to grow only small crops. While it is necessary to 

 maintain a proper supply of available food for the plant, care must 

 be taken that neither of these substances are supplied in too great 

 an excess of the requirements of the plant, as they may act 

 deleteriously to the growth of the plant, or even act as a poison. 

 This is brought about by over manuring and using a rotation of 

 crops that takes up chierly the same foods. Such a system is not 

 only expensive and a waste of valuable manure, but very often gives 

 short crops. Different plants take up different proportions of nitro- 

 gen, phosphoric acid and potash, according to their likings and 

 requirements. Whatever they require most must either be in the 

 soil or supplied to them in proportions according to their needs, by 

 manuring. 



Some plants have a greater capacity for the assimilation of 

 certain constituents of their food than others, although the one with 

 the gr-ater capacity for assimilation may not require so much of 

 that particular kin I of food as the other. For inslance, mangolds 

 have a less capacity for assimilating nitrogen than turnips, but they 

 must be given a rich nitrogenous manure, as they need and take 

 more nitrogen than turnips, which requires a phosphatic manure. 

 Again, wheat sown in the winter lias a longer time to gather 

 nitrogen than barley s-uvn in the spring, which, being a shorter time 

 in the soil, requires a more active nitrogenous manure. 



