Vi.] DORMANT AND ACTIVE PLANT FOOD 177 



with its composition under normal conditions, should 

 give indications of the state of the soil. Various dis- 

 turbing factors come into play ; for example, the presence 

 in the soil of large quantities of a non-essential material 

 like calcium sulphate or sodium chloride lowers the 

 proportion that potash bears to the total ash without 

 necessarily indicating any want of potash ; again, a 

 deficiency of nitrogen is more seen in a general stunting 

 of the whole development of the plant than in a com- 

 parative poverty of nitrogen in the final growth. But 

 by selecting suitable test plants, valuable indications 

 can be obtained as to the need or otherwise for specific 

 manuring. As a rule, cereals are unsuitable test 

 plants, since they are well able to satisfy their require- 

 ments for mineral nutrients from comparatively im- 

 poverished soils ; the straw of barley, however, shows 

 considerable variations from which the condition of the 

 soil as regards its supply of phosphoric acid and potash 

 can be interpreted. The phosphoric acid in the ash of 

 barley straw will vary between 2 and 4 per cent, and 

 the potash between 6 and 24 per cent., and as the 

 straw of barley grown without special manuring can 

 readily be obtained, it forms a convenient test plant. 

 The most sensitive test plants are provided by roots 

 swedes for estimating the phosphoric acid,and,mangolds 

 for estimating the potash in the soils on which they 

 have been grown. The phosphoric acid in the ash of 

 swedes has been found as low as 9 per cent, when the soil 

 was one that responded readily to phosphatic manures, 

 rising to 16 per cent, when the soil was one that required 

 no phosphatic manure. Similarly, the potash in the ash 

 of mangolds will vary between 12 and 40 per cent. 



The method which is now very largely employed 

 to determine the mineral plant food in the soil that 

 may be regarded as immediately " available " for the 



N 



