426 THE FERTILITY OF SOILS IN THE TROPICS 



plantations fifteen years old give as good a return as in 

 their second and third year; a porous sandy soil is pre- 

 ferred for banana culture. 



Isolated cases of abandoned, old banana lands being 

 replanted and yielding good crops, even without the aid 

 of manuring, have been reported in Queensland. But I 

 have no doubt whatever that the crops would have been 

 much better, and certainly much more lasting, if ferti- 

 lizers had been added, as nobody can dispute the cor- 

 rectness of Semler's statement [n] that "no other 

 cultivated plant exhausts the soil to such an extent as 

 bananas." 



This statement is born out (i) by comparing the 

 analyses of the soils from virgin land and exhausted 

 banana land, in which the total amount of potash present 

 is reduced by nearly one-half, and the amount of 

 *' available " potash remaining is less than one-tenth of 

 that found originally (see soil analyses, Table III), and 

 (2) by a study of the actual food requirements of the 

 banana plants, based on the analyses of plants and fruits 

 of the three principal varieties grown in Queensland. 

 carried out in the Departmental Chemical Laboratory 

 (see Table I). 



From this investigation we learn that a crop of 

 Cavendish bananas removes in a fair crop of fruit : 

 123 Ib. of potash, 12*5 Ib. of phosphoric acid, and 437 Ib. 

 of nitrogen; whereas the stalks left on the ground con- 

 tain 150 Ib. of potash, 6 Ib. of phosphoric acid, and 

 41*4 Ib. of nitrogen. Practically speaking, therefore, 

 273 Ib. of potash, or about 5 cwts. of sulphate of potash, 

 per acre, must be at the disposal of the banana plant in 

 a readily available form, to produce its growth and crop 

 in a few months. 



At the time of planning the fertilizer experiments on 

 exhausted lands, these figures were not available, and the 

 manurial basis was fixed more or less arbitrarily, at 

 K = 80 Ib. of potash, P =80 Ib. of phosphoric acid, and 

 N = 40 Ib. of nitrogen per acre. 



The results of the growth during the first year were 

 so striking that the amount of potash was doubled in all 

 further applications, thus making our standard fertilizer 



