82 Fruitgrowing under Irrigation 



lands, but it often needs applications of gypsum to 

 make it available for plant growth. 



Lime may also be applied on many soils with great 

 benefit, especially on heavy clay lands which are 

 rendered looser and generally more productive by 

 the action of the lime in making more available 

 many of the plant foods contained in the soil. 



DEPLETION OF SOIL BY CROPPING. 



The depletion of a soil by cropping depends, of 

 course, upon the nature and extent of the crop. 

 According to Hume, in his work on "Citrus Fruits," 

 800 Ibs. 'of oranges will remove from the soil \ Ib. 

 phosphoric acid, 2 Ibs. potash, and 1 Ib. nitrogen. 

 A crop of 400 Australian cases, weighing about 

 20,000 Ibs., would at this rate deplete the soil to the 

 extent of 12^ Ibs. phosphoric acid, 50 Ibs. potash, 

 and 25 Ibs. nitrogen. In addition to this, allowance 

 must be made for a quantity of plant food necessary 

 to make good leaf, branch and root growth, and for 

 the amount of these ingredients leached out of the 

 soil by rain or irrigation. Assuming the total 

 amount of plant food removed from the soil by 

 these agencies to be three times that taken out by 

 the fruit alone (Hume's estimate) then the quantity 

 of these substances lost to the land -would be : 

 37J Ibs. phosphoric acid, 150 Ibs. potash, and 75 Ibs. 

 nitrogen. Allowing for an 18 per centage of phos- 

 phoric acid, a 50 percentage of potash, and a 20 per- 

 centage of nitrogen, this is equivalent to 2 cwt. of 

 superphosphate, almost 3 cwt. of sulphate of potash, 

 and 4 cwt. of sulphate of ammonia, or to a total of 

 about 9 cwt. of chemical fertilizers. 



Supposing such a crop of 400 cases of fruit were 

 removed from one acre of ground, 9 cwt. of chemical 

 manures would therefore be required to make good 



