38 



chemical fertilizers have to be used to make up the 

 deficiency. As Australian soils generally appear to 

 be somewhat weak in phosphoric acid, phosphates 

 are usually the first kind of manures used to main- 

 tain the crop returns from the land. Commercial 

 varieties of phosphates are of two kinds, the organic 

 classes, such as bone dust and bone superphosphate, 

 and the inorganic forms, made from various kinds of 

 phosphatic rock. Of these bone dust is the slowest 

 in its action, lasting about two seasons in the soil 

 before being entirely dissolved. By not being so 

 water soluble as other kinds of phosphates, the 

 manurial properties of bone dust are not so readily 

 leached out of the soil by heavy rains or irrigation, 

 and, therefore, although slower, last longer in the 

 soil for the roots of plants to feed on. The mineral 

 superphosphates are usually water soluble to a great 

 extent, hence their action on plant growth is rapid, 

 and for this reason their effect is not lasting, there- 

 fore must be applied annually to keep up growth 

 and crop returns. 



Potash as a fertilizer is also needed on most soils 

 that are being continuously cropped. As has been 

 previously mentioned, potash is usually contained 

 in considerable quantities on many classes of clay 

 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. 

 Accoring to Hume, in his work on Citrus Fruits, 

 800 Ibs. of oranges will remove from the soil J Ib. 

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

 A crop of 400 Australian cases, weighing about 



