74 Fruitgrowing under Irrigation 



SOIL FERTILITY. 



Soils having in the first place been formed out of 

 various classes of rock, it is evident that all mineral 

 salts necessary to sustain plant life must have been 

 existing in the original rock, but were not available 

 for plant food until the weathering influences of the 

 sun, air, frost, and rain had reduced them to a form 

 in which they could be absorbed by the roots of the 

 plants. 



A soil may be ever so rich in potash, phosphates, 

 and other salts that are necessary to build up plant 

 tissues, yet unless a proportion of these salts exists 

 in a form that is soluble to the roots of the plants, 

 vegetation 'will not thrive, and the soil will appear 

 as unfertile. However, the action of sun, air, and 

 water on the sail, being always in operation, has the 

 effect of gradually changing most mineral salts into 

 forms that are more soluble, and thereby more 

 readily absorbed by plant rqots. Therefore, left to 

 purely natural agency, an unfertile soil may, in the 

 course of time, be changed into a fertile one, and by 

 the operation of familiar causes, especially by very 

 heavy rains, the soluble salts may be mostly washed 

 out of an erstwhile fertile soil and so rendered 

 barren. 



Soil experts are now generally agreed that the 

 fertility of a soil does not depend upon the total 

 amount of phosphates, potash, nitrogen, or lime it 

 contains, but upon the amount of these ingredients 

 that are existing in a form that the plants can 

 assimilate. This theory would explain the reason 

 why some of our light Australian soils, such as the 

 Pinnaroo lands, which, when chemically analyzed 

 by chemists were pronounced too poor for successful 

 wheat culture, were nevertheless on trial found to 

 produce good crops of wheat, as the small proper- 



