138 Mineral Nutrition of Plants 



from increasing periods of cultivation of South Australian grassland in 

 the red-brown earth zone. The major part of the decline took place 

 during the first five years out of a total of twenty years of cultivation. 

 Similar observations have been made in this country. 



The effect of aggregate size upon yield of Synapis alba (white mus- 

 tard) grown in Mitscherlich pots filled with soil aggregates of different 

 size classes was studied by Schuylenborgh (62). The aggregates and 

 crumbs were obtained by screening a cloddy clay soil. With respect to 

 structural state and aggregate size class, Schuylenborgh found that dry 

 weight yields increased in the order: (compact structure) < (4-5 

 mm.) < (2-4 mm.) < (finer than 1 mm.) < (1-2 mm.) < (crumb 

 structure).* These results are in general agreement with those obtained 

 by Kwasnikov (see Krause, 45) who conducted a somewhat similar 

 experiment. Doyarenko (see 45) and Schuylenborgh (62), respectively, 

 regard the aggregate size classes 1-2 mm. and finer than 1 mm. as 

 highly desirable. Unfortunately, the last category fails to specify the 

 amount of aggregation within this size class. It cannot be said that 

 any one aggregate size class is best for all soils which possess the ten- 

 dency to form stable aggregates. It seems more likely for soils of dif- 

 ferent intrinsic physical and chemical properties that different states of 

 aggregation will be required for the growing plant. In this respect, 

 moreover, different plants may have different requirements. 



The crumb structure is less permanent than that associated with the 

 water-stable aggregates of size classes < 1 mm. and 1-2 mm. For 

 many soils in certain climates it may be too costly to attempt to create 

 this structural state. Superior yields on the crumb structure and finer 

 aggregates were attributed by Schuylenborgh to the greater surface 

 areas there exposed for root exploration. This may be the explana- 

 tion for the results obtained with this particular soil but, omitting the 

 crumb structure, there is no evidence to indicate that some other size 

 classes than those finer than 2 mm. might not be superior for other soils, 

 e.g., those which form soft aggregates. 



The influences of particle size upon the availability to plants of soil 



* Crumbs have weak cohesion and lack distinct dimensions and shape; they 

 possess a honeycomb structure formed by the combination of primary soil parti- 

 cles with larger aggregates (Schuylenborgh). 



