The improvement of shrub pastures of Kochia and Atriplex under moderate to 

 heavy but not too severe stocking has been frequently noted by pastoralists, and was 

 recorded in the investigation of the effect of grazing on Atriplex vesicaria (Osborn, 

 Wood and Paltridge, 1932). The greater vigour of grazed stands has been attributed 

 to pruning of the bushes leading to the production of young shoots. This probably 

 results in the conservation and more efficient subsequent use of soil water reserves, 

 but does not in itself account for the increased establishment of young plants. The 

 Chenopodeaceous shrubs are less palatable to sheep than much of the annual herbage, 

 and it is possible that part of the increase in number and production of shrubs may be 

 due to the control of competing annuals by grazing. In the present experiment, how- 

 ever, the weight of forage other than bluebush on all the grazed plots except the very 

 heavily stocked plot, has at least equalled that on the control plot over the period 

 1946- 1951. 



Previous investigations with the perennial grass Phalaris tuberosa (Richardson, 

 Trvunble and Shapter, 1932) showed that repeated defoliation trebled the amount of 

 nitrogen in the edible above-ground portion of the plant compared with that con- 

 tained in previously ungrazed herbage. Approximately one half of this increase was 

 secured by the herbage at the expense of the nitrogen in the root system and other 

 portions of the plant not available to livestock. The remainder of the increase was 

 assumed to be the result of increased uptake of nitrogen from the soil. 



A major factor responsible for the marked improvement of the bluebush on the 

 grazed plots relative to the ungrazed is considered to be the increase of soil fertility 

 through the grazing animal, a principle that is widely accepted on pastures in areas 

 of higher rainfall, particularly where phosphatic fertilizers are applied (Trumble and 

 Donald, 1938; Sears and Goodall, 1948), but which so far as the authors are aware 

 has not been recognized in arid regions. 



The Fertility Status of Arid Soils 



Although climatic factors are of paramount importance in determining the produc- 

 tivity of semi- arid and arid regions, it is now evident that the limiting role of soil 

 fertility must also be considered. The level of fertility is in part a reflection of pre- 

 vailing climatic conditions which limit the physical and chemical processes of soil 

 formation as- well as biological and microbiological activities. The breakdown of 

 rock minerals proceeds slowly, profile characteristics are generally not well devel- 

 oped, and soluble salts may accumulate; soil organic matter and soil nitrogen are in- 

 variably at a low level. In some cases the geological parent materials of the soils 

 are low in essential nutrients and the soils developed from them are correspondingly 

 deficient. 



In the arid pastoral areas of southern Australia the principal soil groups are arid 

 red earths, stony tableland soils and arid calcareous soils. Atriplex vesicaria and 

 Kochia planifolia are the more important shrubs associated with the two former groups, 

 while Kochia sedifolia is associated with the arid calcareous soils. The soils of 

 the North-west Pastoral District have been developed on Cretaceous shales, and 

 Jurassic sandstones of low nutrient status (Jessup, 1951), whereas in the North-east 

 Pastoral District the soils are associated with richer Proterozoic and crystalline 



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