pondingly greater. It is also possible that by the stimulation of greater microbio- 

 logical activity in the manured soil, the rate of decomposition of the soil organic 

 matter was hastened, leading to the release of further nutrient supplies. 



The grazing animal can add little to the total store of fertility in arid soils in 

 the absence of important leguminous constituents of the pastures, but by increasing 

 circulation of nutrients, it may add greatly to the productivity of shrub pastures 

 during favourable seasons. This may also stimulate the regeneration of young shrubs 

 from seed. The drain on soil fertility by the removal of nutrients in animal products 

 does not represent a serious loss when spread over many acres, and insofar as nitro- 

 gen is concerned, would not amount to more than ^^Ib. per acre per annum in the 

 above example. 



The improvement of soil fertility by the fertilizing effect of sheep accounts 

 satisfactorily for the observed effects on the shrub pastures in the grazing manage- 

 ment investigation. During seasons of low rainfall the pasture did not benefit from 

 the return of nutrients ; and the weight of forage per acre was depleted on the heavily 

 and very heavily grazed plots. During this period nutrients accumulated in the soil 

 in a fairly readily available form in the plots grazed at moderate to very heavy rates 

 of stocking. Under the favourable climatic conditions of 1946, the established shrubs 

 responded to the accumulated sheep manure. Young seedlings developed vigorously 

 and were able to become firmly established, leading to a large increase in bush den- 

 sity. Under the subsequent generally favourable conditions of 1947- 51, the circu- 

 lation of nutrients was maintained and the production of herbage continued at much 

 higher levels than on the ungrazed plot. 



Hence the productivity of shrub pastures may be regarded as a function of graz- 

 ing intensity and effective seasonal rainfall; and high intensities of stocking can 

 only be maintained during favourable seasons, since the fertility cycle pasture — 

 animal — soil — pasture can only operate when there is sufficient soil moisture avail- 

 able for sustained plant growth. It is probable that repeated heavy defoliation of 

 shrubs during a run of drought seasons would lead to widespread mortality of the 

 bushes. This concept of grazing intensively the arid shrub pastures when seasonal 

 conditions favour their active growth makes possible substantial improvement in 

 practical management. 



There is of course a limit to the number of sheep which may be carried on shrub 

 pastures during seasons of high effective rainfall. An equilibrium must be maintained 

 between growth and consumption of forage. The small size of the grazing areas em- 

 ployed in these investigations has permitted the support of sheep at rates greatly in 

 excess of the recognised carrying capacity of shrub pastures in the North-west Dis- 

 trict where the average stocking rate is 25 sheep per square mile (Jessup, 1951). The 

 mean size of paddocks in this area is, however, 32 square miles with an average of 

 only one watering point per paddock (Jessup, ioc. cit.). Under these circumstances, 

 the upper limit of carrying capacity beyond which permanent degeneration of the 

 shrub stand may occur is fixed by the stock concentration within a Hrnile radius of 

 the water, supply. 



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