acre inch of water transpired; by comparison, with material grown on north-west arid 

 red earth, a transpiration ratio of 570 indicated a production of only 3.5 cwt. dry 

 forage per acre inch of water transpired. 



The fertility cycle under arid shrub vegetation 



Comparatively little is known of the fertility cycle in arid soils, and any dis- 

 cussion is necessarily of a speculative nature. The soils of the North-west Pas- 

 toral District have low organic carbon and nitrogen contents of the order of .25 and 

 .025 per cent respectively (Crocker and Skews, 1941; Jessup, 1952). The processes 

 of breakdown and decay of plant remains are extremely slow, and despite a carbon 

 nitrogen ratio of 10 : 1, it appears that the mineralization of the soil organic matter 

 and particularly the production of nitrate is inhibited, partly by climatic conditions, 

 but perhaps also by unfavourable soil conditions including possibly the presence of 

 the root systems of perennial plants (Theron, 1951). 



It appears legitimate, therefore, to regard the fertility cycle under natural con- 

 ditions as an almost closed system with the nutrients circulating at an extremely 

 slow rate, with a large proportion of the 'available' supply locked up at any given 

 time in living or dead plant material and partly decomposed remains. There is, 

 nevertheless, a small annual turnover of nutrients permitting limited production when 

 soil moisture conditions allow plant growth. Nutrients may accumulate in the soil 

 during a cycle of dry years when virtually fallow conditions prevail; then with the 

 occurrence of favourable rains a relatively lush growth occurs which gives a mis- 

 leading impression of fertility. But should another favourable season follow the 

 first the soils are incapable of sustaining production and very little growth is made. 

 This occurred at the Yudnapinna Station in 1947 following the previous good season 

 in 1946. 



The impaqt of the grazing animal on the fertility cycle of arid shrub pastures 



The introduction of grazing animals to shrub pastures leads to a marked accel- 

 eration in the fertility cycle by increasing both the amount of plant material returned 

 to the soil and its rate of decomposition. The rate of turnover of nitrogen in particu- 

 lar is accelerated because some 75 per cent of the amount ingested by merino sheep 

 is returned to the soil in urine, and rapidly becomes available to plants. A further 

 20 per cent, returned in the dung, undergoes more rapid decomposition than non- 

 ingested plant material. Provided conditions of rainfall are favourable, plant growth 

 is stimulated by the fertilizing effect of the grazing animal; and the cycle is repea- 

 ted at a rate depending on the stock concentration per unit area, on favourable tem- 

 peratures, and on availability of soil moisture. 



It is estimated that the large- framed merino wethers employed in the grazing 

 management investigations under review (1501b. live weight) void annually 25- 30 

 pounds of nitrogen. On the most heavily grazed plot during 1946- 51 the amount of 

 nitrogen returned to the soil by the animal was of the order of 6- 71b. per acre per 

 annum. This represents a considerable increase of the soil nitrogen available, in 

 terms of the relatively low production level on a per acre basis. On the more heavily 

 grazed portions of the plot, the amount of nitrogen returned would have been corres- 



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