PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



175 



lichens, blue-green algae and other cryptograms 

 (53) and by Medicago spp. and other N-fixing 

 flowering plants (11). If the contribution turns 

 out to be significant, then the interaction of tram- 

 pling and these species may also be of some im- 

 portance. It seems probable, however, that varia- 

 tion in this source of plant nutrient, along with 

 income in rainfall (69) and its subsequent redis- 

 tribution is likely to remain of minor importance 

 in relation to other effects of stock. 



Soil Nutrient Losses 



The quantities of nutrients considered so far 

 in processes of recycling and redistribution have 

 been three to four orders of magnitude smaller 

 than the bulk sources available in the soil and, 

 where relevant, the atmosphere. Even the amount 

 of nutrients contained in the standing crop of 

 vegetation is small in comparison with these bulk 

 sources. Thus, while the importance of the recy- 

 cling portion of the total plant nutrients should 

 not be underestimated, the comparative serious- 

 ness of losses from the bulk source of nutrients 

 in the soil requires some emphasis. 



Soil loss was considered in terms of nitrogen 

 and phosphorus by Charley and Cowling (15). 

 They indicate that Australian rangeland soils are 

 low in these essential elements by world standards 

 (table 1) and demonstrate from profiles examined 

 in an Atriplex vesicaria shrub-steppe a marked 

 decrease in concentration of available nitrogen 



and phosphorus with depth (table 2). They also 

 calculate that erosion of the top 10 cm. of soil 

 would result in the loss of 27 percent of the total 

 nitrogen, 21 percent of the total phosphorus and 

 38 percent of the organic matter contained in the 

 soil to a depth of 45 cm. rising to 35 percent for 

 nitrogen and 45 percent of organic matter when 

 the available portions only are considered. 



Soil Stability 



Stock, by trampling and grazing, reduce soil 

 stability and make soils more susceptible to ero- 

 sion. Soil erosion resulting from wind action is 

 considered here as a direct interaction of micro- 

 environment with stock;, erosion resulting from 

 water is considered as an interaction of the water 

 balance component with the soil stability com- 

 ponent. 



Reduction Of Protective Vegetation 



Soil erosion by wind following the reduction of 

 perennials in rangeland vegetation by grazing has 

 been noted by several authors (9, 10, 16, 29, 1$). 

 The importance of perennial species in protecting 

 the soil from wind erosion has also been recog- 

 nized, because they alone persist when the cover- 

 ing of annual species on the intervening soil sur- 

 face has disappeared (29, 4.1, Jf3, 1$). 



The role of perennial species on erosion suscep- 

 tible soils is the maintenance of the rangeland re- 



Table 1. — Comparison of Australian and overseas arid zone soils as the percentage of surface 

 soil samples according to nitrogen and, phosphorus contents 





No 





Nitrogen (percent total) 





Mean 

 nitrogen 

 (percent) 



Soils 



samples 



<0.01 



0.01-0.03 



0.03-0.05 



0.05-0.1 



0.1-0.5 



Australian 

 Overseas 



77 

 38 



4 

 



22 

 16 



23 

 29 



n 

 18 



10 



37 



0.06 

 0.11 









Total 



phosphorus 



(p.p.m.) 





Mean 



phosphorus 



(p.p.m.) 







0-100 



100-250 



250-500 



500-1,000 



>1,000 



Australian 

 Overseas 



70 

 70 



24 

 7 



39 

 6 



30 



It 



7 

 54 





 19 



240 

 710 



Source: From Charley and Cowling (15). 



