172 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



0.8 r 



0.7 



— 0J6 



" 05 

 E 



> 



§E 03 



0.4 



1 

 1 



\ 









\ 



\ 

 \ 









\ 









\ 









\ 









Mean 



▲ 



~~A ~ ' 



A 

 A 



■--B 



. scald sorptivity 











0.2 



0.1 ■ 



TRAMPLING INDEX 



Figuee 5. — Variation of sorptivity with trampling inten- 

 sity. Trampling intensity was rated according to the 

 trampling index defined in the caption of figure 4. The 

 average initial moisture content of the soil was 4.6 

 percent by weight. The mean of four determinations 

 of sorptivity for scald soils of similar moisture content 

 is given for comparison. Scalds are near level, exten- 

 sive, bare surfaces resulting from soil erosion <see 

 Discussion and Conclusions, p. 180). They have char- 

 acteristically low sorptivity values. 



top 30 cm. of soil more effectively than the end- 

 of -season growth of the annual, C otocephalus 

 sonderi F.v.M., and the perennial grass, Dan- 

 thonia caespitosa, which were present in both 

 plots. 



A similar example of this effect of grazing 

 was shown by Jones (33). However, his results 

 show in addition that moisture from the soil 

 under the degraded community eventually evap- 

 orates, resulting in similar amounts of soil mois- 

 ture in both profiles. The implication is that the 

 soil water has been more efficiently used in the 

 presence of the saltbush where more of it has been 

 exchanged for dry matter production. 



The main effects of stock on water balance are 

 the reduction in water available for plant growth, 

 largely by effects on infiltration rate, I, and hence 

 runoff, R, but also by a reduction in the exploit- 

 able soil water by the vegetation, M. 



The implications of the effects of stock for in 



situ evaporation (/Edt in equation 1) and for 

 erosion are considered in the sections on radia- 

 tion microclimate and soil stability, respectively. 



Nutrient Balance 



Effects of stock on nutrient balance have been 

 studied less than those on water balance. Not only 

 are such studies inherently more complex but 

 there has prevailed the feeling that in arid en- 

 vironments water is likely to be the determinant 

 factor. However, several reports (4-, 14-> 15 1 %%•> 

 30, 33, 50, 51) have drawn attention to the ex- 

 istence of considerable variation in anion and 

 cation concentrations horizontally and vertically 

 in Australian rangeland soils. This variation may 

 be important in the interpretation of some of 

 the observed plant responses because of the low 

 inherent fertility of the soil (27, 70) (see table 

 1). 



Rate of Mineralization 



Studies on sown pastures in more humid cli- 

 mates indicate that some plant nutrients eaten by 

 sheep are more rapidly converted to mineral 

 form than if the plant material had been allowed 

 to decay in the soil (7, 8). Trumble and Wood- 

 roffe (65) recorded an increase in density and 

 vigor of grazed pearl bluebush (Kochia sedifolia) 

 compared with ungrazed controls during a period 

 following a year of exceptional rainfall of about 

 one in 16 years recurrence. They concluded that 

 the increase was due mainly to: "an increased 

 circulation of nutrients from the vegetation to 

 the soil via the grazing animal and thence back 

 to the plant," with urine returning approximately 

 75 percent of the nitrogen ingested and feces, 

 20 percent. Other factors may also have contrib- 

 uted. These include the pruning effect as sug- 

 gested for the zone of increased vigor of Atriplex 

 vesicaria which occurs at varying distances from 

 watering points depending on grazing intensity 

 (JfS). This effect is considered to result in the 

 more effective use of soil moisture by the shrubs. 

 There is also a possible reduction in competi- 

 tion by ephemeral vegetation which, when pres- 

 ent, is preferentially grazed by sheep. 



A dependency on rainfall of the response to 

 grazing found by Trumble and Woodroffe (65) is 

 indicated in their results by the absence of a 



