130 TILLAGE MOVEMENTS OF SOIL WATER [chap. 



of North America and parts of Australia and South 

 Africa, in order to deal with land with an average 

 rainfall of from 15 to 10 inches. There the amount of 

 water reaching the soil is insufficient to provide for the 

 needs of a crop every year. Failures are too numerous 

 to allow the farmer to make a living ; but it is found to 

 be possible to obtain a crop with some certainty if it is 

 preceded by a year of bare fallow, during which the 

 greater part of the rainfall is accumulated for the 

 succeeding year's crop. The bare fallow may be 

 necessary in alternate years or may be taken every 

 third year. For example, the regular course of cultiva- 

 tion on the wheat lands of South Australia, in the zone 

 with an annual rainfall of from 12 to 20 inches, begins 

 with a year of bare fallow, during which the ground is 

 worked over three or four times, as often as is necessary 

 to keep the surface loose. Then follows the wheat which 

 is harvested by a machine that simply strips off the 

 heads and leaves the straw standing. Sheep are then 

 turned in and graze upon the straw and upon such 

 weeds as may come up spontaneously during this the 

 third year of the rotation. With better farming, 

 especially where the rainfall is rather greater, peas 

 may be sown on the stubble to be grazed off in the 

 third year, thus enriching the soil in nitrogen as well as 

 providing increased food for the sheep. But the ground 

 is left by the wheat too dry for a substantial fallow crop, 

 which in its turn would endanger the accumulation of 

 the scanty rainfall for the ultimate wheat crop. In 

 Australia the wheat is autumn sown, and the farmer's 

 routine consists in spending the late autumn and early 

 winter in ploughing and sowing for wheat. During spring 

 and summer he is occupied in going regularly over the 

 fallow land, with appropriate intervals for sheep shearing 

 and harvest. On the Great Plains the land is frost- 



