Yield Trends in The Wheat Belt of South Australia During 1896-1941

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Abstract
The history of the wheat industry in S. Australia is reviewed to provide a background for discussion of the trends observed in yields. The period chosen for examination was 1896-1941, and the analysis extends to practically the entire wheat belt. The basic territorial unit used for assessing yield was the hundred, the mean area of which in S. Australia is approx. 118 sq. miles. As a preliminary to the evaluation of the trends it was necessary to estimate and to correct for the effects of variations in seasonal rainfall. The statistical technique used was that of partial regression, and reasons are given for the choice of rainfall variates. The major soil groups under cultivation are described and mapped. The elimination of P as a limiting factor in yield coincided with the beginning of the period under review, so that in classifying the forms of trends observed it was convenient to divide the hundreds into 2 groups, according to whether they were opened for cultivation before or after the advent of superphosphate. The N status of the major wheat soils is discussed, and after consideration of relevant literature, it is concluded that the N required by the crop has been drawn almost entirely from soil reserves under the exploitative systems of cropping generally employed. The wheat belt is broadly divisible into 3 parts (1) Sandy, stony, and mixed mallee soils and related types in which N becomes limiting after 20-40 yrs. of cropping, and yields subsequently decline owing to exhauston of the reserves; (2) loamy mallee soils and red-brown earths, where yields increased over the period 1896-1941, but at diminishing rates as N becomes limiting; and (3) sandy and loamy mallee and transitional mallee-solonetz soils, where yield increases linearly throughout, mainly because exploitative cropping has not been in progress long enough to make its influence apparent. These regions constitute only a small proportion of the total area. The economic restoration and maintenance of the N status of the wheat soils are discussed briefly.

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