Fertility investigations on the black earth wheatlands of the Darling Downs, Queensland. I. Moisture accumulation under short fallow

Abstract

The accumulation of soil moisture during the period of summer fallow between successive winter crops of wheat has been examined for a number of localities on the Darling Downs, and for the seasons 1951-1954. The moisture gain to a depth of 4 ft between the harvest of one crop and the planting of the succeeding crop ranged from – 0.3 to 5.0 in., with an average of 2.2 in. representing 17 per cent. of the rainfall. Available moisture at planting averaged 4.8 in., the excess of 2.6 in. over fallow gain being due to the available moisture already in the soil at the preceding harvest. In this locality, therefore, moisture accumulation commonly occurs prior to harvest, owing to rainfall in excess of crop requirements at this period. The average values for available moisture at planting mere well below the capacity of these soils for storage to 4 ft. Values following long fallow averaged 6.5 in., and were as high as 9.9 in. in one instance. Correlations have been sought between fallow moisture gain and available moisture at planting, and the associated soil and climatic factors. The more important findings were : (1) Total rainfall during the fallowing period was not correlated with fallow moisture gain. (2) Fallow moisture gain and available moisture at planting were correlated with March-April rain. (3) Fallow moisture gain was correlated negatively with available moisture at the preceding harvest and positively with available moisture at planting. (4) The clay content of the soil and depth of surface mulch were not significantly related to fallow moisture gain nor to available moisture at planting. (5) No significant difference in moisture accumulation was observed between sites stubble-mulched and those stubble-burnt. Explanations in terms of climatic and soil characteristics are offered for the results obtained. Features considered important are the characteristics of the rainfall and the high-evaporation conditions, the low rate of moisture penetration in wet soil, and the high moisture-holding capacity of the surface soil layers.