FARM LANDS OK NEW SOUTH WALES. 51 



Analysis of Silt from Hawkesbury River Water. 



Per cent. 



Insoluble in hydrochloric acid ... ... ... ... ... = 89 '96 



Soluble in hydrochloric acid — 



Oxide of iron and alumina (FejOg and A^Og) ••• ••• — 4 - 77 



Lime (CaO) = 049 



Potash (K 2 0) = 0-12 



Phosphoric acid (P 2 6 ) = 0-08 



Volatile matter ... ... ... ... ... ... ... = 4 68 



Nitrogen ... ... ... .. ... ... ... ... = 0105 



Weight per acre, one foot in depth = 3,307,332 lb. 



Assuming an average based on local observations of 2 inches of deposit 

 after the subsidence of a flood, the silt left by the Hunter River in flood 

 would amount to 567,186 lb. in weight per acre, and would supply the land 

 with a top-dressing of fertilising constituents to the following amount per 

 acre : — 



Lime 8,7911b. 



Potash 510 ,, 



Phosphoric acid ... ... ... ... ... ... ... 1,020 ,, 



Nitrogen ... ... ... ... ... ... ... ... 476 ,, 



— a manuring which contains sufficient of the necessary plant-food to supply 

 .the requirements of most crops for nearly ten years. 



On the assumption that the same volume of water was flowing for the 

 same length of time at the same rate in both cases, the amount of deposit 

 left by the flood will be proportional to the amount of suspended matter. 

 Taking 2 inches to be the depth of deposit left by the Hunter River, then 

 the Hawkesbury River would deposit under similar conditions as to time, 

 volume and rate of flow r \y-inch silt. The weight of this deposit in the case 

 of the Hawkesbury flood would be 27,561 lb. per acre, or about one-twentieth 

 the weight of that left by the Hunter River flood. 



This would provide the land with a top-dressing per acre of the following 

 fertilising constituents : — 



Lime 135 1b. 



Potash 33 ,, 



Phosphoric acid ... ... ... ... ... ... ... 22 ,, 



Nitrogen 29 ,, 



All the above estimations as to the amount of fertilising material de- 

 posited are based on the assumption that the depth of the deposit in the 

 case of the Hunter River flood averaged 2 inches. This appears a very 

 high average, but there are unfortunately no hydrographical data available 

 by means of which this can be checked. It would be a matter of consider- 

 able national importance if further data concerning the rate of flow, volume 

 of water, height of flood, and amount of silt deposited, &c, could be obtained 

 during the period of such floods as those now discussed. This is a matter 

 which might well engage the attention of engineers. 



The fact must, of course, not be lost sight of that although such floods 

 are distinctly beneficial in increasing the fertility of land over which they 

 flow, their immediate action is more frequently detrimental than otherwise. 

 Instances have even occurred in which farms have been practically ruined 

 by heavy deposits of sand brought down by flood currents of more than usual 

 strength and volume. In any case floods are uncertain in their coming, and, 

 besides the risk of their destroying growing crops, they render the land unfit 

 for the plough for several weeks, so that a whole season may be lost. 



