-2 2 2 Journal of Agriculture , Victoria. [lo April, 1912. 



SOIL MOISTURE AND CHOPPING. 



By John W. Pater son, B.Sc, Ph.D., Exfcrimcntalist. 



A twenty-bushel crop of wheat (grain, straw and roots) contains at 

 harvest about 450 lb. of water. This water is present as water in the crop. 

 Chemical analysis further shows that it contains about 170 lb. of hydrogen 

 chiefly in its starch and protein compounds. This hydrogen would nearly 

 all come from water which was decomposed as water by the plant while 

 growing. It requires 1,530 lb. of water to yield 170 lb. of hydTOgen. 



Adding 450 and 1,530 we get 1,980 lb. of water. One inch of rain- 

 fall per acre weighs just over 100 tons, so that 1,980 lb. of water equals 

 0.0088 inches, or 0.88 points. If water only played the part of phosphoric 

 acid or any ordinary food material to plants then 0.88 points of rain would 

 satisfy the requirements of this crop for water. 



The insufficiency of such a ranifall is abundantly manifest. About 

 2,000 times as much rain must fall on the ground annually, and such a 

 rainfall must have a suitable periodic distribution, to give a yield of wheat 

 such as is here contemplated. 



What then was the use of all this rain of which the crop has nothing to 

 show at harvest, but which was nevertheless required to produce the desired 

 yield? There are three answers : — 



(i) Part of it was used in the vital processes, and afterwards discarded 

 from the stems and leaves during the growth ot the plant. 



(2) Part of it was necessary for the soil changes whereby food materials 

 are prepared for the plant. 



(3) Part of it was inevitably lost from the .soil. 



First: — As to the water discarded by the plant. — The crop of wheat 

 under consideration would weigh dry about i| tons. In producing this 

 dry matter it is essential that very much water must pass through the plant 

 during growth. It has been variously estimated that frqm 200 to 600 lb. of 

 water must pass through the plant for each i lb of dry increase. Taking 

 400 as a middle figure, it would thus be necessary for i J x 400 ^= 600 tons 

 of water to be absorbed from the soil and evaporated from the leaves to 

 produce the crop of wheat. 



This amount of water equals 6 inches of rainfall. Satisfactory figures 

 are wanting. It is certain however that different crops, and probably in 

 an important degree different varieties of the same crop, make widely 

 different demands in regard to water per unit of dry matter formed. The 

 subject is an important one as bearing upon the best crops, and the best 

 variety of crop to cultivate in dry districts. 



Economy in the use of water is not howe\-er everything in a crop ; 

 ability to obtain the water under various conditions is not less important. 

 The power of any crop to obtain water depends upon several factors among 

 which are : — - 



(i) The percentage of water in the soil. 



(2) The readiness with which its water is gradually given up by the 

 particular class of land. 



(3) The amount of pure air (oxygen) at the rootlets. 



(4) The soil temperature. 



(5) The concentration (osmotic value) of the weak salt solution forming 

 the soil moisture. 



(6) The variety and vigour of the crop. 



(7) Climate — particularly as regards sunshine, humidity and wind. 



