VII.J 



PHYSICAL EFFECTS OF MANURE 



221 



the growing period of the plant. The rainfall for 

 this period, ?>., for the four months March to June, 

 was 13 inches in 1879 and only 2-9 inches in 1893. 

 The average yield on the dunged plot is about 

 3 bushels more than on Plot 7, but in the dry year its 

 superiority amounted to 14 bushels, whereas in the very 



Table LXX.— Effect of Farmyard Manure in Dry and Wet 

 Seasons. Wheat. (Rothamsted.) 



wet year the two plots sank to the same low level. In a 

 bad season the bacterial changes, which render the plant 

 food in dung available for the crop, go on very slowly. 



It has been suggested that farmyard manure may 

 have an effect upon the water-content of the soil by 

 reducing the surface tension of water with which 

 it comes in contact. If the surface tension of the 

 soil water were thus reduced, it would be less readily 

 lifted to the surface and therefore less available to 

 shallow-rooted plants, but more conserved in the lower 

 layers of the soil. Although an extract of dung 

 possesses a lower surface tension than pure water, the 

 facts concerning its behaviour in the soil are very 

 obscure as yet, and the figures just quoted as to the 

 relative distribution of moisture under dunged and 

 unmanured plots lend no support to the theory. 



The application of farmyard manure to grass land, 

 not only has a fertilising and water-retaining effect, but 

 is also valuable from the way it acts as a mulch and 

 affords the springing grass in the early months of the 



