162 SOIL CONDITIONS AND PLANT GROWTH 



root range of the plant and the water supply where it lies horizontally, 

 but proving much less harmful where it dips at any considerable angle. 

 The soil itself is good although it has rather too much fine silt, and it 

 becomes very productive when the effect of the rock is counteracted. 

 The Dicker soil, while not of the best type as its fine silt is too high, 

 is far from being hopeless, but it unfortunately lies on a deep bed of 

 stiff clay which keeps it wet in winter and parched in summer. It is 

 therefore very poor, and even with the best management never gives 

 great results. 



The second case, over-drainage, is illustrated by the Shopwyke soil 

 in the same table. It is a fair soil, containing too much fine silt to be 

 in the first rank, but it is spoiled by lying on a deep bed of gravel only 

 nine inches or a foot below the surface the subsoil sample could only 

 be taken in one or two instances. Consequently it dries out badly in 

 summer and does not repay much expenditure in the way of manures. 



The Wye soil is given as an instance of the normal case where a 

 soil becomes rather heavier in its lower depths, with the result that the 

 movement of water is somewhat impeded without being stopped. 

 Thus the subsoil furnishes a reserve of water for the surface, yet even 

 in wet weather it does not hold up too much water and in dry weather 

 does not constitute too great a barrier against the upward capillary 

 movement The Wye soil contrasts with the Dicker soil, the usual 

 case in a clay, where the subsoil contains much more true clay than 

 the surface. 



Chemical Analysis of Soils. 1 



Recourse is had to chemical analysis to discover the amounts of 

 potential and actual plant food in the soil, and the rate at which 

 potential food is likely to become available. But as the problem is 

 vague, so the methods are empirical and the interpretation of the 

 results often very difficult. 



Organic Matter. The analyst should note whether the organic 

 matter is fairly well decomposed, or whether it still shows definite plant 

 structure, also whether or not it is acid to litmus paper. He can then 

 interpret his observations as shown on pages 67, 71 and 143. 



Nitrogen. Unlike the other soil constituents nitrogen and car- 

 bonates are determined absolutely. The amount of nitrogen is closely 

 related to the loss on ignition, of which in a large proportion of cases 

 it is about 3 per cent. As a guide to fertility it is therefore subject to 

 the same limitations ; a high nitrogen content may be associated either 



1 See Appendix for methods of analysis. 



