THEIR ACTION AFFECTED BY CIRCUMSTANCES. 341 



a moment at the general conditions which are necessary to ensure the 

 success of these or any other saline manures. 



1°. They must contain one or more substances which are necessary 

 to the growth of the plant. 



2°. The soil must be more or less deficient in these substances. 



3°. The weather must prove so moist or the soil be so springy as to 

 admit of their being dissolved, and conveyed to the roots. 



4°. They must not be applied in too large a quantity, or allowed to 

 come in contact with the young shoots in too concentrated a form — the 

 water that reaches the roots or young leaves must never be too strongly 

 impregnated with the salt, or if the weather be dry, the plant will be 

 blighted or burned up. 



5°. The soil must be sufficiently light to permit the salt easily to 

 penetrate to the roots, and yet not so open as to allow it to be readily 

 washed away by the rains. In reference to this point the nature of the 

 subsoil is of much importance. A retentive subsoil will prevent the 

 total escape of that which readily passes through a sandy or gravelly 

 soil, while an open subsoil again will retain nothing that has once made 

 its way through the surface. 



/. Cases in which the nitrates have failed. — A knowledge of the 

 above conditions will enable us in many cases to explain why the ni- 

 trates, and other generally useful substances, have failed to exhibit any 

 beneficial e.Tect. 



1°. Thus on the light soils of Berkshire the nitrate of soda failed for 

 barley, causing it often to be blighted or burned up. This, no doubt, 

 arose from the drought which may act in one or other of several ways. 

 Either it may prevent the salt from being dissolved at all, and thus hin- 

 der its action altogether for the time,— -or it may retard the solution till 

 the plant has attained such a state of maturiry, that it is no longer ca- 

 pable of being equally benefitted by the introduction of the salt into its 

 roots— or after being dissolved, and having partially descended into the 

 soil, the drought may cause it to ascend again with the water which 

 rises to the surface in consequence of the evaporation, and may thus 

 present it to the plant in so concentrated a form as to injure the young 

 shoots — or, finally, the action of the sun upon the green leaf, into which 

 a portion of the salt has already been conveyed by the roots, may be so 

 powerful as to concentrate the saline solution, or to increase its decom- 

 position to such an extent as to cause injury, and consequent blight to 

 the leaf itself. 



2'^. Again, at Cheadale, in Cheshire, {Mr. Austin), the nitrate of soda 

 is said to have had a good effect on wheat and grass where the subsoil 

 was clay, but none where the subsoil was gravel, or the soil light and 

 sandy. Here the supply of water in. the soil may have been such as to 

 fit it for entering readily into the roots in a proper state of dilution, when 

 the retentive subsoil kept it within reach of the roots, — and yet sufficient, 

 at the same time, to wash it away altogether where the soil and sub- 

 soil were too open to be able to retard its passage. 



3°. But the occasional occurrence of droughts or the mere physical 

 distinctions of lands as light or heavy, are not sufficient to account for all 

 the recorded differences in the effect of the nitrates. Thus on the clays 



