WHEN APPLIED AS A MANURE. 347 



luxuriates most when it is within the immediate reach of the driving 

 spray of the southern sea. It may well be, therefore, that among our 

 cultivated crops one may delight more in common s^U than another, — 

 and if we consider how much alkaline matter is contained in the tops 

 and bulbs of the turnip and the potatoe, we are almost justified in con- 

 cluding that generally common salt will benefit green crops more than 

 crops of corn, and that it will promote more the developement of the 

 leaf and stem than the filling of the ear. 



If this be so, we can readily understand how a soil may already con- 

 tain abundance of salt to supply with ease the wants of one crop, and 

 yet too little to meet readily the demands of another crop. The appli- 

 cation of salt to such a soil will prove a failure or otherwise, according 

 to the kind of crop we wish to raise. 



3°. Failures have sometimes been experienced also on repeating the 

 application of salt to fields on which its first effects were very favour- 

 able. In such cases it may be presumed that the land has been already 

 supplied with salt, sufficient perhaps for many years' consumption — 

 and that it now requires the application of some other substance. 



If it be desired, experimentally, to ascertain whether the land already 

 contains a sufficient supply of common salt, the readiest method is to 

 collect half a pound of the soil in dry weather, to wash it well with a 

 pint or two of cold distilled water, and then to filter through paper, or 

 carefully to pour off* the clear liquid after the whole of the soil has been 

 allowed to subside. A solution of nitrate of silver (common lunar-caus- 

 tic of the shops) will throw down a white precipitate, becoming purple 

 in the sun, which will be more or less copious according to the quantity 

 of salt in the soil. If this precipitate be collected, dried in an oven, 

 and weighed, every 10 grains will indicate very nearly the presence of 

 4 grains of common salt. The quantity of this precipitate to be expect- 

 ed, even from a soil rich in common salt, is, however, very small. If 

 half a pound of the drj"- soil yield a single grain of salt, an acre should 

 contain about 1000 lbs. of salt where the soil is 12 inches deep — where 

 it has depth of only 6 inches, it will contain nearly 500 lbs. in every 

 acre. 



8°. Chlorides of Calcium and Magnesium. — These compounds are 

 rejected in large quantities as a refuse in some of our chemical manu- 

 factories — and tbey are contained, especially the latter, in considerable 

 abundance in the refuse liquor of our salt pans. They have both been 

 shown to be useful to vegetation (see Appendix), and where they are 

 easily to 'be obtained, they are deserving of further trials. Like com- 

 mon salt, it is generally in inland situations that they are fitted to be 

 the most useful. Where salt springs are found in the interior of Ger 

 many, the refuse obtained by boiling down the mother liquors after the 

 separation of the salt has been often applied with advantage to the land. 



Theory of the action of these chlorides. — Common salt and the chlo- 

 rides of calcium are not unfrequenily found in the sap of plants — they 

 may be supposed, therefore, to enter into the roots without necessarily 

 undergoing any previous decomposition. But we have already seen 

 (Lee. v., § 5), that the green leaves under the influence of the sun, 

 have the power of decomposing common salt — and no doubt the other 

 15* 



