SOIL EXHAUSTION AND ROTATION IN CROPS. 79 



exception of soda, as has been established bj^ such an amount 

 of experimental evidence, that there can remain no doubt of 

 it whatever. Sulphuric acid, or the sulphates, as thej are 

 found in nature, are very liable to be removed from the soil. 

 The sulphate of lime is the form in which sulphuric acid 

 chiefly occurs in land. This dissolves in about five hundred 

 times its weight of water ; and where the soil is so situated 

 that heavy rains fall upon it, leach through and go out of it 

 again, the sulphuric acid is rapidly washed away. Almost 

 everywhere, except in the poorest soil, you find the water a 

 little hard, when you use it with soap. This hardness is due 

 to the presence of lime, and in most cases you find the water 

 contains a little sulphate of lime, which is the same as plaster 

 of Paris. This continually dissolves from the soil and passes 

 into the springs and rivers. If the soil is not porous, but of 

 such a nature that it can hold the rain which falls upon it to 

 a large extent, the case is different, and the loss is not so 

 rapid as from soil where the water runs freely through ; but 

 we have in this way a constant loss of sulphuric acid from the 

 soil. 



Unless there is an unfailing supply of sulphates in the soil 

 itself, furnished, for example, by the chemical alteration of 

 some other sulphur compound, as iron pyrites, there will in 

 time come to be a deficiency of sulphuric acid from this wash- 

 ing process alone, and although this element of crops is the 

 least prominent of them all in respect to quantity it is likely 

 to be soonest exhausted. The moment when the available 

 sulphates in the soil become less than is required for a full 

 crop, it will be impossible to realize such a crop without 

 making good the deficiency. 



The soil in a given case may be unfertile, may become ex- 

 hausted, simply because this one ingredient is removed by the 

 processes of washing and cropping. Lime and soda are also 

 washed out from the land, slowly to be sure, but continually, 

 and in quantities whose aggregate is very large. There are 

 other elements, like phosphoric acid, which we do not lose by 

 washing to any appreciable amount. You do not commonly 

 find this substance in the water of wells or springs, except in 



