Various earthy and saline substances

1300. Various earthy and saline substances have been proved to constitute an important part of the food of plants, and it has also been proved that different plants not only require different kinds of these substances, but also that they are found in some soils and not in others. Thus if plants requiring phosphate of lime are grown in a soil which does not contain that salt, they become weak and unhealthy; but if phosphate of lime could be added to the soil artificially, it would instantly become so changed in its nature that the plants which before would scarcely live in it will grow luxuriantly.