AN IMPERMEABLE SUBSOIL. 93 



will be very likely to render the soil poorer for some time, 

 instead of richer. If a farmer is aware that his soil 

 would be improved by being deeper, he must make the 

 improvement by adding to its depth a little each year. 



312. When a loose sand rests upon clay ; or a clayey 

 soil upon calcareous marl, or upon sand ; indeed when 

 ever the subsoil will serve as an amendment to the soil, 

 the two may be mixed with great advantage. 



The evils of a subsoil impermeable to water are the 

 stagnation of water and the excessive humidity of the 

 soil. Generally, a very slight declivity is sufficient to 

 induce the water to trickle along below the soil upon the 

 surface of the subsoil, until it finds some means of escape. 

 But even in this case, there is likely to remain in the 

 soil superfluous moisture, which ought to be carried away 

 by draining. 



313. When the slope is not sufficient to lead the water 

 to run off, the ground becomes boggy and the evil is 

 declared by signs intelligible to every-body, by the spring 

 ing up of rushes, sedges and other bog plants. But 

 when the slope allows the water to trickle away slowly, 

 the evil is not so apparent. The most certain sign, per 

 haps, is the presence of the weeds called horsetail, and 

 scouring rush, (species of equisetum^) which need a sub 

 soil always wet for their horizontal roots to run upon. 



It may be laid down as a rule that wherever horsetail 

 appears, the ground needs draining. 



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