IMPROVEMENT OF THE SOIL BY PLANTING OF TREES. 429 



§ 8. Improvement of the soil by the planting of trees. 



It has long been observed by practical men, that when poor, thin, 

 unproductive soils have been for some time covered with wood, Lheir 

 quality materially improves. In the intervals of the open forest, they 

 will i)roduce a valuable herbage — ?r when cleared of trees they may 

 for some time be made to yield profitable crops of corn. 



This fact has been observed in almostevery country of Europe, but 

 the most precise observations upon the subject with which I am ac- 

 quainted are those which hav^e been made in the extensive plantations 

 of the late Duke of Athol. These plantations consist chiefly of white 

 larch (^Larix Eiiropcea.) and grow upon a poor hilly soil, resting on 

 gneiss, mica-slate, and clay-slate (Lee. XI., § 8.) In six or seven 

 years the lower branches spread out, become interlaced, and com- 

 pletely overshadow the ground. Nothing, therefore, grows upon it 

 till the trees ar6 24 years old. when the spines of the lower branches 

 begirming to fall, the first considerable thinning takes place. Air and 

 light being thus re-admitted, grasses (chiefly holcus mollis and lana- 

 tus) spring up, and a fine sward is gradually produced. The'ground, 

 which previously was worth only 9d. or Is, per acre as a sheep pas- 

 ture, at the end of 30 years becomes worth from 7s. to 10s. per acre. 



The soil on this part of the Duke's estate is especially propitious to 

 the larch — and. therefore, this tree both thrives best and in the great- 

 est degree improves the soil. - Thus in oak copses, cut every 24 years, 

 the soil becomes worth only 5s. or 6s. per acre, and this during the last 

 six years only. UnTler an ash plantation, the improvement amounts 

 to 2s. or 3s. per acre ; under Scotch fir, it does not exceed 6d. an acre 

 — while under spruce and beech the land is worth less than before. 

 (Mr. Stephens, in the Transactions of the Higliland Society, xi., p. 

 189 ; also Loudon's Encylopccdia of Agriculture, p. 1346.) 



The main cause of this improvement, as of that which is produced 

 by laying down to grass, is to be found in the natural manuring with 

 recent vegetable matter, to which the soil year by year is so long 

 subjected. Trees differ from grasses only in this, that while the lat- 

 ter enrich the soil both by their roots and by their leaves, the former 

 manure its surface only by the leaves which they shed. 



The leaves of trees, like those of grasses, contain much inorganic mat- 

 ter, and this when annually spread upon the ground slowly adds to the 

 depth as well as to the richness of the soil. Thus the leaves of the fol- 

 lowing trees, when dried in the air, contain respectively of inorganic 

 matter ((Sprengel, Chemiefur Landwirthe, ii., passim) : — 



April. August. November. 



Oak — 5 per cent. 4^ per cent. 



Ash — 6i « — 



Beech •..— 7 " 6i « 



Birch ..— 5 cc _ « 



Elm — 111 « — " 



Willow — 8.i " — « 



White Larch 6 J. per cent. — " — '* 



Scotch Fir — U " — " 



In looking at the differences among these numbers — especially in 



