8 April, 1907] Tree Planting and Forest Preservation. 225 



and lucerne may reach a depth of 3 yards. The roots of old, well- 

 established deeply -rooting trees, on the other hand, may penetrate to a 

 depth of 20 yards ox more, although the greater part of the root-system 

 will be at a considerably less depth. In addition, the root can draw 

 water laterally or upwards from neighbouring moister regions of soil, 

 and the finer the soil the greater the distance over which this action 

 may extend. 



In the case of soils with a friable, open surface, the whole of the 

 rain drains into the soil, unless the rainfall is extremely heavy. Each 

 shower of rain washes downwards a part of the soluble constituents of 

 the soil held by surface absorption on the soil particles. These soluble 

 materials, for the most part, are washed past the crop root-svstem, and 

 join the ground water in the deeper layers of the soil, which, in the absence 

 of trees, drains away to the rivers or to the subterranean water-systems. 

 These, for the most part, pour into the sea the calcium, jjotassium, and 

 magnesium nitrates, sulphates, and even traces of phosphates leached from 

 the soil, together with various other mineral constituents not required for 

 plant food. 



The roots O'f trees catch these mineral constituents to a large extent, 

 suck them in along with the water they absorb, and pass them up to 

 the leaves. It is worthy of note that it is mainly the useful soluble 

 salts which are absorbed with the water, whereas as soon as the plant 

 is clogged with the useless salts further absO'rption of these is checked. 

 In other words, the tree selects toi a certain extent the salts it requires, 

 and sends them up to its leaves, and these salts are precisely those which 

 crops require. During its whole life, the leaf of a tree contains a nearly 

 constant amount of potassium, magnesium, nitrogen and phosphorus, 

 while the calcium usually steadily increases. Although the percentage 

 of the first four elements usually decreases somewhat before the leaf 

 falls, which takes place ultimately whether the plant is deciduous or 

 an evergreen, a relatively large amount remains in the ash of the fallen 

 leaf. As these salts are set free by the decomposition of the leaf and 

 other dejecta membra of the tree in the soil, the ash constituents partly 

 reach superficial root-systems, partly deep root-systems, and partly are 

 washed out of the soil. The amount of mineral manure circulated in 

 this way by a tree during its average life-time is considerably greater 

 than that retained by the tree in its wood, which in most cases contains 

 a very low percentage of nutrient mineral substances if calcium is excluded. 

 It follows, therefore, that belts and clumps of timber on a farm help 

 the farmer to retain on his own land the manure he puts into it, and so 

 reduce the annual loss by drainage of the soluble constituents of the 

 manures applied to the crops. As Mr. Lee has shown in the December 

 number of this Journal, the crops are usually only able to catch and 

 absorb a small fraction of the essential elements supplied to them in the 

 form of manure. 



Belts of timber on the banks of streams are of the utmost importance, 

 not merely in preventing the erosion of the banks, but also because their 

 roots form a filter through which the drainage water from the soil must 

 pass, and be in part deprived of its mineral matter. At the same time, 

 the effect is to hold up the ground water on each side, and prevent over 

 rapid and erosive drainage from the soil. 



The branches and leaves of most trees contain from one to foui parts 

 of ash per 100 of dry weight, whereas the dry wood usual] \ contain less 

 3230. H 



