A correspondent of the Gardener's Chronicle says on this important subject: -

"I have made a few experiments and observations how trees might recover their health and become useful and ornamental to those who possess them. The want of proper nutriment either to vegetables or animals soon shows itself in one way or another, and food applied even in a rough state is to all appearance greedily received when animated beings are in want of it. The trees I intend to make a few remarks upon had fallen into a languid state through want of food, or in other words the soil appears to have been worn out by means of the crop that grew upon it In a hollow part of a wood where some Elms and other trees were planted some years ago symptoms of decay manifested themselves in a very prominent form, such as the leaves turning yellow early in the season and falling off, when others in a more healthy state remained longer to perform the office assigned them. Leaves in such a state make little wood for the tree that bears them; the young shoots apparently never ripened their wood, for many of them died, and the trees altogether had a decaying appearance. The hollow ground was used to put rubbish in that came from the garden and other places.

It was thought at first that the rubbish would kill the trees by burying the roots too deep, but much that was put there was of vegetable origin, which soon decomposed, and being of a porous nature did not prevent the air from reaching the roots, and instead of injuring the trees they soon began to show signs of improvement. Their health recovered rapidly, their leaves expanded in length and breadth, their shoots did the same, and their leaves instead of being the first to droop in the autumn, continued to hold on as long as most deciduous plants do. From these observations we may learn that even old trees may be made to have a healthy old age, and young ones that have set prematurely in their growth, from want of proper soil to grow in, or some deficiency in the constituent parts of the soil, may be made either by liquid or solid food to resume a healthy state, and may live from generation to generation a shelter and an ornament in the place which they occupy. Liquid manure may also be applied with advantage to forest trees. In a plantation where the trees were chiefly Oak, Elm, Maple, Spanish Chestnut, and Birch, and the soil light, the subsoil being of a clayey nature, where liquid manure was applied to some of the trees, they profited greatly by it.

The few Elms that remain in the plantation in the natural soil measure on an average about 1 foot 7 inches in circumference at 3 feet from the ground; the Maples or Sycamores in the same soil measure about 1 foot 9 inches in circumference at 3 feet from the ground. In the same plantation and same soil there are two places where the Maple and Elm have grown much better than the rest. These have had their roots occasionally watered with liquid manure, and the difference in the measurement of the stems shows that they agree well with such treatment. In one of the places the circumference of the Maple 3 feet from the ground is 3 feet 5 inches, and contains about 12 cubic feet of timber, and the Elm measures 4 feet in circumference, and contains about 20 cubic feet of timber; in the other place the Maple measures 3 feet 6 inches in circumference, and contains about 14 cubic feet of timber, and in both places the trees are remarkably healthy, and stand about 18 feet apart It may be a long time before such manure can be spared for growing forest trees, and there are some kinds to which it would do more harm than good if it were applied, such as Pine and Fir trees; but if even the slops thrown away as waste water from gentlemen's and farmers' kitchens and dairies were applied to the purposes of arboriculture, there would be both pleasure and profit derived from the application.

Many trees may be seen growing in soils naturally poor and unfitted to carry heavy timber without assistance, and trees considered by many to be in the last stage of existence may be made to renew their growth; for, give a tree room to grow and food to live upon, and vegetable physiologists will not be able to tell how long it should live and how large it should grow." - P. Mackenzie.

The reader of the Horticulturist has frequently seen paragraphs on the feeding of trees. In a late number, a valued contributor replying to a former article, says, " Do not feed your plants (Evergreen Plants) well; on the contrary, if you have any doubt of the hardihood of a plant, starve it; let it make little growth, but well ripened wood." This is good advice, but we are reminded by "A Hasty Reader" that this upsets the received theory. Pray remark that the writer says, "If you have any doubt of the hardihood" etc. The advice given on both sides is correct; thousands, nay millions of all descriptions of hardy trees are planted in bad soil, in a hole filled with earth more or less adapted to their wants; they soon exhaust their first meal, and send rootlets to the bad surroundings where they meet with nothing to nourish them; they cease to grow with their natural rapidity and vigor. Dig round the roots, insert their natural food, and they recover and progress. No man that has the slightest practical knowledge will attempt to over-force his plantation; at the same time it remains true, as we stated in the Horticulturist ten years ago, that " trees may be fed with as much propriety as chickens." In the operations of nature we see the leaves annually decay and nourish the root; there is no objection to bringing leaf mould and applying it with a shovel as a top dressing; the nourishment designed by nature is carried down by the rain to the roots; there is no reason why we may not assist the process still further and bring the natural food of our plants to the roots where the nourishment will be sooner imbibed.

The danger consists in over-forcing and especially in over-forcing so much that the new growth will not bear the coming winter's cold. This takes place in some of the States of the Union where they have an almost tropical summer and an arctic winter, and we see the growth bears evidence of the injury; the trees present dead limbs, especially on the north side and where no shelter occurs. Sometimes the whole outside of an "opening" will present a phalanx of dead wood.

In our last number, guano is recommended under certain circumstance*, and in moderate dilution, to promote the growth of conifers (hardy); it is simply supplying what nature demands when it is otherwise deficient in the soil. The opinions of our two correspondents, H. W. Sargent, Esq., and the Rev. A. D. Gridley, do not differ so much as "A Hasty Reader," supposes; the one would very properly rather starve a tender conifer than over-force it by food; while the other would feed his hardy trees " with generous food, that they may make a vigorous growth, and always wear the bright hues of health." He would not of course convey the idea that they should be fed so much as always to have the sickly hue of disease, or be subject annually to the destruction of their unripened wood.