The proposed excess of caustic lime will be advantageous in an open field, to kill off all plants, including weeds, and thus prepare it for the new crop as soon as the lime shall have become mild; but it would also kill trees, and render a garden useless for a time. Therefore, in these cases, we can not use the excess necessary to decompose minerals, and must confine the liming to the sole purposes of producing neutrality and of supplying food. This may be done by frequent small applications of fresh lime, or by one large application of carbonate of lime in the form of shell marl, or ground shells, or chalk, or old mortar, or old walls, or scrapings of limestone roads, or rotten limestone, or pure lime exposed fully to the air until, when stirred in water and allowed to settle, the water does not taste of lime.

The organic constituents of plants, or those elements originally derived from the air, are carbon, nitrogen, hydrogen, and oxygen. In fresh manure, ammonia is in excess, from the decomposition of the nitrates contained in the food. But after those have been exhausted the manure rots down the same as muck, etc., successively into ulmic, humic, geic, crenic, and apocrenic acids, (garden mould,) composed exclusively of carbon, hydrogen, and oxygen in different proportions. At each step, the air is decomposed; the nitrogen joins with the hydrogen of the earth acid, forming ammonia, while the oxygen joins with the remainder of the earth acid and forms carbonic acid gas, when the earth acid drops to the next grade. Hence a good supply of the roughest vegetable remains will for a long time furnish ammonia, the most valuable and evanescent ingredient in Peruvian guano. This ammonia from muck, on the contrary, forms most when most required, increasing with the heat of summer, becoming quiet in winter, to be aroused the next season for the next crop.

I have no positive authority that the air is exactly thus acted on, but I give it as an hypothesis that will explain the power of humus to " absorb" ammonia from the air, and at the same time account for the various chemical transformations among these earth acids that do not appear to be agreed on among chemists. Thus Liebig says, "The humic acid of the chemists is the product of decomposition of humus by alkalies. It does not exist in the humus of vegetable physiologists." Now, I found that a specimen of soil taken from a hedgerow, and composed mostly of rotted sods, and very prolific in sorrel, would neutralize 19,000 lbs. of slaked stone lime, if this soil covered an acre of ground eleven inches deep, while the same amount of soil, imperfectly burned, required but 3,000 lbs. Thus there is in this soil a vegetable something, capable of neutralizing 16,000 lbs. of lime per acre.

I propose to the horticulturist to try the experiment whether muck neutralized by lime will not answer in place of guano. If so, he will not care whether wood rots down into humus or humic acid; nor yet whether ammonia is thereby collected from the air, or formed by decomposition of the air. Again, we might infer from Liebig that decomposed vegetable matter is not necessary. His argument may be sound, but I prefer to imitate the fact, that decomposed vegetable matter is invariably reported in all fertile soils, although it may be the conse-quenoe, and not the cause, of fertility. Again, some of the most intelligent and scientific culturists object entirely to the use of magnesian-stone lime. If they are right, the second application should be replaced by pure lime, since an occasional liming is found beneficial, even on rich limestone soils. I have seen land rendered permanently barren by magne-sian lime, but only by excess. I have also seen whole districts raised to fertility by its use, and have used it largely myself in Pennsylvania, where they do not exceed 25 bushels per acre for poor land, and 50 bushels for rich land. They calculate fresh heaped measure.

This would make about 75 and 150 bushels slaked, struck.

The chemical proof of the above would occupy more space than you can devote to any one subject. The best proof is a practical test by a few judicious culturists. This they can do on a small plot, and ascertain whether it deserves the name of theory, at a very small part of the labor that it has cost me to collect and analyze the facts upon which it is founded. If true, it is important; if not true, let us have the objections, that something better may grow out of it.

[The above is from the pen of a gentleman well known in the scientific world; an original thinker, and a pains-taking experimentalist. The article itself is an admirable specimen of condensed thought; there is not a superfluous word in it. The theory propounded conflicts in several material points with that of Liebig and others, but is sustained, in our opinion, by many facts and probabilities. It will no doubt bring forth a response from some of our' readers. We would direct special attention to Mr. Aycrigg's suggestion in reference to the experiment with muck. We venture to say that the result will surprise many. We have not a doubt that vegetable matter is a source of fertility, Liebig to the contrary notwithstanding, and a very important one. Let Mr. Aycrigg's experiment be carefully tried, and the result as carefully noted, and we shall reach some important facts. - Editor].