SIR,--I PROMISED to send you some details of my attempt to grow wheat on the same soil year after year. These I now forward, and hope they may prove interesting. I was led into these experiments by reading Liebig's book on the "Chemistry of Agriculture;" for, assuming his theory to be true, it appeared to me to be quite possible to grow wheat on the same land year after year; as, according to that theory, the carbon, oxygen, and hydrogen, which constitute the great bulk of all cereal crops (both grain and straw), are supplied in abundance from the soil and atmosphere (or perhaps, to speak more correctly, from the latter), and we have only to supply those inorganic substances, which, however numerous, form but a small part of the whole weight of the crop. With the view of testing this theory, and hoping that I might be able to find out what were the elements which built up and cemented the carbon, oxygen, and hydrogen together--or, in other words, which constituted fertility--I begun, in the autumn of 1841, to experiment on a field which had been exhausted by a succession of crops, and which had just been cleared of one of oats. I chose an exhausted field in preference to any other, as the only one in which I could test the truth of the theory. It was very foul, being full of couch grass and weeds of all kinds. It was ploughed up and hastily picked over, for the season was so unfavourable for cleaning the land (from the great quantity of rain that fell) that I was almost induced to abandon the experiment. Previously to sowing the seed, one-fourth of the field was manured with a compost of night-soil and coal-ashes, at the rate of forty tons to the customary acre (7840 yards); the remaining three-fourths having the seed put in without any manure whatever. The winter was very unfavourable for the plants in our cold wet soil, and in the unmanured part of the field many of them perished, and those that survived made very little progress, from having no stimulus at the roots. Thinking it desirable to apply my experimental manures in moist weather, I waited until the 6th May, when I treated that part of the field which had _not_ been manured (three-fourths of the whole) in the following manner. I applied guano to one-fourth, at the rate of two hundredweight to the statute acre, and the same weight of nitrate of soda over another fourth, leaving one-fourth entirely without manure. The wheat manured with the guano and nitrate of soda grew vigorously, and the ears, more particularly in the part manured with guano, were the finest I had ever seen, but when it came to ripen it shrivelled in the ear, and the sample was very indifferent; the soil being evidently deficient in some property necessary for perfecting the grain. The crop also suffered much from the depredations of the birds. The portion manured with night-soil produced to the statute acre 32 bushels of 60 lbs. each.