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The order of the estimated loss of nitrogen by drainage is almost the converse of that of the increased yield in the crops. Plot 10, which gives the least increased yield in the crop, shows the greatest loss by drainage, and plots 7 and 9, which yield the greatest increase in the crop, show the least loss by drainage.
The excess in the soils (over plot 5) is obviously much more in the order of the increased yield in the crops. Plot 10, with the least in the increase of crop and the most in the drainage, shows the least excess in the soil, whilst plots 7 and 9, with the greatest increased yield in the crop, and the least loss by drainage, shows the greatest excess in the soil.
It is clear, therefore, that whilst the excess in the soil has no direct relation to the amount supplied in the manure, it has a very obvious relation to the increased yield in the crop; in other words, to the amount of growth. The last column of the table brings this out more clearly. Excepting in the case of plot 10, with the ammonium-salts alone, there is a general uniformity in the proportion of the excess in the soil over plot 5 to the increased yield in the crop over plot 5, and the variations, such as they are, have an obvious connection with the conditions of growth. Thus plots 11, 12, and 14, all with a deficient supply of potash, show approximately equal proportions retained in the soil for 100 of increase in the crop. Plots 13, 7, and 9, again, all with liberal supplies of potash, show higher, but approximately equal, proportions retained in the surface-soil for 100 of increased yield in the crop.
Upon the whole, it is obvious that the relative excess of nitrogen in the soils of the different plots is little, if at all, due to the direct retention by the soil of the nitrogen of the manure, but it is almost exclusively dependent on the difference in amount of the residue of the crops-of the stubble and roots, and perhaps of weeds.
This leads to the consideration of the actual differences in the crop with equal nitrogen supply and different mineral supply. This is illustrated by the results in Table VI., which shows the effects of mineral manures alone, of ammonium-salts alone, and of ammonium-salts with different mineral manures.
Wheat grown for forty years in succession on the same land. Broadbalk Field, Rothamsted. Results showing the effects of Mineral Manures alone, and when used in addition to Ammoninm-Salts. Quantities per acre. Produce: Dressed Grain in bushels.
400 lbs. Ammonium-Salts 86 lbs. Nitrogen per acre per annum.
Mixed Mineral Manure
For the thirty-two years, 1852 to 1883 inclusive, each of the eight differently manured plots received the same manure each year. I will only call special attention to the average yields over periods of sixteen and thirty-two years.
Plot 5, treated with mineral manure only, gave, during the first sixteen years, an average yearly yield of 17 bushels per acre, during the second sixteen years 133 bushels, and during the whole period of thirty-two years 15 bushels.
Plot 10a, treated with ammonium salts only, gave, during the first sixteen years, an average yearly yield of 23 bushels per acre, during the second sixteen years 17 bushels, and during the thirty-two years an average of 201 bushels. Thus, ammonium-salts alone produced much more than mineral manure alone.
On plot 10b, previous to 1852, in the years 1844, 1848, and 1850, mineral manures had been applied, in the other years previous to 1852 (excepting in 1846, when it was unmanured), and subsequently, ammonium-salts only. The effect of the residue of the previously applied mineral manures is apparent on comparison with the yields on 10a.
On plot 10b we find, during the first period of sixteen years, an average yearly yield of 27 bushels per acre, against 233 bushels on 10a; during the second period of sixteen years 194 bushels, against 17 on 10a; and during the thirty-two years, an average yearly yield of 234 bushels, against only 201 on 10a.
Plot 11, with superphosphate but no potash, in addition to the ammoniumsalts, gave, during the first sixteen years, an average yearly yield of 29 bushels per acre, during the second sixteen years 223 bushels, and during the thirty.two years 26 bushels.
On plot 12, in addition to the ammonium-salts, superphosphate and sulphate of soda were applied; but potash had been applied prior to 1852. The first sixteen years produced an average yearly yield of 34ğ bushels per acre, the second sixteen years of 27 bushels, and the whole thirty-two years of 31 bushels.
On plot 13, the ammonium-salts, superphosphate, and sulphate of potash were applied, and the average annual produce was, over the first sixteen years 34 bushels, over the second sixteen years 291, and over the thirty-two years 31§ bushels.
On plot 14, besides the ammonium-salts and superphosphate, sulphate of magnesia was applied, and some potash had been applied prior to 1852. The average annual produce was, over the first sixteen years 34 bushels, over the second sixteen years 284 bushels, and over the thirty-two years 31ğ bushels.
On plot 7, in addition to the ammonium-salts, superphosphate and the sulphates of potash, soda and magnesia were applied, and gave, during the first sixteen years, an average yearly yield of 353 bushels per acre, during the second sixteen years of 29 bushels, and during the whole thirty-two years of 32 bushels.
Thus, not only the effect upon the yield of wheat of a direct supply, but of a residue from long previous applications of potash, is very noticeable. This is rendered more obvious by reference to the following table (VII), in which the pounds per acre of potash and phosphoric acid removed during two periods of ten years each, in the total produce, and in the grain alone, of the plots last referred to, and some others are given.
TABLE VII.-Potash and Phosphoric Acid in Grain and in total Produce. 1852-61; and ten years, 1862-71. Per Acre in Pounds.
I will illustrate this point by referring only to the potash. Plots 3, 10a, 10b, and 11 show a deficiency of potash in both grain and total produce compared with the amounts in the produce of plots 2, 12, 13, 14, and 7, on all of which there was a sufficiency, or more or less excess, of potash available. comparison of these results with the manuring of the plots, we find that in every case the increase of potash in the total crop depends either on a direct annual potash supply, or on a residue from previous applications. The first ten years shows more potash in the total produce with the direct supply (13 and 7) than with the residue (12 and 14), but the amount in the grain is essentially the same in each case. In the second ten years there is a greater difference in the amounts of potash in the total produce between the plots having the direct and those having only the residual supply, whilst there is scarcely any difference in the amounts in the grain, but such as it is, it is in accordance with the conditions of supply. Hence it is evident that whilst the plant in its vegetative stages assimilates according to the available supply-it may be in excess of actual need-if there is no deficiency, the composition of the final product-the seed-is essentially the same.
We have thus traced the effects of exhaustion, of full manuring, and of nitrogenous and non-nitrogenous manures on one particular soil. It has been seen how very different is the effect of one and the same manuring in different seasons, but the real extent of this variation is more clearly brought out in Table VIII., which shows the best, the worst, and the average produce, over a period of thirty-two years, under very opposite conditions as to manuring.
TABLE VIII.-- Wheat year after year on the same land. Broadhalk Field, Rothamsted. Produce of the best season, 1863; the worst season, 1879; and average of 32 years, 1852-1883.
2 Farm-yard manure.
5 Mixed mineral manure, alone..
6 Mixed min, man, and 200 lbs. Amm.-salts=43 lbs. Nit. 7 Mixed min. man. and 400 lbs. Amm.-salts=86 lbs. Nit. 9 Mixed min, man. and 550 lbs. Nitra.soda=86 lbs. Nit. 8 Mixed min. man, and 600 lbs. Amm.-salts=1291bs. Nit.
We will confine our attention to the amount of dressed grain per acre in bushels. The difference in yield of the various plots in the best and worst of the thirty-two seasons is very marked. The unmanured, the mineral manured, and the heavily nitrogenous manured plots, all suffered severely. In most cases the difference approaches, and in two cases (plots 6 and 7 mixed mineral manure, together with 200 and 400 pounds of ammonium-salts, respectively furnishing 43 and 86 pounds of nitrogen) it actually exceeds the average produce of the plots. From these facts we see how easy it is to form wrong conclusions as to the effects of different manures, if experiments are conducted in only one season or in only a few seasons, and if the characters of the seasons are not studied.
Not only season, but soil and locality must exercise an influence. The Rothamsted results are obtained on one description of soil, and in one locality only. Reference to the following table (IX.) shows the results obtained in experiments conducted at Rothamsted during the same eight years, but in two fields; at the same place in one field for thirty-two years; at Woburn, for seven years; at Holkham, Norfolk, for three years; and at Rodmersham, Kent, for four years. Thus, the experiments were made on very various soils, under various conditions from previous treatment, and in various seasons, yet the general characters of the results are accordant.
TABLE IX.-Results of Experiments on the growth of Wheat by different Manures, on different Soils, in different Localities, and in different Seasons.