24 J. H. Gilbert— Points in connection with Vegetation. 



of a century. Table II shows the yield of nitrogen per acre, 

 per annum, in wheat, in barley, ard in root crops,'" each o TO \vn 

 for many years in succession on the same land, either without 

 any manure, or with only a complex mineral manure, that is 

 supplying no nitrogen. 



Bearing in mind what has been said as to the amount of 

 combined nitrogen known to be annually deposited from the 

 re, the figures in Table II have great interest and 

 significance. Thus, overa period of thirtv-two years, the wheat 

 has yielded an average of 20-7 lbs. of nitrogen, per acre, per 

 annum, without manure. But if we look at the quantities 

 yielded during the first eight, the next twelve, and the last 

 twelve years of that period, it is seen that there has been a 

 gradual, but at the same time a considerable decline in the 

 annual yield. From this it would appear probable that the 

 nitrogen of the soil derived from previous accumulations, is 

 being gradually reduced; Whether or not the whole of the 

 excess of yield over that available from the rain, and other 

 measured aqueous deposits from the atmosphere, is due to pre- 

 vious accumulations within the soil, and is therefore inducing 

 a gradual exhaustion of its stock of nitrogen to that extent we 

 have not conclusive evidence to show. Determinations of nitro- 

 gen in samples of the soil taken at different times during the 

 course of the experiments do, indeed, show an appreciable reduc- 

 tion. It is probable, however, that a part of the rxrcss of yield 

 is due to condensation of ammonia within the pores of the soil 

 beyond that which would be deposited in rain, and in the dew 

 and other minor deposits condensed on the non-porous even 

 surface of a rain-gauge, as already referred to. 



Excluding the first eight years' of the growth of wheat, it is 

 seen that while over the next twenty-four years, 1852-1875, 

 the wheat yielded 19-3 lbs. of nitrogen, per acre per annum ; 

 the barley yielded an average of Itf-V, 11*. over the 'amo period' 

 Again, during the first twelve of the twenty-four years the 

 wheat yielded 22 -6 lbs., and the barley T> 11,'s - while during 

 the second twelve years, the yield in wheat was reduced to 

 lo-> and that in the barley to 14-6 lbs. The similarity in the 

 yield of nitrogen oyer the same periods in ,!,,-., twodoselv 

 alned crops, growing in ,l,llerent fields, is very strikm-r, though, 

 upoo the whole, the indication is that the autumn-^ ,, wheat 

 lias accumulated more than the spring-sown barley 



It is next to be observed that the annual use' -,f a complex 

 Uitroln Teith 6 ^ bUt Ve, ' V Sll ' J " ' "'" '" ! the y> eld ° f 



