FIELD CEOPS. 227 



In regard to the continuous growth of crops of wheat, barley, and mangels 

 on the same land, the author concludes that taking the results as a whole they 

 show " that the wheat crop does not in any way unfit the land for the growth of 

 another wheat crop provided the fertilizer supply is maintained. If there is 

 any toxic effect brought about by the crop leaving behind in the soil substances 

 which have an injurious influence upon another crop of the same kind, this 

 effect is very small in the case of wheat, so small as to be negligible. Of course 

 this does not mean that it is advisable as a matter of practical farming to 

 attempt to grow wheat continuously upon the same land, because . . . there 

 are mechanical difliculties introduced in the way of cleaning the land, and 

 these difficulties are great enough to prevent the wheat on the Broadbalk held 

 from reaching the level of production that we might expect from the fertilizers 

 supplied to some of the plats." 



The average yield for the last 10 years of continuous cropping, without fer- 

 tilizers, was for wheat 10.9 bu., barley 9.3 bu., and mangels 3.68 tons per acre. 

 In a rotation without manure since 1848. swedes produced 0.3 ton in 1908, 

 barley 10 bu. in 1909, clover hay 17.4 cwt. in 1910, and wheat 24.5 bu. per 

 acre in 1911. In the continuous cropping without manures the yield of wheat 

 reached a minimum level of about 12 bu. per acre, below which it seemed to 

 be sinking very gradually, depending upon the amount of nitrogen brought 

 down by rains. The plat which received barnyard manure containing 200 

 lbs. of nitrogen reached a fairly constant level of about 38 bu. per acre, although 

 an excess of nitrogen had been applied each year. During the 59 years 7,800 

 lbs. of nitrogen have been applied and 1,989 lbs. removed in crops. Yet an 

 analysis of the soil showed an accumulation of only 507 lbs., so that estimating 

 150 lbs. as added in rain 5,454 lbs. was unaccounted for and regarded as lost 

 to the crop. Some factor other than nitrogen must, then, have been the limit- 

 ing crop-producing factor. 



It is shown that Azotobacters play an important part in the fixation of at- 

 mospheric nitrogen in the presence of carbohydrates in the soil. Some of the 

 plats that had received manurial treatment, but no nitrogen from 1867, showed 

 only a slight decrease in nitrogen content and the yield of wheat averaged 

 31.2 bu. during a period of 46 years, while when the turnips in the rotation 

 were turned under the soil maintained its initial percentage of nitrogen much 

 longer and produced an average of 32.2 bu. of wheat. The Azotobacter, the 

 author contends, fixed atmospheric nitrogen to a greater extent with the 

 aid of the cai'bohydrates of the turnip crop turned under. Nitrogen detei-mina- 

 tion of the soil showed, with the roots removed, in 1SG7, 0.1224 per cent ; in 1874, 

 0.1147; 1SS3, 0.1161; and 1909, 0.1159; with the roots turned under, in 1867, 

 0.124 ; in 1874, 0.1238 ; in 1883, 0.1228 ; and in 1909, 0.1195 per cent. 



In a 4-year rotation running for 60 years, turnips without manure produced 

 in 1908 21.6 cwt, and under the influence of phosphoric acid applications 179 

 cwt. in the same year. Barley was also helped by phosphoric acid in its rota- 

 tion, and the author connects this action with the shallow root system of these 

 plants in which phosphoric acid acts as a stimulant to root growth and tUlering. 

 Phosphoric acid seemed to have a specially beneficial effect in a wet or cold 

 soil and season as shown with the barley. In 1893, a very dry year, phospho- 

 ric acid apparently raised the yield from 11.6 to 18.1 bu. per acre, and in 1894, 

 a very wet and cold season, from 10.4 to 34.9 bu. 



Another indication of the need of certain crops for special food elements 

 was in the case of mangels, potatoes, and legumes in their relation to potash, 

 the yield being increased and often the quality of the product. Potash seemed 

 to have a reverse effect in a wet season from phosphoric acid, apparently 

 increasing the wheat yield from 11 ty 10 bu. per acre in a wet season, but 



