102 THE SOIL OF THE FARM. 



of our root crops, which, when well supplied with min- 

 erals, and a small amount of ammonia, get what more 

 they require of this element from the atmosphere. The 

 following are the views of this writer with regard to a ro- 

 tation: *'That some crops demand all the nitrogen they 

 require to be supplied to them; others require a small 

 amount, which enables them to obtain a good deal in ad- 

 dition from the atmosphere; while others again can ob- 

 tain the whole of the nitrogen they need from the atmos- 

 j)here." This explanation api^ears so simple and clear 

 that it seems quite a pity to say anything that could 

 throw a doubt upon its accuracy. 



I will now endeavor to show what loss of the nitroo'cn 

 in the manure has taken place in our own exj^eriments on 

 the growth of potatoes at Rothamsted. In order to 

 measure the effect of nitrogen, and also ascertain 

 whether any, and if so what amount of loss has taken 

 place, our plan has been to grow the crop continuously, 

 with mineral manures alone. AYe consider that by this 

 means the crop avails itself of all the sources of nitrogen 

 at its disposal, whether they be derived from the soil or 

 the atmosphere. When, in addition to the same min- 

 erals, nitrogen in some soluble form is applied to tlie po- 

 tatoes in another experiment, we consider that the increase 

 in the crop over that grown by minerals alone, is 

 due to the nitrogen of the manure; and further, if we 

 deduct the amount of nitrogen in the crop grown by 

 minerals alone, from the amount contained in the crop 

 grown by minerals and nitrogen, the residue, when com- 

 pared with the amount of nitrogen applied in the manure, 

 will give us the measure of the loss. I must observe, how- 

 ever, that this experiment requires to be continued for a 

 good many years before any safe conclusions can be 

 drawn; first, because of the great influence of favorable 

 or unfavorable seasons; and, secondly, because it is only 

 by the aid of time that we can ascertain whether the 



