244 A VISIT TO EOTHAMSTED. 



Init tlie other two supplied, no doubt a similar deficiency of crop 

 would have been the result. This shows very conclusively that 

 a deficiency in any one constituent of a fertile soil cannot be 

 compensated by an excess of the others. It requires some care 

 on the part of the farmer to determine what are the proportions 

 of these constituents required for each particular soil, in order 

 to produce a full crop, and this he can learn by experimenting 

 in the manner above described. The proportions on plot 9 are 

 not supposed to be the best possible, though they seem to have 

 come very near it. If there is any deficiency, it is in the nitro- 

 gen, as shown by the increase on plot 11 ; an increase, how^ever, 

 which was not great, and quite disproportionate to the large 

 addition of ammonia salts which produced it. A much smaller 

 increase of ammonia salts would probably have been quite as 

 effective. The limit of profitable manuring has here been passed. 

 The point at which profitable manuring ceases is when the crop 

 is as heavy as will stand against wind and weather, and when 

 the cost of additional manuring is as great as the value of the 

 additional crop. 



There are many other incidental lessons to be learned from 

 the above experiments. Plot 1 shows that farmyard manure is 

 an excellent application for grass, and that it is more permanent 

 in its effect than other manures. The 200 lbs. ammonia salts 

 given annually to (1 h) in the second decade are intended to 

 exhaust the residues of mineral wealth left by the farmyard 

 manure during the first decade, and it is shown that this 

 exhaustion is progressing at the rate of about 1 cwt. per annum. 

 A converse experiment is going on on plot 6, where ammonia 

 salts, which were applied for thirteen years, have been discon- 

 tinued, and minerals are now being given to use up any residual 

 nitiogen which may be lying there. It will be seen that that 

 amount is exceedingly small, the average annual increase during 

 seven years being only about | cwt. Another experiment of a 

 similar kind has recently been started on plot 15. Here, in- 

 stead of ammonia salts, an equivalent amount of nitrate of soda 

 has been applied, but as yet, of course, it is not apparent whether 

 the residual nitrogen, resulting from an application of nitrate of 

 soda, is greater or less than that derived from its equivalent of 

 ammonia salts. Plot 15 a, however, compares favourably with 

 plot 6 a, and would seem to show that nitrate of soda is a better 

 manure for grass than an equivalent amount of ammonia salts, 

 and this is a result which is supported by general experience. 

 The ordinary practice of top dressing grass with nitrate of soda 

 in preference to ammonia salts is thus shown to be founded on 

 the superior efficacy of the former. The reason of the superi- 

 ority of nitrates over other forms of nitrogenous manures has also 

 l)een made a subject of investigation at Rothamsted. It has been 



