240 FARMYARD MANURE [chap. 



no other fertiliser. Other experiments at Rothamsted 

 show how lasting is the effect of farmyard manure, i.e. 

 how very slowly some of the nitrogenous compounds 

 get oxidised by bacteria and converted into a form 

 available for the plant. In two cases, on the grass field 

 and on the barley field, farmyard manure at the rate of 

 14 tons to the acre was applied for eight years and 

 then discontinued. The effect of that application can 

 still be traced more than forty years later, in the higher 

 crop given by these manured plots than by the plots 

 which had remained unmanured the whole time. Of 

 course, these long-continued effects of farmyard manure 

 are not great in themselves, and will only be perceptible 

 when the land has been reduced to the lowest ebb of 

 fertility by continual cropping without manure. Since 

 the greater part of the nitrogen contained in farmyard 

 manure is not in a condition to be utilised by the plant 

 until it has first been attacked by bacteria and converted 

 into ammonia or nitrates, it may happen that the plant, 

 though it has been well supplied with farmyard manure, 

 cannot obtain the nitrogen therein quickly enough, 

 when the season is otherwise favourable to growth. 

 For example, we see from the Rothamsted experiment 

 with mangolds that in years of large crop the plant 

 which receives some active source of nitrogen like 

 nitrate of soda will grow heavier crops than those which 

 receive farmyard manure alone ; although the amount 

 of nitrogen in the manure applied to the latter, in 

 addition to that which is already stored up in the soil, 

 may be far in excess of the plant's requirements. From 

 many experiments it can be demonstrated that when 

 the grower is aiming at a very large crop, for example 

 of potatoes or mangolds, it is more economical to obtain 

 this by using a mixture of dung and active fertilisers 

 than by increasing the amount of dung alone. The 



