174 Fertilisers [pt. iii 



for plant nutrition. Complex compounds such as pep- 

 tone are of little use to the plant even when they are 

 soluble in water; only the simpler compounds are of 

 nutritive value. The decomposition in the manure heap 

 is brought about by micro-organisms, and seems to be 

 similar in its earlier stages to the hvdrolvsis effected in 

 the laboratory by acids, giving first aminoacid and 

 then ammonia. Subsequent events depend on the con- 

 ditions in the heap. Under anaerobic conditions {i.e. in 

 absence of air) there seems to be little further change. 

 Under the mixed conditions of aeration which obtain in 

 actual heaps there is a loss of nitrogen, which, however, 

 does not take place under strictly aerobic or strictly 

 anaerobic conditions. If the heap becomes dry moulds 

 develop and assimilate some of the ammonia produced, 

 building it up into forms not available to the plant. 

 The changes may be summarised as follows: 

 Under aerobic conditions, i.e. when air is admitted, 

 some of the carbohydrates of the straw decompose : this 

 change is advantageous if the manure is to be applied in 

 the spring, because otherwise there is a loss of soil 

 nitrates. Some of the nitrogen compounds are decom- 

 posed, giving rise to ammonia: this also is a useful 

 change. If the conditions are not wholly aerobic (and 

 they never are, except in laboratory experiments) there 

 is always a loss of nitrogen which counteracts the ad- 

 vantages of the preceding changes. If the heap becomes 

 too dry moulds develop and take up some of the 

 ammonia. 



Under anaerobic conditions, i.e. when air is excluded, 

 the straw suffers less change, but the complex nitrogen 

 compounds break down especially at a temperature of 

 about 26 C. There is no evolution of gaseous nitrogen. 



