46 CHAPTER IV 



analysis. These results, considered in conjunction with the 

 total reserve supplies, in most cases make clear the manurial 

 requirements of the soil. 



(b) Field Experiments. — In this method a representative 

 portion of the land is divided into a number of equal plots, 

 certain of which are fertilised, and others (every second or 

 third plot) left untreated as " checks." Standard one- 

 constituent fertilisers are used, e.g., blood meal as a source 

 of nitrogen, superphosphate as a source of phosphoric oxide, 

 and sulphate of potash as a source of potash. These are 

 applied both singly and in combination. The effect of each 

 fertiliser treatment is gauged by the increase of yield, as com- 

 pared with the untreated plots. If the land chosen is uniform, 

 the plots properly laid out, and the underlying principles of 

 experimental work observed, this is probably the most satis- 

 factory method of ascertaining a soil's deficiencies. The 

 results of the first year will often indicate positive require- 

 ments. The farmer is advised, before undertaking a heavy 

 expenditure on fertilisers, to test these in some simple manner 

 on his land. The chief points for him to observe are to choose 

 uniform and representative ground, to have unraanured plots 

 as checks contiguous with the treated plots and, if possible, 

 to duplicate or even triplicate the series. 



Eelative deficiencies or excesses of those plant foods with 

 which we are most concerned in fertiliser practice, if marked, 

 can frequently be detected by merely studying the appearance 

 of crops growing on the soil in question. 



Nitrogen appears to be closely associated with the rate of 

 growth. Insufficient nitrogen results in a weak, stunted 

 growth and a poor colour. Excess, on the other hand, pro- 

 motes a vigorous vegetative growth of good colour, often at 

 the expense of fruiting and the strength of the stems ; de- 

 creases the plant's resistance to disease (e.g., rust in cereals), 

 delays maturity, and tends to increase the nitrogen content of 

 the crop. 



Phosphoric oxide seems to be associated particularly with 

 root development and the fruiting process. A sufficiency of 

 phosphoric oxide to some extent counteracts the ill effects of 

 an excessive nitrogen supply. It promotes root development, 

 hastens maturity, and stimulates fruiting; thereby ensuring, 

 in the case of grain crops, a larger proportion of grain to straw. 

 Potash is said to be closely concerned with the formation 

 of carbohydrates in the plant. A sufficiency of potash 

 apparently strengthens the vegetative tissues, thereby increas- 



