WHEAT-GROWING AND ITS PROBLEMS 289 



food, water, warmth, air, light, and there must be an absence 

 of poisons and pests. In the case of wheat there is a seventh 

 special condition — the straw must be sufficiently strong to stand 

 up or the plant will be beaten down by the rain and wind and 

 cannot make proper growth or be harvested readily. As these 

 conditions become more favourable the crop may increase, but 

 any one of them may set a limit to its size. 



One of the first problems investigated at Rothamsted was 

 the food requirements of wheat. Experiments were begun in the 

 Broadbalk field in 1843 to ascertain what substances must be 

 added to the soil as manure. It was soon found that nitrogen 

 and potassium compounds and phosphates were necessary, but 

 even after sixty years there is no evidence that the other 

 essential constituents — magnesia, iron, etc. — need be added, 

 these always occurring in sufficient quantities in the soil. No 

 great advance has since been made in this direction, and we 

 have even yet but little direct knowledge of the part actually 

 played by these various constituents in the metabolic processes 

 going on in the plant. The general effects, however, are now 

 fairly well ascertained. Thus it is known that nitrogen com- 

 pounds tend to promote leaf development and vegetative 

 growth, that phosphates promote root development in the early 

 stages of the plant's life, while in the later stages they hasten 

 the ripening processes ; also that potassium compounds give 

 the plant increased vigour, generally enabling it better to 

 withstand adverse conditions of drought, wetness, rust attacks, 

 etc. When no nitrogen compound is supplied there is of course 

 no growth beyond the seedling stage, even though the other 

 food-stuffs — phosphates, potassium compounds, etc.— are present 

 in abundance, and the other conditions of growth are favourable. 

 If a small quantity of an ammonium salt or a nitrate is added 

 growth takes place, and as the nitrogen compound is increased, 

 so the amount of growth increases. Some of the Rothamsted 

 results are as follows : 



