120 The Potato 



color. In extreme cases the foliage may become too loose 

 and open and porous in texture, and so render the potato 

 plant more liable to the attacks of disease and sun-scald. 

 Lack of available nitrogen for the plant is shown by small 

 size and light yellow color of the foliage. Commercially, 

 nitrogen used in fertilizers comes from a variety of sources 

 which vary greatly in value and in the quickness with 

 which they become available. Nitrate nitrogen, the form 

 to which other forms must be changed in the soil before 

 the plants can take it up in the soil solution, is the quickest 

 available. Nitrate of soda is the principal commercial 

 form. It comes from deposits in Chile, hence the old 

 name of "Chile saltpeter." It is so rapidly soluble in 

 water that it may sometimes be dissolved and lost to the 

 plant if heavy rains fall before the feeding roots have had 

 time to form. For this reason, it is either applied after 

 planting as a top-dressing or is used for only a part of 

 the nitrogen of a mixture, the remainder being in slower 

 acting forms. Because of this rapidity of action, nitrate 

 of soda is particularly valuable for use in early spring, on 

 early potatoes before the soil warms up enough for the 

 processes by which the other forms of nitrogen change to 

 nitrate. It contains about 15 per cent of nitrogen. Sul- 

 phate of ammonia, a by-product of coke and gas manu- 

 facture from coal, is next in order of availability. It 

 contains the largest percentage of nitrogen of any material 

 — 20 per cent. It has good mechanical condition for 

 mixing or sow^ing. One drawback is that its use tends to 

 make land sour and that it is not very efficient on sour 

 soils. Nitrate of lime and cyanamid are comparatively 

 new commodities, being made from the gaseous nitrogen 

 of the air by the use of electricity. While not extensively 

 used, it is probable that in time they will become very 



