NITROGEN 177 



monium nitrogen or the nitrate nitrogen. Some plants 

 appear to have a preference, but in many cases it depends 

 on the conditions of the soil. Most garden plants can use 

 the nitrate form better with a pH of 6.5 or lower, but above 

 this pH in some cases they use the ammonia form more effec- 

 tively. Nitrogen is often applied in an organic form as bone 

 meal, tankage, cotton seed meal, or raw bone meal. These 

 forms act slowly because they are not available until they 

 are decomposed by bacteria as shown in the chart (Fig. 25) . 

 Manure carries some nitrogen and, as explained above, it 

 appears to stimulate the nitrogen-fixing bacteria. 



A lawn will need very small applications of nitrogen, since 

 the high organic content left by decaying roots furnishes food 

 for the nitrogen-fixing bacteria, and also the lawn holds 

 moisture so well that the leaching effect is reduced. If nitro- 

 gen is applied it should be done in the fall or in the early 

 spring before the shoots begin their active growth. At this 

 time the soil is low in nitrogen and growth may be limited. 



The size of flowers can be increased with nitrogen, since 

 it reacts on flowers in much the same way as on leaves. 

 Leafy vegetables can be increased in size with nitrogen. The 

 nitrogen content of the soil is usually lowest after protracted 

 heavy rains in mid-summer or later, when it may become so 

 low that the plant reacts as indicated by lack of good green 

 color. 



The problem of an adequate supply by nature of nitrogen 

 salts for the soil is not understood. The atmosphere contains 

 79 per cent of free nitrogen but plants cannot use it until 

 it is fixed in a molecule. The salts of nitrogen are very solu- 

 ble and are readily leached from the soil. In fact under some 

 conditions as much may leach from the soil as is used by the 

 plants. A crop of corn may use more than a hundred pounds 

 of nitrogen per acre, whereas the farmer seldom applies more 

 than fifteen pounds per acre as fertilizer. Each crop except 

 the legumes, which bear nodules on their roots containing 



