II32 



THE BOOK OF GARDENING. 



Chrysanthemum, if the soil is allowed to get thoroughly dry, the 

 plants never afterwards seem to regain their former healthy 

 vigour, nor will they produce such fine blossoms as if a steady 

 and continuous growth is maintained. 



The final product of nitrification is nitric acid ; but the nitrifying 

 organisms cannot develop in the presence of a free acid, hence 

 the benefit of liming sour soils, such as water-meadows, peaty 

 soils, or very rich old garden soils. The lime corrects the 

 sourness of the soil by neutralising the free acid, and then, if the 

 other conditions of heat, oxygen, moisture, and food are favour- 

 able, nitrification may proceed. There must be an excess oi 

 lime applied over and above the amount necessary to correct 

 the acidity of the soil in order to neutralise the nitric acid as it 

 is formed. 



Whenever the soil is in a condition unfavourable to nitrification, 

 there is danger that not only may nitrates not be formed, but 

 that there will be a loss of nitrogen from those nitrates which 

 are present. This loss is due to a process known as denitrifi- 

 cation — a process which is also dependent on micro-organisms. 

 The denitrification organisms flourish under one condition, which 

 IS directly opposed to the corresponding condition favouring 

 nitrification — namely, the absence of oxygen. Under that con- 

 dition the nitrates may be reduced or changed back to nitrites, 

 and the nitrites are often further reduced till they lose their 

 nitrogen altogether by having it pass off into the air as gaseous 

 nitrogen. 



Denitrification may take place, therefore, in \vater-logged soils, 

 in badly-drained pot-cultures, and in the inner parts of manure 

 or compost heaps, when air is measurably excluded. An organic 

 manure, therefore, which is effective for plant growth when 

 applied in small quantity may thus become injurious when made 

 use of in excess. The supply of atmospheric oxygen to a soil is 

 effectually prevented if the soil is kept in a saturated condition 

 with stagnant water. This condition alone is sufficient to set up 

 an energetic denitrification, by which process the growing plants 

 must considerably suffer. 



A recently-published experiment by Professor Breal furnishes 

 a further excellent example of the active denitrification which 

 takes place in a soil kept saturated with water. He placed some 

 garden soil in a percolator, and consolidated it by pressure ; the 

 column was about i5in. high. Water more than sufficient for 

 saturation was then poured upon the soil ; when the w^ater had 

 run through it was poured back again over the soil, and this 

 treatment was continued for some time. The soil at the 

 commencement of the experiment w^as in an active state of nitri- 

 fication, and the drainage-water was at first rich in nitrates ; but at 

 the end of three weeks the nitrates had entirely disappeared from 

 the drainage-water, though no water had been removed from the soil. 



