134 



IRISH GARDENING 



SEPTEMBER 



with a wealth ol' colour in Auj^ilsI is rcpif- 

 scnlL't-i in i;arilcns bv many \aiiotioN. \ar_\inL;' 

 in colour from wliitc to rose and clocp red, anil 

 some have double llowers. 



Si. Dabeoc's Ucalh, Habtccia polifolia is a 

 particularly handsome dwarf everi^recn, bearinj;' 

 lar_i;e red. pure while, ov led and while llowers. 

 This is a beautiful plant for massini;- and is 

 always y^reatly .idmired. 



Other excellent peat plants are the Bilberries 

 belong'ingf to the i^eniis \ accinium. 



\'accinium Myrtillus and \ . \ ilis-ld.ea are 

 luo desirable native species of comparati\ ely 

 low i^rowlh, while \'. corymbosum, the Swamp 

 Bilberry of North America, and \'. pcnnsyl- 

 vanicum are useful shrubs, growiny some three 

 to five feet high. The cranberries, Oxycoccus 

 macrocarpus and O. palustris are useful trailing" 

 plants for various positions, while EpigiXia 

 ripens, a dwarf-creeping evergreen with white 

 tlowers, tinged with pink, is a lovely subject for 

 a shad}' position ; it is known as the North 

 America Mavllower. 



Nitrogen Fixation. 



By Patrick Di ikv, Associate Royal College Science 

 for Ireland. 



NirivOGEN is one of the elements 

 essential for the life and growth o( 

 plants ; and it is the elemenl in which 

 most soils are deficient, only a verv small 

 quantity of it being" present in a form which 

 can be utilised by^ green plants. Nevertheless, 

 nitrogen is very common in nature, the air 

 being composed of four-fifths of it, but in a 

 form in which ordinary plants can make no use 

 of it. It must be in the form of a nitrate or an 

 ammonium salt before plants can absorb it into 

 their tissues. As nitrates and ammonia com- 

 pounds are very soluble they are readily washed 

 out of the soil ; so that in order to grow a crop 

 successfully nitrogen must be applied in one of 

 these forms at about the tii"ne when growth has 

 started. Nitrate of soda, sulphate of ammonia, 

 calcium cyanamide, and nitrate of lime are the 

 nitrogenous manures now most commonly 

 used. Nitrate of soda occurs as a deposit in 

 certain rainless reg^ions of South America. 

 The supplies in those regions are now g-etting 

 worked out, and so far no new deposits have 

 been located in any other part of the earth, so 



that it seems iinly a matter o\' some years till 

 this source becomes exhausletl. Sulphate ot 

 ammonia is i.ibt;iined chiellv irv>m gas works, 

 where it occurs as a b\e-product in the manu- 

 facture iif coal-gas. The ciuantity produced is 

 not by any means sullicient in itself to supply 

 the amount ol" nitrogen requiied by our crops 

 from year to year. The otlier two nitrogenous 

 manures, calcium cyanamide and nitrate ol 

 lime, are produced by causing the nitrogen ol 

 the air to unite chemically with calcium carbide 

 and oxygen respectively in an electric furnace. 

 There is a probability that this method will in 

 lime alfitrd large quantities ol nitrogen in a 

 suitable form for plants, liut at present the out- 

 put is conipaiali\el\' small owing to the great 

 expense ol' producing the electric current 

 necessary for the process. 



I*'rom the foregoing" considerations it will be 

 seen that the problem ol' nitrogen, as it effects 

 crop growers, would be a very grave one it 

 there were no other sources of suppl_\- than the 

 ones named. There is one other important 

 source by means of which nitrogen is made 

 available for plants, and it is with this we 

 propose dealing here. This source consists ol' 

 certain species of bacteria which are found in 

 most soils, and which have the power of taking 

 up nitrogen from the air and building it into the 

 substance of their bodies. When they die 

 nitrogen in their bodies becomes changed by 

 ammonification and nitrification into a form in 

 which it can be taken up and used by green 

 plants. Because of this power of using up the 

 nitrog"en from the air these bacteria are spoken 

 of as nitrogen fixers. They can be conveniently 

 divided into two groups— viz., {a) those which 

 are free-living or which live apart frorn green 

 plants in the soil, and {b) those which live in 

 association with plants of the pea-family or 

 leguminos;!^. 



(rt) It has been long known that g"round 

 which has been allowed to lie fallow for a time 

 increases in its store of nitrogenous compounds 

 during the interval. At first this phenomenon 

 was explained by the fact that nitrates are 

 formed in the air during thunderstorms and 

 other electric rnanifestations, and are washed on 

 to the soil by rain and dews. Later investiga- 

 tions have shown, however, that the amount 

 of nitrogen derived in this way is comparatively 

 small, and it became necessary to find out what 

 other source or sources was responsible for the 



