IT is well known that the presence of nitrate in the root environment tends to reduce nitrogen fixation in nodule-bearing species of leguminous plants. A careful quantitative study of this effect in the case of lucerne (Medicago sativa) and soy beans (Glydne hispida) has been made by Giobel (1926). A large number of papers have been published recording the effect of nitrates and ammonium salts in reducing the number of nodules produced on legumes, and nodulation has been observed to be prevented at high concentrations of these nitrogenous salts. Literature on these subjects is quoted by Giobel (1926) and by Fred, Baldwin, and McCoy (1932), and need not be given here in extenso.It is difficult to compare the results obtained by different workers, partly because of the varied methods of expressing the amounts of nitrogen compounds applied, and because of the often insufficient recording of experimental details. Part of the difficulty is also due to the fact-sometimes too little appreciated-that after a dose of mineral nitrogen has been applied at the commencement of an experiment, the initial concentration of mineral nitrogen in the medium is not maintained, but is progressively decreased by assimilation during the growth of the plant. Since nodules can be produced throughout the growth period, those nodules which appear towards the end of the experiment have been exposed to a lower concentration, actual or effective (see pp. 178, 179) of nitrate, than have the earlier nodules. The duration of an experiment, and the amount of growth made by the plants, must thus affect the results obtained. In comparing results of different experiments, it is necessary to remember, too, that the mineral nitrogen may have been applied in a single dose, or repeatedly.It is not surprising to find that different authors have obtained variable results in attempts to ascertain the doses of mineral nitrogen Journ. Agric. Sci. xxvi 12

IX. Summary and Abstract1. Inoculated lucerne was grown alone and in association with Italian rye grass, in pots of sand watered with food solution and given three different doses of sodium nitrate.2. The dose of nitrate did not affect the dry weight or nitrogen content of lucerne when grown alone, save that the highest dose checked the root growth somewhat.3. When lucerne and Italian rye grass were grown in association, the growth of the grass varied directly with the dose of nitrate applied, and the growth of the lucerne varied inversely to it. Checking of the lucerne growth was probably due to root competition with the grass.

Summary. About 1840, the beginning of the era of scientific agricultural chemistry, many chemists believed that ammonia was the principal, if not the sole form in which nitrogen was taken up by all plants. This view was abandoned, and towards the end of last century it was generally believed that with the possible exception of the Leguminosae, the higher plants took up their nitrogen almost solely from nitrate. This belief was in large measure founded upon the results of excessive attention paid to the conditions of soil which was not bearing vegetation. The discovery that leguminous plants were able by the help of specific bacteria to utilise atmospheric nitrogen was not thought to extend to any of the other higher plants. Though widespread use had been made by practical farmers of leguminous plants in association with non‐legumes, the idea of commensalism between legumes and non‐legumes did not arise amongst agricultural scientists until the present century was well advanced. The acknowledged benefits attained through the growth of legumes were over long ascribed to nitrification of decayed roots resulting from some previous (not to a simultaneous) legume occupancy. This theory may have been correct for the conditions of sinqle crops on arable soils, but was inadequate to account for the comparative failure of grassland and mixed forage crops to respond profitably to fertilising with quickly acting mineral nitrogenous manure. The rde of nitrate in the soil is not clearly understood; nitrate is most likely an end‐product of micro‐organic decomposition of organic materials. Its presence is detectable in notable amounts, and most clearly, in the absence of plants. This does not necessarily imply, as it was once thought, that it is preferentially absorbed by plants; it is suggested that plants can absorb some of the less highly oxidised forms of nitrogen which are the precursors of nitrate. In other words, the finding of nitrates in considerable amounts in soil indicates that there has been a local surplus of nitrifiable nitrogen compounds which plant roots have been unable to reach and consequently to absorb. No single compound of nitrogen can be named as the primary component of the nitrogenous nutrition of plants. Evidence is presented that non‐leguminous plants can profitably utilise compounds of nitrogen built up by the symbiotic life of nodule bacteria within their proper leguminous host plants. Some insight into the nature of the transferred compounds has been gained, though the conditions in vitro do not admit of facile extension to natural conditions. The mode of transfer from legume to non‐legume is still obscure, but the existence of a transfer can be taken to be well established; it represents a stage in a double symbiosis of which the importance has not been fully appreciated. It is probable that in the nitrogenous nutrition of plants some factors are involved which are not yet formulated. These accessory factors may be found to derive ultimately from the animal, aided by activity of legume nodule bact...

Summary and abstract1. Italian rye-grass grown in the presence of lucerne in sand with no added nitrogen contained, after 18 weeks’ growth, some 2¼ times as much nitrogen as did grass of the same age similarly grown but in the absence of lucerne.2. In another experiment, where lucerne and grass were grown together in sand:(a) Where 0·33 gm. of sodium nitrate was added per pot, the grass after only 13½ weeks’ growth contained 2½ times, and after 18 weeks’ 5½ times, as much nitrogen as was supplied as nitrate.(b) Where 1·0 gm. of nitrate was added, the grass after 13½ weeks contained slightly more nitrogen than was added as nitrate, and after 18 weeks it contained 2¼ times as much.

T h e w o r k d e a ls w i t h t h e b e h a v io u r o f m ix e d s t r a i n s o f n o d u le b a c te r ia t o w a r d s e a c h o t h e r a n d to w a r d s t h e i r le g u m e h o s t.I t in t r o d u c e s t h e c o n c e p t o f d o m in a n c e in c o m p e t it io n b e tw e e n s tr a in s . T h is d o m in a n c e is i n d e p e n d e n t o f d e g re e o f e ff e c tiv e n e s s a s r e g a r d s n itr o g e n f i x a tio n .W h e r e tw o s t r a i n s o f n o d u le b a c t e r i a a r e b o t h p r e s e n t in t h e s u r r o u n d in g s o f t h e i r h o s t 's r o o t s y s te m , a c ti v e c o m p e t it io n b e tw e e n t h e m m a y c a u s e t h e s t r a i n h a v i n g t h e h ig h e r in i t i a l g r o w th r a t e a lm o s t c o m p le te ly t o c h e c k m u l t i p l i c a t i o n o f t h e o t h e r s t r a i n o u ts id e t h e p l a n t . T h is d o m i n a n t s t r a i n w ill t h e n b e r e s p o n s ib le f o r n e a r l y a ll t h e n o d u le s .I n p e a s a n d s o y b e a n s , w h e r e g r o w T h e r e a r e la r g e d iffe re n c e s in t h e r a t e s o f a p p e a r a n c e a n d fin a l n u m b e r s o f n o d u le s p r o d u c e d b y d if f e r e n t s t r a i n s s u p p lie d i n p u r e c u lt u r e , p a r t i c u la r ly w i t h c lo v e r . th o f t h e r o o t s y s te m is r a p i d a n d o f c o m p a r a t i v e l y s h o r t d u r a t i o n , t h e n o d u le -p r o d u c in g c a p a c i t y o f t h e p l a n t m a y b e p a r t i a l l y o r w h o lly s a tis f ie d b y t h e n o d u le s p r o d u c e d w ith i n t h e fir s t fe w w e e k s , so t h a t f u r t h e r in f e c tio n , w h e th e r b y t h e s a m e o r b y a d if fe re n t s t r a i n , is c h e c k e d o r inT h e r e l a t i v e n u m b e r s o f n o d u le s p r o d u c e d b y t h e tw o s t r a in s s im u l ta n e o u s l y a p p li e d t o t h e r o o t s is c o n d itio n e d b y t h e sp e c ific in f e c tiv ity p e c u li a r t o e a c h s t r a i n , u n le s s s o m e o t h e r f a c t o r , s u c h a s c o m p e t it io n o u ts id e t h e p l a n t , m a s k s t h i s e ffe c t.