No. 4.] SOIL EXHAUSTION. 287 



crops as rye, mustard, buckwheat, etc., no such bacterial 

 adai)tati()ii occurs, and the soil nitroijfen will not be in- 

 creased from atmospheric sources. The leguminous plants 

 constitute the best catch-crops, on account of the peculiar 

 nutritive adaptation existing between the nodular bacteria 

 and the atmospheric nitrogen. A crop of these plants can 

 be sown, and just before reaching maturity they can be cut, 

 and, if necessary, fed to stock. The roots containing or- 

 ganic matter and a store of nitrogen can be plowed under. 

 We hav^e practised this system in our greenhouse to good 

 advantage, — a practice which, as far as we know, is not 

 made use of in greenhouses to any extent. During the 

 summer the greenhouse, which is devoted to winter cucum- 

 bers and lettuce, generally lies idle, and by sowing a crop 

 of white lupine (which will develop in about six weeks under 

 these conditions) or some other legume we succeed in add- 

 ing to our soil a needed supply of organic matter and nitro- 

 gen. Experiments have shown that a crop of legumes 

 plowed under is practically equal to a normal supply of 

 nitrogen to the soil (see Figs. 2 and 3). 



A certain stage of development in the crop is necessary 

 in order to obtain the largest supply of nitrogen. This 

 stage probably coincides in most cases with that when the 

 seed are maturing. There has been a considerable num- 

 ber of leguminous plants grown for test purposes at the 

 Hatch Experiment Station in Amherst during the past ten 

 years or more, such as the white lupine, horse bean, serra- 

 della, alfalfa, soy bean, mclilotus, Canadian pea, the various 

 clovers, etc. Unfortunately, however, the majority of these 

 winter-kill in our climate, and only a few of them can be used 

 for winter soil-covers. Among those best suited for our 

 climate is the common red clover and the melilotus or sweet 

 clover. The latter, when sown in Jul}^ or at the time of 

 the last cultivation of the soil, is capable of attaining a 

 height of twelve or fifteen inches the following May, at 

 which time the crop can be cut and utilized, and the nitro- 

 gen-containing roots can be plowed in. The red clover is 

 also useful as a soil-cover, but does not always make suffi- 

 cient growth in time for spring planting, being considered 

 by some to be less desirable on this account than the meli- 



