On the Theory of Mamn^es. 9 



for the turnips, he calculates at 12,730 lb. of carbon, 2801b. 

 of azote, and 6104 lb. of saline matter. "We thus find that 

 while the manure provides half as much again carbon as needed, 

 and five times the quantity needed of inorganic matters, there 

 is not above -^^ part of an overplus in the nitrogen, to allow 

 for what may never reach the roots of plants (being carried off 

 by evaporation or washed away), and for the ammonia which 

 circulates in the vital juices, assisting in the transformations 

 needed to prepare the food for assimilation, and stimulating 

 the activity of the vital principle. Wherever nitrogen is fur- 

 nished in abundance from the substances deposited in the soil 

 as food, whether in the form of ammonia or nitrates, the plants 

 are found to assume a dark green healthy appearance ; the 

 evidence, well known to practical men, of luxuriant vigour of 

 growth, this colour being always assumed in the healthy con- 

 dition of the plant ; though perhaps the alkaline effect of the 

 ammonia on the chromule of the leaf may only denote its pre- 

 sence, and the capability of action, other circumstances being 

 favourable, as the colour has be^ sometimes found to appear, 

 without the. usual consequences of luxuriance in growth follow- 

 ing. It might be more beneficial, when the manure is com- 

 pounded of substances not known to abound in nitrogen, to 

 make such as jDotatoes, containing little nitrogen, to precede 

 wheat. The analysis of turnips, as given by Professor Johnson 

 in his Elements, would cause a greater quantity of nitrogen to 

 be suspected in the rotation than that of Dr. Madden's state- 

 ment. He states the gluten and albumen in 25 tons of tvirnips 

 at 1400 1b.; according to Dr. Prbut's estimate of 15'55 per 

 cent, about 217| lb. of nitrogen. Boussingault's estimate of 

 •17 per cent of azote in the turnips would make only about 

 95 1b. 



When the nitrogen of manures is so small in comparative 

 amount, it seems to strengthen the opinion that part of this also 

 is got from the air. Some crops, undoubtedly, derive a great por- 

 tion of their nitrogen from the air. Boussingault has found it so 

 in Jerusalem artichokes ; a more familiar instance, however", is 

 to be found in the bean, which, in an average crop, carries off a 

 great deal more of nitrogen than wheat, and should proportion- 

 ally exhaust the fertility of the ground in a greater degree ; yet, 

 while the wheat is one of the most exhausting crops we have, 

 the bean is rather a fertiliser. The oat also is a very exhausting 

 crop, and contains still less nitrogen than the wheat. The crops 

 of oats which follow beans are more luxuriant than ordinary, as 

 if the bean had been depositing rather than extracting nitrogen. 

 If Professor Johnson's estimate of the quantity of nitrogen car- 

 ried off by turnips be correct, it is another instance of ground 

 being fertile after what should have been a scourging crop. If 



