“I suppose, because the manure for wheat was applied in the autumn, and the rains of winter and spring dissolved more of the plant-food than would be the case if the manure was applied in the spring. If the manure had been applied on the surface, instead of plowing it under, I believe the effect would have been still more in favor of the autumn-manuring.”
When the nitrogen is in an available condition, spring barley can take up and utilize a larger proportion of the nitrogen than winter wheat. Neither the wheat nor the barley can get at and take up half what is applied, and this, notwithstanding the fact that a heavy dew or a slight rain furnishes water enough on an acre to dissolve a liberal dressing of nitrate of soda or sulphate and muriate of ammonia. The truth is, the soil is very conservative. It does not, fortunately for us, yield up all its plant-food in a year.
We have seen that when wheat or barley is dressed with soluble ammonia-salts or nitrate of soda, a considerable amount of the nitrogen is left in the soil—and yet this nitrogen is of comparatively little benefit to the succeeding crops of wheat or barley, while a fresh dressing of ammonia-salts or nitrate of soda is of great benefit to the crop.
In other words, when wheat is sown after wheat, or barley after barley, we do not get half the benefit from the manure which it is theoretically capable of producing.
Now, the question is, whether by a judicious rotation of crops, we can avoid this great loss of manure?
There was a time when it was thought that the growth of turnips enriched the soil. I have heard it said, again and again, that the reason English farmers grow larger crops of wheat and barley than we do, is because they grow so many acres of turnips.
“So I have often heard,” said the Deacon, “and I supposed the broad turnip leaves absorbed nitrogen from the atmosphere.”
There is no evidence that leaves have any such power; while there are many facts which point in an opposite direction. The following experiments of Lawes and Gilbert seem to show that the mere growth of turnips does not enrich land for grain crops.
Turnips were grown on the same land, year after year, for ten years. The land was then plowed and sown to barley for three years. The following table gives the results: