This matter is necessary to combine with, and fix the ammonia in the soil, and give it out to plants as they require it.
It is possible that nitrates may superabound in the soil from the oxydizement of the nitrogen of a superfluity of ammonia.

The people say that all land may become _oosur_ from neglect; and when _oosur_ can never be made to bear crops, after it has been left long fallow, till it has been flooded with rain-water for two or three seasons, by means of artificial embankments, and then well watered, manured, and ploughed.

When well tilled in this way, all but the very worst kinds of _oosur_ are said to bear tolerable crops.

In the midst of a plain of barren oosur land, which has hardly a tree, shrub, or blade of grass, we find small _oases_, or patches of low land, in which accumulated rain-water lies for several months every year, covered with stout grasses of different kinds, a sure indication of ability to bear good crops, under good tillage.

From very bad _oosur_ lands, common salt or saltpetre, or both, are taken by digging out and washing the earth, and then removing the water by evaporation.