The plants most frequently used are white mustard, rape, buckwheat, spurry, rye, and several kinds of leguminous plants, especially vetches, lupins and serradella.
On the light, poor sands of Saxony Herr Schultz, of Lupitz, made use of serradella, yellow lupins and vetches as green manures for enriching the land in humus and nitrogen, and found the addition of potash salts and phosphates very profitable for the subsequent growth of potatoes and wheat.
Above this belt the firs gradually disappear and are succeeded by the shortleaved Pinus montezumae, or Mexican " ocote " - one of the largest species of pine in the republic. These continue to the upper tree-line, accompanied by red and purple Pentstemon and light blue lupins in the open spaces, some ferns, and occasional masses of alpine flowers.
That the Leguminosae (a group of plants including peas, beans, vetches, lupins, &c.) play a special part in agriculture was known even to the ancients and was mentioned by Pliny (Historia Naturalis, viii.).
Soil which had been cultivated for many years as pasture was sown with lupins for fifteen years in succession; an analysis then showed that the soil contained more than three times as much nitrogen as at the beginning of the experiment.
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