----------------+--------------------------------------

--------
|        NITROGEN IN AN ACRE OF CLOVERS.
+---------------+----------------+-------------
| Lbs. in tops. | Lbs. in roots. | Lbs., total.
----------------+---------------+----------------+----------

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Crimson Clover  |    125.28     |     30.66      |   155.94
Mammoth Clover  |     67.57     |     78.39      |   145.96
Red Clover      |     63.11     |     40.25      |   103.36
----------------+---------------+----------------+----------

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A large part of the nitrogen found in these plants was undoubtedly taken by the roots from the soil air.

Besides adding humus and nitrogen to the soil the legumes, being mostly deep-rooted plants, are able to take from the subsoil food which is out of reach of other plants.  This food is distributed throughout the plant and when the plant is plowed under the food is deposited in the upper soil for the use of shallow-rooted plants.

BENEFITS

The benefits derived from green crop manuring then are as follows: 

We add to the soil organic matter or humus which is so helpful in bringing about the conditions necessary for root growth.

By using the legumes for our green manure crops we may supply the soil with nitrogen taken from the air.

We return to the surface soil not only the plant food taken from it but also plant food brought from the subsoil by the roots of the green manure plants.

CHARACTER OF BEST PLANTS FOR GREEN CROP MANURING

The plants best adapted to green crop manuring are deep-rooted, heavy-foliaged plants.  Of these the legumes are by far the best, as they collect the free nitrogen from the air which other plants cannot do.  This enables the farmer to grow nitrogen which is very expensive to buy.

THE TIME FOR GROWING GREEN MANURE CROPS

Green manure crops may be grown at any time that the soil is not occupied by other crops, provided other conditions are suitable.  Land which is used for spring and summer crops often lies bare and idle during fall and winter.  A hardy green manure crop planted after the summer crop is harvested will make considerable growth during the fall and early spring, and this can be plowed under for the use of the following summer crops.  If there is a long interval of time during spring or summer when the land is bare, that is a good time for a green manure crop.

Green manure crops are often planted between the rows of other crops such as corn or cotton at the last working of the crop for the benefit of the crop which is to follow.

It is advisable to arrange for a green manure crop at least once in three or four years.

LEGUMINOUS GREEN MANURE CROPS

Cowpea. (Field pea, stock pea, black pea, black-eyed pea, clay pea, etc.) (Fig. 79.)