One-crop system, 221.
effect on fertility, 221.

Onion, 6.

Organic matter,
in soils, 32, 62, 220.
value of, 61.

Osmose, 18.

Ovary of flower, 130.

Ovules, 130.

Oxygen in plants, 66.


Paris green to destroy chewing insects, 117.

Parsley family, 86.

Parsnip root, depth of growth, 13.

Parsnips, 4.

Pasture, soils for, 30.

Pea family, 86.