but a row containing parsnips, cabbages, and lettuce would be a very faulty combination.  One part of the area should be set aside for all similar crops.  For example, all root crops might be grown on one side of the plot, all cabbage crops in the adjoining space, all tomato and eggplant crops in the center, all corn and tall things on the opposite side.  Perennnial crops, as asparagus and rhubarb, and gardening structures, as hotbeds and frames, should be on the border, where they will not interfere with the plowing and tilling.” ("Principles of Vegetable Gardening,” page 31.)

Usually where large acreages are worked there is a tendency to devote a greater portion of tile land to one crop and sometimes a failure in this crop will mean ruin to the farmer, whereas, where small areas are used, there is generally a diversity of the higher-priced crops and a failure in one is not so likely to be disastrous.

To get the greatest production from the soil two crops can be grown in the same soil at the same time—­one of which will mature much earlier than the other, thereby giving its place up just about the period of growth when the second crop would need more room.  This is known as companion cropping.

“In companion cropping there is a main crop and a secondary crop.  Ordinarily the main crop occupies the middle part and later part of the season.  The secondary crop matures early in the season, leaving the ground free for the main crop.  In some cases the same species is used for both crops, as when late celery is planted between the rows of early celery.

Following are examples of some companion crops: 

Radishes with beets or carrots.  The radishes can be sold before the beets need the room.

Corn with squashes, citron, pumpkin, or beans in hills. 
Early onions and cauliflower or cabbage. 
Horseradish and early cabbage. 
Lettuce with early cabbage.”

("Principles of Vegetable Gardening,” page 184.)

If fruit trees be planted, vegetables may be grown in rows.  As soon as the early vegetables mature they are removed, and a midsummer crop planted.  These are followed by a fall or winter crop.

Radishes, lettuce, and cabbage grow at the same time and on the area formerly used for one crop.  Early potatoes and early cauliflower are followed by Brussels sprouts and celery, two crops being as easily grown as one by intelligent handling.  The best beans are grown among fruit trees.

The principles of “double-cropping” are summarized by Professor Thomas Shaw, in The Market Garden.

“Onion sets may be planted early in the season and onion seeds may then be sown.  Between the rows cauliflower may be planted.  Later between the cauliflower, two or three cucumber seeds may be dropped.  The onion sets up around the cauliflower may be taken out first, and the cauliflowers in turn may be removed in time to let the cucumbers develop.