Oxygen, heat and moisture are the requirements for germination, but successful results are largely dependent upon proper soil selection and preparation. Many kinds of seeds cannot germinate in stiff clay soils which are devoid of humus. Air can scarcely enter such soils. The largest seeds may germinate in extremely heavy soils, but most of the garden seeds demand a soil of the best physical condition and of fine texture. The failure to get a good stand of plants is often due to clods and coarse particles of earth, which cause the soil to dry out quickly and which prevent it from coming into direct contact with the seeds. This is a, matter of prime importance. In a well-prepared soil each particle is surrounded with a film of water, and when a large number of these are in contact with the seed ample moisture is supplied for germination. A fine seed bed, therefore, is necessary for a high percentage of germination. This applies to sowings made under glass as well as in the open. Seed beds are generally improved by the application of rotten manures, and sand may also be used to advantage in heavy soils. Well-prepared soil is loose and friable and takes water readily, while a certain degree of firmness is also essential. When fertilizers are used before sowing, they should be mixed thoroughly with the soil, to prevent a considerable quantity from coming in direct contact with the seeds and causing serious injury.
Proper moisture conditions previous to sowing may be secured in various ways. Fall plowing is often a great advantage in this respect, particularly in the heavier types of soil. There should be no unnecessary loss of moisture after spring or summer plowing, which can be avoided by prompt and thorough harrowing. When seed is to be sown rather late in the season the moisture can be conserved by harrowing at frequent intervals. Another excellent plan, although somewhat troublesome but practicable on a small scale, is to mulch the beds heavily with strawy manure, the coarse particles of which are removed by raking immediately before sowing. Soils which have received heavy annual dressings of manure are seldom too dry for the successful germination of seeds. Although moisture is essential, a surplus is just as disastrous as an insufficient amount.
A suitable temperature is required for each class of seeds. Lettuce, onion, beet, cabbage, cauliflower and many other kinds of seed will germinate at a temperature of 50 degrees F., or even less, although higher temperatures will cause no harm. The seeds of many vegetables, as the tomato, eggplant, bean, pepper and the cucurbits, require much higher temperatures, and they soon rot in cold, damp soils.
Experience counts for more than anything else in determining the proper dates for sowing different kinds of seed. A great many factors must be considered, but one of the most important is market conditions. When will a given crop be most likely to command the best prices, and how many weeks or months will be required to get the crop ready for that particular time? Weather conditions must be regarded. Lettuce, cauliflower, cabbage, onion, radish, spinach, and peas may be sown as soon in the spring as the ground can be prepared. This will not do for pepper, eggplant, tomato, bean, melon, squash and seeds of other tender plants, for these must not be planted in the open until the ground is thoroughly warm and there is no danger of frost,
It is always better to sow after rain rather than before, and this is especially true in the heavier soils. The soil soon bakes after a rain, and a hard surface crust is fatal to the germination of delicate seeds. Such crusts exclude air, and thus prevent the necessary physical and chemical changes in the soil, and make it difficult for the tiny plant to force its way through the soil to the light. If heavy rains fall soon after sowing, the crust can often be broken to advantage by rolling as soon as the ground is dry enough. It is usually desirable to sow in freshly stirred soil, because of better moisture conditions.
Many questions must be taken into account when determining the proper amount of seed to use on a given area. Among them may be mentioned: (1) The viability of the seed or its power to grow. This should be previously determined, and the rate of sowing regulated accordingly. (2) The date of sowing. It may pay to take chances in planting some crops, as beans and sweet corn, before the ground is warm enough to make certain of a high percentage of germination. By using seed freely a good stand may be secured (3) The physical character of the soil. More seed should be used in heavy soils, because the percentage of germination will be necessarily less than in light soils. (4) The size or vigor of the young plants. Carrot and parsnip seedlings are very delicate and feeble; and many may be lost before they are well started. Therefore the safe practice is to insure a good stand by heavy seeding. (5) If to be transplanted, the time when this work will be done should be considered. The seed may be sown much more freely if transplanting is to occur in about three weeks from sowing. (6) The demand of the market. At times the market may demand small carrots, onions and other products, or exactly the reverse, and sowing must be regulated accordingly. (7) It is rather expensive to thin some crops. For example, the thinning of onions is very slow and tedious, and therefore great care should be taken to sow just the right amount of seed. (8) Ravages of insects. Insects are usually very destructive to certain plants, as melons and cucumbers, and by using plenty of seed there will be greater certainty of saving enough plants to make a satisfactory stand. It generally pays to use seed freely and thin when necessary.
This is an important operation in growing many garden crops, and it is often practiced in starting plants under glass, but more frequently in open-ground culture. Thinning is a process of selection; the weakest plants should be discarded, and only the most vigorous left to mature. This is one of the strongest arguments for thinning. Thinning secures a uniform stand. Because the operation is tedious and expensive successful gardeners endeavor to avoid it as much as possible by the even distribution of the proper quantity of seed.
There are no infallible rules to determine the proper depth for sowing. Certain writers have advocated the regulation of soil covering by the diameter of the seed; that is, by making the covering two, three or four times the diameter of the seed. Such rules may be of some value when sowing in the greenhouse or hotbed, where moisture and soil conditions are under control, but they are likely to be misleading when applied to field conditions. The size of the seed should be considered, and also the character of the soil. In light, sandy soils the depth might be several times as great as in heavy soils. Summer and early fall sowings require greater covering than early spring sowings, because the surface layer of soil is drier, and it is necessary to place seed at a greater depth to secure the necessary amount of moisture. Some of the smallest seeds, as celery, are often merely pressed into the soil. A very slight covering is sufficient for many seeds when the best conditions are provided. The reader should consult the chapter on the culture of the various classes of vegetables to obtain more definite information on the proper depths for planting.