This section is from the book "Commercial Gardening Vol4", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
When Tomatoes are grown under glass it is essential to keep the atmosphere pure and fresh. This can only be done by proper ventilation Houses properly built will have plenty of ventilators along the top, near the ridge board, those on one side alternating with those on the other. Ventilators will also be let into the sides; and in large houses there may be a second set about midway between the ridge and gutters. During the summer months all the ventilators may be open to their full extent. In this way there will be a good but not draughty -circulation of fresh air, the movement of which carries the pollen from flower to flower, thus ensuring better fertilization of the pistils. If houses are kept too close and "muggy" the air is not so pure and the plants do not set their fruits so well or grow so quickly; they are also more liable to fungoid disease. Early in the season, of course, when the plants are young and only just beginning to make growth, ventilation must be regulated with a good deal of care. Cold draughts must be avoided, and, if a little air is given to keep the atmosphere fresh, the ventilators on the side away from the wind should be opened, those on the windward side being kept closed. The mildness or otherwise of the weather is always the dominating factor in ventilating Tomato houses, and the grower who is himself sensitive to weather changes will always see that his plants are kept in a genial atmosphere.
If the soil in which Tomatoes have been planted is a naturally rich and sandy loam, very little manuring will be required during the season beyond a mulching or topdressing of a similar compost, with a little well-decayed manure or leaf mould added to it. The remains of an old mushroom bed or old hotbed make an excellent topdressing, as the material keeps the roots moist, and at the same time yields up a certain amount of soluble food in the process of further decomposition. This is almost equivalent to giving repeated applications of weak liquid manure, and is perfectly safe. Liquid manure made from the droppings of cows, chickens, rabbits, sheep, etc, and given in a much-diluted state two or three times a week when the flowers are setting, will be of great assistance, but few market growers go to the trouble of making it.
When chemical manures are used they should be applied with care and judgment, and not recklessly, as one often sees. The chemical manures most generally recommended for Tomatoes are nitrate of soda, superphosphate of lime, kainit, and sulphate of iron. How to use these to the best advantage puzzles most growers, and the manures are often applied, as in the Channel Islands, on the "hit-or-miss" principle.
To apply chemical manures judiciously the grower must consider his particular soil and the appearance of his plants. Indeed the behaviour of the latter will be a very safe guide as a rule. If the foliage is large, luscious, and of a deep healthy green it may be concluded that the soil does not require any special nitrogenous manures like nitrate of soda or sulphate of ammonia. Leaves will only assume a fine healthy tint when there are already plenty of nitrates in the soil. If, however, the leaves are of a yellowish tint, and by no means luscious, it is a sure sign that nitrates are lacking in the soil. A pound, or at most 2 lb., of nitrate of soda or sulphate of ammonia may then be given to every square rod of soil (30 1/4 sq. yd.), taking care to spread it evenly over the surface, and having it watered in as soon as possible. Where these manures are not available, a good dressing of soot and a thin mulching of well-decayed manure will supply the necessary food in a perfectly safe form. If the plants do not respond readily to these manures, it may almost be taken for granted that the soil is also deficient in potash and phosphates both valuable foods (see Vol. I, pp. 156, 158). Phosphates may be applied by means of bone ash or bone meal at the rate of 4 or 5 lb. to the square rod; guano, 1 to 2 lb. to the rod; superphosphate of lime, 4 to 6 lb. to the rod; and basic slag, 4 lb. to the rod. The latter is an excellent phosphatic and lime manure, and should be mixed with the soil at the time of planting, as it is slow acting in its nature.
To secure a supply of potash, one of the cheapest manures is kainit. This, however, should not be applied to growing plants, as it contains so much common salt. It is best worked into the soil a few weeks before planting or potting takes place. The same remarks apply to muriate of potash, another highly concentrated potash manure. Sulphate of potash has come to the front of late years as an excellent manure, and when necessary may be given at the rate of 2 to 3 lb. to the square rod. Wood ashes, in addition to yielding up phosphates, are also valuable as a safe potash food, 1 ton containing as much as 150 to 200 lb. of potash. By burning vegetable refuse the grower can always have a supply of this manure at his disposal.
If a word of caution is necessary in regard to the use of chemical manures it may be given in regard to nitrate of soda. The frequent, and often unnecessary, use of this quickly soluble nitrate causes not only a softness and sappiness in the stems and leaves that predisposes them to attacks of fungoid diseases, but also engenders a softness and tenderness in the skin of the fruits that causes them often to crack and to travel badly to market.