Nitrogen is very important to insure the growth of vine without which a good crop may not be expected. Lands vary more widely in nitrogen content than in phosphorus and potash. Sandy soils are commonly deficient in this element and often difficult to keep supplied. Here liberal applications are needed. Up to a hundred pounds[5] of actual nitrogen may prove profitable where other conditions justify. Heavier soils, well managed and manured during rotation, require less nitrogen and fair results may be obtained with no fertilizer where investment must be kept to a minimum.
Form of nitrogen to be used is largely a matter of economy though nitrate for part of it may be desirable early in the season when soil is cold and nitrification slow. Nitrate is desirable for side dressing but even here ammonia and other forms are now considered suitable when the soil is warm.
Why do tomatoes sometimes run to vine with failure to set fruit? This is an old, old query and, since 1918, has been, directly or indirectly, the occasion of more research projects than any other horticultural topic. Kraus and Kraybill[6] set the ball a-rolling with a paper which called attention first to the observations of Klebs in Germany in which he emphasized the fact that external conditions influence conditions within the plant which in turn influence performance--a veritable chain of causation. Kraus and Kraybill then undertook to relate performance (vegetative growth and fruitfulness) to internal conditions, chiefly carbohydrate and nitrogen content of the plant tissues. These, in turn, were traced back to treatments applied to the soil.
They suggested four combinations of vegetation and fruitfulness in plants as follows:
1. Non-vegetative and non-fruitful. Plants whose carbohydrate supply has been cut off, say by removal of leaves which make carbohydrates. These plants were low in carbohydrate and high in nitrogen.
2. Vegetative and non-fruitful. These plants were well supplied with both carbohydrates and nitrogen. They were of the sort we describe as having "run to vine."
3. Vegetative and fruitful. These plants were well supplied with carbohydrates, but not so liberally supplied with nitrogen, thus, providing a balance between the two that was favorable for a good crop.