Although we started our variety trials in channels without any substrate, it became evident through other experiments that better results could be obtained by growing the lettuce plants, usually seeded in Jiffy 7's, on a layer of 2-inch of perlite in the channels. This often resulted in larger and better shaped heads and nearly always in a lower incidence of tipburn. Therefore, we recommend the use of perlite as a substrate, and most of our variety trials were done with perlite in the growing beds.

We found as important advantages of the nutrient flow technique that fungus diseases were only rarely encountered; that a very clean product is harvested; and that each crop could be planted immediately after the previous one had been harvested. In spite of omitting any disinfect ion, diseases have been no problem at all, even though many successive crops were grown on the same perlite.

In order to increase the profitability of greenhouse growing, as little growing space as possible should be wasted. Since the nutrient flow technique enables us to do so, we thought initially that it would be possible to utilize space better by growing lettuce in vertical pipes. However, time and time again it appeared that growth was very uneven because of differences in light conditions at various heights. Therefore, a different approach was tried by eliminating as much as possible the large amount of space which is wasted when the plants are young.

In a conventional crop planted at, say, 7.5 inches x 9 inches the young plants occupy only a small fraction of the growing area and they will gradually fill in the open spaces. The nutrient flow technique, however, offers the possibility to gradually move the plants apart, at least in one direction, by the use of movable growing channels. This is (was) explained in various illustrations.

The greenhouse used in this experiment was about 25 feet X 48 feet and was divided into four sections about 9, 11, 12, and 15 feet long. Lettuce seedlings were planted at 9-inch distances in growing channels which consisted of aluminum downspouts cut lengthwise through the narrow sides and lined with black polyethylene. These channels were placed next to each other in the first section which had a manifold with outlets at 3.5 inch distance. When the open space was filled up by the young plants --- after one or two weeks --- these channels were moved to section two where the outlets of the manifold were 4.75 inches apart. The first section was occupied by a group of channels in which seedlings were planted which had been seeded one or two weeks after the first batch of plants.

Another one or two weeks later the first batch of plants was moved to the third section where manifold outlets were 6 inches apart, the second batch was moved to the second section and the first section was again occupied by young seedlings.

This procedure was repeated a fourth time, which filled up all four sections in the greenhouse with successive age groups with spacings of 3.5, 4.75, 6, and 7.5 inches between channels and 9 inches within each channel. By doing it this way instead of having all plants at distances of 7.5 X 9 inches from the start, the number of plants in the greenhouse was increased by more than 35%. By modifying the spacings it may even be possible to achieve somewhat larger increases. Essential for this method is that seeding and harvesting take place periodically, starting the crops out at one end of the greenhouse and keeping them moving at intervals to the other side where they are harvested. Once this procedure has been established, it can continue year-round.

By utilizing the right varieties at each time of the year, by continuous cropping, and by high density culture considerable quantities of butterhead lettuce can be harvested from a certain area. By making use of movable growing channels, as outlined above, the number of plants per square foot can be increased from 2.2 to 3.3 in the actual growing area. In practice, when paths are included, the numbers of lettuce heads per square foot are more likely to be from 2.0-2.75.

If four successive seedings which fill up the four sections of the greenhouse are considered to constitute one crop, 8-10 consecutive crops per year can be grown under our conditions. This would mean that 20-25 lettuce plants per year can be harvested per square foot. Although in this way, a staggering number of plants per acre would be harvested over the whole year - close to a million - there is no reason to believe that this cannot be achieved.