Hydroponics: Growing Vegetables Without Soil


Plant Science Day, August 5, 1998

By Martin P.N. Gent

Department of Forestry and Horticulture; The Connecticut Agricultural Experiment Station;
123 Huntington St.; P.O. Box 1106; New Haven, CT 06504

Hydroponics is a method of growing plants in which nutrients are supplied in a solution that is fed to the roots. Hydroponics may include the use of sand, rock wool, peat, or some other inert medium to support the roots. All the essential mineral elements required for plant growth are supplied in a solution that is used to water the root medium.. I use the term more particularly to mean growing plants with their roots suspended in a water solution. Lettuce can be grown by hydroponics in water solution. There are benefits to this growing method. The leaves are more succulent and thicker than those of plants grown in soil. The plants remain clean as there is no dirt to be trapped in the leaves. It is easy to transplant and space the plants as they grow. In addition, the composition of the plant tissue is more sensitive to the composition of the nutrient solution, as there is no inert medium to buffer the changes in solution composition. There is one drawback to hydroponic lettuce. When the sunlight intensity is low, as in mid-winter, when grown by hydroponics. In part this is because the energy from photosynthesis is not available to reduce nitrate to organic forms of nitrogen. In part it may be because nitrate accumulates in the nutrient solution when it is not required by the plants. In northern Europe in winter the nitrate concentration in lettuce grown in hydroponics can approach a level considered hazardous for human health. The European Economic Community allows up to 4500 ppm nitrate in fresh leaf tissue. In Holland scientists found that replacing nitrate with sulfate a few days before harvesting lowers tissue nitrate. However, this method would not be appropriate if lettuce at various stages of development were fed by the same solution, as is usually the case in commercial production. Do high concentrations of nitrate occur when lettuce is grown by hydroponics in Connecticut? Can this accumulation of nitrate be curtailed by modification of the nutrient solution? I have conducted experiments over the past three years to answer these questions.

In the spring and summer of 1996, I compared growth and composition of hydroponic lettuce grown in nutrient solutions which differed in the concentration of all the mineral elements. A solution containing 440 parts per million (ppm) or 7 milli-Molar nitrate, was compared to a half-strength solution in which all nutrients were diluted. In both solutions, the ratio of the various elements in the nutrient solution were the same as the ratio of elements found in the plant tissue. Thus as the plants took up the nutrients from solution, all the elements would be depleted to the same extent. Lettuce grown in the half-strength solution had less nitrate in plant tissue. However, plants supplied with the higher concentration of nutrients grew faster, and in both cases, the nitrate concentration in leaf tissue was relatively low, compared to reports for lettuce grown in Holland in winter. It was not clear that the plants in half-strength solution were growing slowly due to lack of nitrogen, as several of the other elements in the dilute nutrient solution were drawn down to a low concentration.

In 1997, I altered the ratio of nitrogen to the other elements in the nutrient solution using a three-part nutrient solution. The first two solutions contained all the necessary elements to support plant growth, supplied at the high concentration used in 1996, except the concentration of nitrate was one third less. The third solution, containing nitric acid, was added to one treatment to increase the nitrate to the concentration used in 1996. The concentrations of all other mineral elements were maintained in a constant proportion by feed-back injection of nutrients based on measurement of solution conductivity. I grew many plantings of lettuce using this protocol in spring and summer and fall of 1997. To expose the plants to a greater range of sunlight, I grew lettuce in a greenhouse covered with a shade cloth that lowered the light intensity to one half that outside, and continued plantings in winter and spring of 1998.

I expected the lettuce plants to take up nitrate and completely deplete the nitrate in the nutrient solution with one third less nitrate, as it should be the limiting nutrient. This did not happen in fact. Although uptake of nitrate by the plants reduced the actual nitrate concentration in the solution to less than 200 ppm, it remained far above the concentration that limits nitrate uptake into plants, which is on the order of 1 ppm. In the standard solution, the nitrate concentration was higher. It varied between 200 and 800 ppm. For both solutions, the concentrations were higher in winter than in summer. The concentration of all the other mineral elements remained at a level similar to that in the solution as it was originally made up.

When the plants reached a marketable size, the nitrate concentration in leaf tissue, varied from 1100 to 4400 ppm. Overall, the nitrate concentration in leaf tissue was about one third less in plants fed the nutrient solution with one third less nitrate. The tissue nitrate concentration was sensitive to temperature and sunlight as well as the nitrate concentration in solution. At the lowest sunlight levels in mid-winter when the greenhouse was shaded, the difference in solution nitrate was large because the concentration in the standard solution rose to 800 ppm, while with less nitrate supplied it remained below 200 ppm. When the tissue nitrate was plotted against solution nitrate, it increased in proportion up to about 400 ppm nitrate in solution, then leveled off at about 4000 ppm nitrate in the tissue.

In contrast, when lettuce was grown under a sunlight intensity typical of spring and summer in Connecticut, solution nitrate was nearly always below 400 ppm, regardless of amount of nitrate supplied. The concentration of nitrate was higher in the standard solution, 200 to 400 ppm, than in the solution which supplied one third less nitrate, 40 to 120 ppm. Under bright sunlight, the response of tissue nitrate to solution nitrate in solution was the same as under dim sunlight. The accumulation of nitrate in the standard nutrient solution, when supplied to plants grown under low sunlight, resulted in a high concentration in the leaves. This suggests the solution with one third less nitrate would be a better choice for production of hydroponic lettuce in Connecticut. Although the tissue nitrate was sensitive to the solution composition, the protein in leaf tissue, was affected relatively little by season or solution composition. It varied only between 35 and 50 mg nitrogen per gram dry weight.

The growth rate was defined as the final weight per lettuce plant divided by the number of days between transplant into the greenhouse and harvest. Growth was most highly correlated to sunlight intensity. It was as high as 50 grams per plant per week in summer, compared to 10 to 20 grams per plant per week in winter. The growth of lettuce was inhibited in summer but not in winter when the supply of nitrate was one third less than in the standard solution. In winter under a low sunlight intensity, there was no significant effect of the nitrate supply on the rate of growth, but in spring or summer under high sunlight intensity, the growth rate of plants supplied with one third less nitrate was 30 to 40 grams compared to 50 grams per plant per week when grown using the standard nutrient solution.

In summary, lettuce grown in hydroponics in a greenhouse in Connecticut never had nitrate concentrations in leaf tissue as high as the action level developed by the EEC, even when grown in a shaded greenhouse in mid winter. Although the tissue nitrate of lettuce grown in an un-shaded greenhouse was higher in winter than summer, it was generally less than 3300 ppm. Supplying one third less nitrate , in relation to the other mineral elements in the nutrient solution, effectively lowered the nitrate in leaf tissue. However, this technique should be applied with care, as it inhibited the rapid growth of lettuce under the high sunlight intensity in summer.