A Growers’ Guide
By Eric Sideman, Ph.D.
Director of Technical Services, MOFGA
Local production for local consumption is a guiding principle for sustainable organic growing, but winter months are challenging for us in New England. Most growers hang up their tools and park their equipment, and consumers are left buying vegetables that have traveled thousands of miles in gas guzzling trucks.
One alternative to this scenario is the production of lettuce and other greens that do well under the low light levels and cool conditions of a winter greenhouse. This production method is catching on, because consumers quickly discover that the quality of local produce that they are accustomed to in the summer may even be greater in winter greenhouse vegetables. Growers, too, quickly recognize a profitable system.
The Maine Organic Farmers and Gardeners Association recently held a full-day workshop on this production method and invited experts who addressed nearly all aspects of production and marketing (see coverage in this issue of The MOF&G). One topic that was not covered was that of nitrogen accumulation by some crops when grown under low light intensity. This topic was very hot a few decades ago, when European growers as well as researchers at the New Alchemy Institute were first investigating winter production in greenhouses. I do not want the warnings of those reports to be ignored by farmers today, so I put together a review of the biology of nitrogen accumulation, the associated risks and the cultural practices that reduce accumulation.
Nitrate itself is relatively nontoxic to mammals, being readily absorbed and readily excreted. Under certain conditions, however, nitrate is reduced (loses electrons) to the compound nitrite in stored crops and in your gastrointestinal tract. The nitrite ion (an ion is a positively or negatively charged molecule, as opposed to a neutral molecule) is a health risk. Infants are particularly at risk for a few reasons. First, illness involving the gastrointestinal (GI) system in infants may permit the bacteria responsible for the reduction of nitrate to nitrite to move higher in the tract. Second, the pH in infants’ GI system is less acidic than in adults, encouraging such bacterial growth. Furthermore, infants’ hemoglobin and enzyme make-up differs from that of adults, so when the nitrite reaches the bloodstream, it reacts directly with infants’ hemoglobin to produce methemoglobin, which impairs oxygen transport and causes “blue baby syndrome.”
Adults are at risk too. When nitrate is digested, part is converted to nitrite and N-nitroso compounds (nitrosamines). Many nitrosamines are carcinogenic in lab animals.
Nitrate and nitrite occur widely in human and animal food, both as intentional additives (as in cured meats) and as undesirable contaminants. Nitrates are present in all plants and are an essential source of nitrogen for normal plant growth. Foods of plant origin will invariably contain some nitrate, therefore, but crops in the spinach family, leafy vegetables, bok choy, and endive can accumulate potentially unhealthful concentrations of nitrate. Some European countries have set maximum allowable levels at about 1,000 mg of nitrate per kilogram of fresh weight of the vegetable (1000 ppm). Unfortunately in six out of seven samples of lettuce taken from greenhouses in England during the winter of 1987-8, the New Alchemy Institute found nitrate in excess of 1000 parts per million. The worst cases occurred in conventional hydroponic houses.
Nitrates are also found in water, but vegetables may contribute 54% of the average dietary intake of nitrates. Reducing the risk from vegetables is up to growers. Understanding the nitrogen pathway through plants will help reduce its accumulation.
Nitrogen and Nitrate Accumulation
Nitrogen is an essential element in protein, and thus is essential for all plants and animals. Plants absorb nitrogen from the soil (or other growing media) mostly in the form of an ion called the nitrate ion (NO3 –). Before the nitrogen can be combined with the carbon-based compounds produced by photosynthesis to make proteins, the nitrate ion is changed (reduced) to the nitrite form (NO2 –). This chemical reaction is facilitated by an enzyme called nitrate reductase.
In general the amount of nitrogen assimilated through this pathway equals the amount of nitrate taken up by the plant from the soil. But when the nitrate uptake exceeds assimilation, nitrate ions can accumulate in cells, specifically in the vacuoles. Some “nitrate accumulators,” such as members of the spinach family, lettuce and other greens, are prone to this. Nitrate accumulation is greatest in the leaves and stems of plants.
Crops grown with excessive available nitrogen fertilizer accumulate nitrates, especially if the nitrogen fertilizer is primarily in the nitrate form. Vogtmann and his students showed that different fertilization regimes can affect the extent of nitrate accumulation by lettuce. Fertilization with farmyard manure compost leads to much lower nitrate concentrations in the plant tissue than fertilization with a soluble NPK fertilizer. The lower concentrations of nitrate observed with the compost treatments probably resulted from the slower and more balanced release of nitrogen available for plants through microbial activity in composted manure, compared with the rapid release of nitrogen from soluble fertilizer.
Nitrate accumulates not only when its concentration is excessive in soils, but also when plants are growing so slowly that they don’t metabolize it into proteins. So, in addition to having too much nitrate available, slow plant growth, even in soils with only moderate amounts of nitrogen, can also result in nitrate accumulation.
Crops grown under low light levels tend to accumulate nitrate even when they are not overfertilized. In the winter, when light levels are low and growth is slow, a relatively low concentration of nitrate in the soil can provide sufficient surplus to cause unhealthful concentrations of nitrate in plants. In the 1988 New Alchemy survey of greenhouse lettuce, nitrate concentrations ranged from 2000 to 3000 ppm, except in lettuce from one house where the crop was growing in Promix and the grower had neglected to fertilize for months.
Increased light levels can increase nitrate reductase activity in plants. (Nitrate reductase is the enzyme that facilitates the reduction of nitrate so it can be available for assimilation into protein.) In the dark or under very low light levels, the enzyme levels are very low, even when adequate levels of nitrate exist in plant tissue. That is why nitrate accumulates.
Interestingly, this influence of light causes a diurnal pattern of nitrate uptake and nitrate accumulation. Nitrate concentrations in the leaves and especially in the petioles of spinach increase during the night in the initial hours of darkness.
One other factor to consider is that long or poor storage can increase the amount of nitrite in crops. Vegetables that accumulate very high concentrations of nitrate may also contain significant amounts of nitrite as the result of microbial reduction of the nitrate during storage.
Practices Growers Should Consider
The practices that have been tried to reduce nitrate accumulation address include the source of nitrogen, level of light, growth rate of the plant and storage of the crop.
The rate, form and timing of fertilizer N applications all influence the uptake of nitrate by crops. Most organic growers use compost in their soil mixes, but anyone using a soluble N source—including some quick-release organic fertilizers—should be warned.
Since nitrate concentrations build during the night and decrease during the sunny part of the day if plants are growing, afternoon harvesting is recommended. Experiments at the New Alchemy Institute showed a 15 to 20% decrease in nitrate in lettuce harvested in the afternoon compared with those harvested in the morning.
When Steingrover exposed spinach plants to light during the night, nitrate uptake was reduced and the nitrate concentration in leaves did not increase. Even a single night with low light levels just before harvest could reduce nitrate concentrations.
Another practice that worked but may be impractical for large scale growers is removing the source of nitrate for a day before harvest. When New Alchemy Institute staff removed lettuce plants from the soil and washed the roots, then placed the roots in containers of water for a day before harvest, quality was retained but significantly less nitrate was measured in the crop.
The time of year, even during out-of-season production, had a great influence. In a greenhouse at the New Alchemy Institute, nitrate concentrations in crops reached a maximum in mid-November and declined thereafter. This was surprising since light levels are lower in January and February. It was explained by soil temperature because, as I pointed out, nitrate uptake and assimilation need to be balanced in order to avoid nitrate accumulation. Either assimilation has to be fast enough to keep up with uptake, or uptake has to be slow enough not to overload assimilation.
In November the soil is relatively warm, but light levels are getting very low. Thus, nitrate uptake is relatively good but assimilation slows way down, and the nitrate accumulates. In January and February, the soil gets so cold that nitrate uptake is minimal, and even though the use of the nitrate is very low (if it is even assimilated at all), the risk of accumulation is also minimal. In March the soil begins to warm, but the light levels apparently promote assimilation enough to limit accumulation. So late fall is the time of year that presents the greatest risk of nitrate accumulation.
It was years ago that organic growing methods using compost were shown to pose much less risk of nitrate accumulation than using soluble NPK did. Let’s keep our reputation and use the best practices in greenhouse production.
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