Changes with season of nutrients in salad greens grown in high tunnels Martin P.N. Gent Dept of Forestry & Horticulture The Connecticut Agricultural Experiment Station PO Box 1106, New Haven, CT 06504 Phone: 203-974-8489 E-mail: Martin.Gent@ct.gov Growth and composition of leafy vegetables varies with season or time of year. This seasonal variation is related to changes in light and temperature that affect metabolism in leaves and roots, and the rate of chemical transformation of nutrients in the soil. The variation of nutrient availability with season may depend on whether the crop is grown with nitrate-based fertilizer or organic fertilizer. Similar yields are generally obtained with organic and inorganic fertilizer. In field trials, compost or mineral fertilizer gave equal yield for lettuce, cabbage, carrots, tomatoes, and peppers. Sometimes compost amendment increases the concentration of nutrients in leaves, compared to inorganic fertilization. Different species of salad greens react differently to fertilizer. Nitrate in mustard and turnip greens rose with N application up to 100 lbs acre, whereas nitrate in lettuce reached a maximum at 50 lbs, and composition of kale did not have a simple response to applied N. Although the leaf nitrogen did not vary among lettuce, spinach, and cabbage when grown with little or no fertilizer, it varied substantially among these species when excess N fertilizer was applied. Because high tunnels allow growth later in the year, and at times when there is more extreme variation of day to night temperature, growth in high tunnels may exaggerate seasonal variation in composition of lettuce and other salad greens. I examined the growth and composition of crops grown in high tunnels and harvested at approximately one month intervals from September to June. Seven species of greens were grown either with organic fertilization, namely leaf compost amended with cotton seed meal, or inorganic fertilization, namely perlite watered with a complete nitrate-based nutrient solution. Environment Day-time average temperatures in high tunnels ranged from 40 to 70 o F in winter to 90 o F or more in summer. Night-time temperatures were below freezing from late November to early March. These cold temperatures were the primary limitation to growth in winter. Night-time temperatures in the high tunnels were only 2 or 4 o F degrees warmer than ambient, whereas day- time temperatures were 20 o F warmer than ambient throughout the year. Day-time temperatures were correlated with sunlight, but on a weekly basis, the maximum and minimum temperature lagged about one month behind maximum and minimum sunlight integral. Temperatures were warmer in fall than in spring at a given light level. Growth Growth in high tunnels was most rapid in May and September, 17 to 21% per day averaged over all species tested. Reasonable growth rates were also seen in April and October, 12 to 14% per day. Growth in the open field was much slower at this time of year, typically 5 to 10% per day. Growth in high tunnels slowed substantially in the fall, from about 15% in early October to 5 to 7% in late December. Quality of leaves was good, except when plants experienced sudden extreme cold after a period of mild weather. Growth in winter was extremely slow, between 0%