Fertilizer can extend the growing season of tomatoes grown in high tunnel

-- Jenna Meeks

Do you ever feel there just aren’t enough hours in the day? How about enough days in the growing season for your home-grown tomatoes? The use of fertilizer may extend the growing season for tomato plants grown in a high-tunnel, a greenhouse like structure heated by solar radiation (Figure 1).

High-tunnel (Photo: Jenna Meeks)

One way to measure the benefits of fertilizers on plant growth is by analyzing leaf reflectance data. Reflectance is determined using a specific instrument which can see more than the visible rays detected by human eyes. It also encompasses the invisible region and this is where cellular differences in plant leaves can be seen as variations in reflectance.

The normalized differential vegetation index (NDVI), calculated using two wavelengths, red in the visible region and infrared in the invisible region, is used as a proxy for plant vigor. Therefore, NDVI is a tool to determine if fertilized tomato plants are healthier than unfertilized plants as the normal growing season comes to an end.

I studied the effect of four fertilizer treatments: a) Control (no fertilizer); b) 200 pounds of nitrogen as NPK fertilizer; c) a substitution of 75% of the N requirement with compost and; and d) a substitution of 25% of the N requirement with compost. Reflectance values were recorded from each plant four times with an Alta II Reflectance Spectrometer and the data were analyzed in Microsoft Excel.

The control plot displayed a lower NDVI than the fertilizer treatments, suggesting fertilized plants maintain their health for a longer period than unfertilized plants (Figure 2). The treatments with compost showed higher vigor than the control plot for half of the sampling dates, yet the 200lb N treatment was consistently higher than the control. These analyses are helpful to growers as they develop their fertilizer plan and crop rotations in their high-tunnels.