Horticulture: June 2004

What do investing in the stock market and fertilizing your garden have in common? The answer is timing. While I don't pretend to know much about the stock market, I do appreciate the importance of timing when it comes to applying nitrogen fertilizer to a growing garden.

A successful crop of tomatoes produced by applying small doses of nitrogen on a weekly basis.

Under most situations, all the required nutrients, as recommended by a soil test, should be applied and incorporated into the soil prior to planting. The only exception is nitrogen. Only a portion (30 to 40 percent) of a crop's total nitrogen requirement should be present in the soil at planting. For most vegetable crops, this is equivalent to 40 to 50 pounds of actual nitrogen per acre or about two-thirds cup of 34-0-0 per 100 square feet.

Too much nitrogen applied prior to planting or shortly after planting can result in delayed fruiting. Every summer, I receive calls from gardeners frustrated by the lack of fruit on their tomato plants. The plants are large and the foliage is dark green, but the plants yield poorly. This is a classic example of an excessive amount of nitrogen fertilizer being applied too early to the plants.

As plants grow, their demand for and ability to use nitrogen increases, peaking during reproduction (fruiting). Matching fertilizer application to a crop's demand and ability to properly assimilate nitrogen is the most efficient way to fertilize. This is best accomplished by using a drip irrigation system to deliver small, frequent (usually weekly) doses of soluble nitrogen fertilizer to the plant's root system, a process referred to as "fertigation."

Applying small amounts of fertilizer through a drip system is much more uniform than attempting to do so by hand. If your beds are covered by plastic mulch, fertigation is your only option as the film acts as a barrier to surface applied fertilizer.

Applying weekly doses of fertilizer is also easier on your pocket book. The nitrate form of nitrogen, the form most utilized by plants, is highly water soluble and is susceptible to leaching below the root zone as a result of heavy rain or irrigation. The risk of losing a significant amount of nitrogen as the result of any one rain or irrigation event is minimal when nitrogen is applied in small doses.

Once you've decided to fertigate your garden, you'll need to select a fertilizer injector and obtain a fertigation schedule. For information on using an injector and a fertigation schedule for 16 popular vegetable crops, contact the Noble Foundation at (580) 224-6500 and request a copy of Permanent Raised Bed Gardening, Volume II.