I know I have written about the loss of nitrogen in our soil before, but with all the rain we've had, I thought it would be a good idea for a reminder.
An article based on a survey by the University of Missouri soil test laboratory revealed a trend that most soil tested from vegetable gardens showed soil phosphorus (P) and potassium (K) are becoming very high, and soil pHs are also on the rise.
The reason for the elevated P and K is most likely because vegetable gardeners have the tendency to seasonally or annually apply "balanced fertilizers," those that have nitrogen (N), P and K. While the nitrogen is leached by water or can evaporate or disperse in vapor into the atmosphere, P and K adhere to the soil tightly, thus they "build up." Use of organic fertilizers like manure or compost won't change a build-up from occurring, as most organic fertilizers are balanced as well.
If your garden does not seem to be growing as well as in the past, have your soil tested, and then focus on only supplying the needed nutrients prescribed by the test. Increasing organic matter, if less than 5 percent, will also help. For established vegetable gardens where nutrients have built up, the focus may be on just one nutrient, nitrogen.
Synthetic fertilizers that are slow or time released are desired by many gardeners, as they are less likely to "burn" plant tissue (either roots or foliage). Some specialty turf products are now available with only nitrogen. However, balanced formulations are more common.
An option is to select slow or time-release fertilizers that are highest for N and lowest in P and K. Often, these fertilizers are marketed for turf applications but can also work well in gardens. Formulas in the range of "22-5-6" and "25-2-5" are available and will work.
There are several organic options. First, when you have the time, grow a legume cover crop and till it under about a week or two before planting. Of course this takes quite a bit of planning and might be good to plan this for next year's garden.
Legumes fix nitrogen with the root nodules and it is accumulated in the green growth, which, when plowed down, is sometimes referred to as "green manure" and is a good source of nitrogen. Good sources of nitrogen organic fertilizers without P or K are corn gluten (9-0-0), blood meal (13-0-0), and feather meal (12-0-0).
The MU Guide 6220 "Organic Gardening Techniques" is a good source of information on organic fertilizers frequently used by gardeners and is available free online.
Most of the time, a deficiency of nitrogen will cause the plants to lose some of their color. They will not be a dark green as usual, even having more of a pale-green tint to them. If you do find the garden needing extra nutrients after the plants have started growing, side-dressing is the method to help them.
Side-dressing applies fertilizer to the soil at the side of a row of seed or plants. It can be done at planting or as an extra application later in the growing season to help provide a uniform supply of nutrients throughout the season. This is especially important if you are using chemical fertilizers because of their solubility and their tendency to "leach" down into the soil beyond the root zone before the growing season is over.
The usual rate for a side-dressing in the garden is 5 tablespoons per 10 feet of row of a high-nitrogen fertilizer, such as discussed above. Place about a 6-inch band of fertilizer on each side of the row, rake it in, then water. A combination of chemical fertilizer, organic fertilizer and mulch also works well.
The chemical fertilizers give the initial boost required by young plants, organic fertilizers provide nutrients uniformly throughout the season, and mulch keeps the soil more evenly moist and the nutrients more uniformly available.
Side-dressing is especially useful when growing sweet corn. Side-dress sweet corn when it is about 16 inches tall and again when it tassels out. Since corn is a heavy feeder, this will help you get a bumper crop.
Peter Sutter is a lifelong gardening enthusiast and a participant in the MU Extension's Callaway County Master Gardener program. Gardening questions can be sent to [email protected].