GARDENING : Get the Most Out of Air’s Nitrogen

Fertilizer is on every gardener’s list of things to pick up at thestore this time of year.

Ironically, nitrogen is the fertilizer nutrient generally needed most--yet the air contains about 80% nitrogen, 35,000 tons over every acre.

The problem is that the nitrogen in air is not in a form usable by most plants.

Legumes are plants that can use the nitrogen from the air. Peas, beans and lupines are garden legumes, while out in fields and lawns you will find clovers and vetches. Honeylocust and blacklocust are trees that are legumes.

In order to use atmospheric nitrogen, legumes must be infected with special strains of bacteria.

You can make sure that your peas and beans are infected with the right bacteria by buying a legume inoculum when you buy your pea and bean seeds.

You apply this black powder (the bacteria are mixed in dry peat) either by shaking it with moistened seeds before sowing, or by dusting it directly over seeds in the furrow.

As soil moisture stirs seeds out of dormancy, it also awakens the bacteria, which then proceed to infect newly emerging legume roots.

The roots respond by forming nodules, which are obvious to the naked eye on roots of plants dug up in midsummer.

Within the nodules, bacteria convert gaseous nitrogen from the air into soluble nitrogen, which is pumped up into the plant. In return, the host plant gives up a portion of its energy-yielding carbohydrates to the bacteria. The relationship is symbiotic--beneficial to both the host plant and the bacteria.

In one study with soybeans, inoculated plants yielded 67% more beans than did uninoculated plants.

There is some specificity between the strain of bacteria and kind of legume, although for vegetable gardens, a combined pea and bean inoculum is what is commonly sold.

A different strain would be needed for soybeans or for clover, though.

Soil inoculated with appropriate bacteria rarely needs reinoculation. Inoculation is needed for new gardens, or soils in which peas or beans have not been grown for many years, and in soils subject to heavy chemical use (either fertilizer or pesticides).

All parts of legume plants are rich in nitrogen. As an infected plant ages, old nodules and roots slough off and add nitrogen to the soil.

If the plants’ leaves and stems are dug into the soil after the plants mature, the soil is further enriched.

In traditional agriculture, legumes are used to supply nitrogen for succeeding crops. Modern agriculturists are attempting to utilize this symbiosis by growing corn and soybeans elbow in the same field.

To capitalize on free nitrogen from legumes on a back-yard scale, limit your use of nitrogen fertilizer on these plants. Too much nitrogen fertilizer is wasteful because it suppresses the bacterial symbiosis.

Check the roots of a few plants in midsummer to see if they are well-nodulated. If they are not, either the plants need inoculation, or you have overfertilized with nitrogen.

In your lawn, make use of this symbiosis by allowing or by sowing some clover with the grass. Under ideal conditions, clover has supplied up to 100 pounds per acre of nitrogen to grass in a pasture.

Even if the clover supplies only a quarter of that amount (which may be more realistic for a lawn), this is equivalent to spreading the nitrogen equivalent of more than 5 pounds of 10-10-10 fertilizer per thousand square feet of lawn.

In your vegetable garden, maximize use of “free” nitrogen by inoculating legumes and using them as part of a crop rotation. If your garden is large enough, set aside a portion of the area each season just for this purpose.

The actual amount of nitrogen added to the soil will depend on the plant and the environmental conditions.

On the average, a legume might add about one-quarter of a pound of nitrogen per hundred square feet.

This is equivalent to almost three pounds of 10-10-10 fertilizer, which is a typical recommendation for fertilizer nitrogen for a vegetable garden.