Elements in making plants grow

Published 6:00 am Sunday, February 24, 2019

Terry Rector

Of the 16 chemical elements all plants must have to grow, three are in the air in large amounts. Thus we don’t need to fertilize plants with carbon, hydrogen and oxygen. Plants breathe in plenty of those three. 

And since air is about 78 percent or so nitrogen gas, you might think plants should get plenty of nitrogen that way also. Trouble is atmospheric nitrogen is not in a form plants can intake. 

Now, there are plants we know as legumes that convert this nitrogen gas from the air into forms that plants do use. 

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Legume plants have structures on their roots that utilize certain soil bacteria species in the conversion of unusable air nitrogen to usable soil nitrogen.  Most plants, however, are not legumes. 

Non-legume plants growing right alongside legumes can share in the legume nitrogen. That’s why years ago some gardeners mixed peas and corn in the same row; the pea legumes made nitrogen for both. 

The other way legume nitrogen gets passed around occurs when legumes die or are plowed into the ground.  The plants’ leftover nitrogen stays in the soil for a while. 

Another source of natural nitrogen plants use is animal manure, from wild animals in the wild and farm animals in gardens and some crop and hay fields. 

Manure from chicken houses supplies nitrogen in many hay fields in Mississippi and swine manure is still an important nitrogen fertilizer for corn in Iowa.

The decay of all plants and animals produces nitrogen that plants can use. That is why organic matter in soil matters. 

In undisturbed, unfertilized soils such as those of forests, decay and organic matter are essentially the only nitrogen sources. 

Unfertilized grass areas are likewise dependent on the natural nitrogen sources.  We help Nature along, and vice versa, by adding a little nitrogen to garden soil by way of compost, aged gin trash and peat moss.

The two forms of nitrogen important for plants are ammonia and nitrate.

Legumes produce nitrate. Manure and decaying plants and animals produce ammonia. 

The Nitrogen Cycle is the conversion of ammonia back into the nitrate that plants actually utilize. 

The most common manmade nitrogen fertilizer, ammonium nitrate, is created by mixing atmospheric nitrogen with hydrogen under pressure and heat.  Essentially, ammonium nitrate is made from air and natural gas. Nowadays, the fertilizer is often altered or diluted to avoid the threat of ammonium nitrate being used illegally as an explosive. 

Another manufactured fertilizer is ammonium sulfate, made from ammonia and sulfuric acid. This one is especially useful in soils that are too alkaline since the sulfur will, over time, make them a tad more acidic.

As usual, too much of a good thing can turn out bad. If we add too much nitrogen fertilizer to plants, we can wind up with lots of lush, green growth and few tomatoes or zinnia blooms.  Excess nitrogen keeps plants in the growth stage too long instead of moving into the reproductive stages of blooming, fruiting and making seeds. 

Terry Rector is spokesman for the Warren County Soil and Water Conservation District.