Agriculture Reference
In-Depth Information
Nitrogen
Lastly, we come to nitrogen. It's the basis for all amino acids and proteins in our bodies
and the bodies of all living things. Interestingly, while the air is 78% nitrogen, we can't get
nitrogen directly from the air and neither can plants. Luckily, certain microbes can. While
rainfall and lightning bring some of this nitrogen down into the soil, nitrogen-fixing bacter-
ia and actinomycetes provide most of it. As we've seen, they take nitrogen out of the air
and convert it into other forms of nitrogen they can use and plants can use.
Nitrogen exists in many different molecules, generally combined with hydrogen or oxy-
gen. While plants take up complex nitrogenous proteins and amino acids, the two main
forms they use are the relatively simple ammonium (NH
4
+
) and nitrate (NO
3
-
). Most food
plants use ammonium in small amounts, especially towards the fruiting and flowering
stage. It's mainly produced when microbes decompose organic matter such as leaves. Since
ammonium has a positive charge, it's held on cation exchange sites, although relatively
weakly.
Some of the soil ammonium is converted to nitrite (NO
2
-
) and then nitrate (NO
3
-
) by
different groups of bacteria. The hydrogen coming out of this process kicks cations off the
exchange sites. We don't want too much of that to happen at once, so we don't want to add
too much nitrogen at once.
If the soil is dominated by fungi, much of this ammonium won't get converted to nitrate.
This is great if you're trying to grow perennials, trees and shrubs, which prefer more am-
monium. If you're trying to grow annual vegetables and grasses, they mostly want a soil
dominated by bacteria, and consequently more nitrate.
Nitrate is the main form most crop plants use and this is what certain microbes manufac-
ture from other forms of nitrogen. Most of this work happens during the growing season as
temperatures get warmer, just when plants need more nitrogen. This is good because nitrate
would leach away very quickly if the microbes made too much. A lot of nitrate nitrogen
leaches away after application of chemical nitrogen, or also with the application of too
much fresh manure. Nature knows the right amount to make available, but to get the best
crop, it's often necessary to supplement some nitrogen.
But chemical nitrogen fertilizers are overused in conventional gardening and agriculture,
especially early in spring before the microbes are ready to use the nitrogen. This is when it
leaches down through the soil to pollute the ground water and consequently isn't available
for the last few weeks before fall harvest. While nitrogen fertilizers are often overused, it
doesn't mean we shouldn't be thinking about how to supply sufficient nitrogen to the soil
and microbes.
Nitrogen in the soil is like the electrolyte in a battery. We need enough to provide energy
to the soil to get everything else working. Like phosphate, nitrogen also transports nutrients
