Nitrogen Changes in the Soil, and it Changes Fast
At a recent meeting the question came up about the fate of nitrogen fertilizer applied through the irrigation system. If it is applied as urea, how long does it take to convert it to nitrate? If applied as ammonium, how long does it take to convert to nitrate? Urea and nitrate pretty much move wherever water moves and is very susceptible to leaching. Because of the positive charge on ammonium, it is not as mobile as nitrate, but once bacteria transform it to nitrate, it moves with water.
This is an important question, since if more water is applied than is needed by the plant, the nitrate is going to move out of the root system and no longer be available to the plant and ends up heading to ground water. Reading the literature, growers get the sense that all this transformation takes time, maybe a long time.
It turns out that soils in coastal California have a pretty rapid conversion of nitrogen. Francis Broadbent at UC Davis did a bunch of studies back in the 1950's and 60's and found enzyme hydrolysis of urea to ammonium occurring within hours. Other researchers have looked at nitrification, the conversion of ammonium to nitrate by soil bacteria, occurring within days and much of the conversion occurring within a week depending on soil temperature (see chart below).
So there is all this nitrate present and the key is what happens to it. It turns out that most plants when actively growing absorb nitrate at about 5 pounds of nitrogen per day. So with a 100% efficiency, applying 20 pounds of nitrogen, all of it would be taken up in four days. Of course, nothing in nature is that efficient. But the point is a big slug of nitrogen applied is not going to be taken up immediately and if more water is applied after that than is needed by the crop, it likely is pushed out of the avocado root zone.
Of course all the nitrogen a plant uses does not come from applied fertilizer. The bulk is coming from soil organic matter that is slowly decomposing. This nitrogen is being released at a rate that is probably in balance with the growth of the tree.
The applied fertilizer, however, is much more unstable and needs to be handled accordingly. The rule of thumb is to break the irrigation application into thirds. In the first third, run the irrigation to fill the lines and wet the soil. In the second third, run the fertilizer. This spreads it through the system and onto the ground. The last third is clear the irrigation system of the material and to move the fertilizer into the root zone. Then given time, the tree will take up the applied nitrogen. At the next irrigation then the bulk of that nitrogen will have been taken up and little will be pushed through the root system.
Low and High Nitrogen Avocado Leaves
Chart showing rapid conversion to nitrate with soil temperature