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Understanding Soil Nitrogen


Soil Savvy

By Gurbir Dhillon

hole in soil
A soil sample location.

Understanding soil nitrogen and plant interaction/uptake can help crop growers make decisions around healthy soils.

Most of the nitrogen in soils occurs in the form of organic molecules. Inorganic nitrogen forms generally account for 1-2% of total soil nitrogen unless land managers apply large amounts of nitrogenous fertilizers to soils.

Crops uptake the inorganic forms of nitrogen, including nitrate (NO3) and ammonium (NH4+) ions from the soil solution. While a relatively equal mixture of these forms of nitrogen is considered optimum for the growth of most crops, the effect of both these forms on the soil pH is variable. Negatively charged nitrate ions exchange with bicarbonate (HCO3) or hydroxyl (OH) ions, while positively charged ammonium ions exchange with hydrogen ions around roots in the soil solution.

Thus, nitrate ions raise the pH while ammonium ions lower the pH of soil solutions. These pH changes may potentially affect plant uptake of other nutrients such as phosphates. The inorganic forms, particularly nitrates, are soluble in water, unlike the organic forms, and can be lost through processes such as leaching through drainage water. The inorganic forms may also be lost through their conversion into gaseous forms through the process of volatilization. Nitrogen in organic forms is largely unavailable for plant uptake but helps retain N in soils since the organic compounds are not susceptible to the processes, including volatilization and leaching. More details about these processes will be discussed in the following articles of this series.