Final answer:
Plant roots absorb ammonium easily from soil when it is produced through the decomposing organic material. Some soil bacteria can convert this ammonium into nitrates via the nitrification process. Moreover, symbiotic nitrogen-fixing bacteria can supply ammonium by converting gaseous nitrogen.
Step-by-step explanation:
Plant roots more easily absorb ammonium that has been produced from the decomposition of organic material. This process, called ammonification, involves decomposers that break down the organic compounds from dead organisms and waste, releasing nitrogen in the form of ammonium ions (NH4+) into the soil. These ions can be taken up directly by the plants, and some can be transformed into nitrates by soil bacteria in a process known as nitrification.
Nitrifying bacteria carry out this conversion through a two-step process where firstly, ammonium is converted to nitrite (NO2-) by bacteria such as Nitrosomonas, and secondly, nitrite is converted to nitrate (NO3-) by bacteria like Nitrobacter and Nitrospira. Although many plants prefer nitrate, ammonia from ammonification can be readily utilized by some. Additionally, symbiotic nitrogen-fixing bacteria, like Rhizobium associated with legumes, convert gaseous nitrogen (N2) into ammonium (NH4+), providing another source of absorbable nitrogen for plants.