In forests, an assortment of microorganisms break down organic material at and below the soil layer. These microorganisms uptake and release various nitrogen compounds which eventually are used by trees. A common process is for the microbes to absorb inorganic forms of nitrogen and then produce organic forms, which the trees need. The concentration of inorganic forms of nitrogen (NH₄+, NO₃-) and organic forms (amino acids) were recorded (g/kg) in soils in 10 forest study plots in the spring. The amount of tree growth (number of board-feet) in the plot was estimated by season's end.

What are the individuals? What kind of variable is this? categorical? If we wanted to compare the number of trees among all ten plots, what would we calculate?