3.4.1 Plant nutrients
Plant nutrients are necessary in varying amounts for the growth, reproduction and well-being of growing plants.
Of the major nutrients of plants, nitrogen and phosphorus are important growth-limiting factors in primary production (i.e. they are likely to run out before any other element needed by the plants). Both nitrogen and phosphorus enter watercourses from natural leaching by water of the soluble nitrates and phosphates found in soils and rocks, as well as from sewage effluent and agricultural sources. Phosphorus is the limiting growth factor for certain types of blue-green algae, and both phosphorus and nitrogen limit the growth of green algae.