Page 128 - Soil and water contamination, 2nd edition

6

Nutrients

6.1 INTRODUCTION

Nutrients are defined as the raw materials that are assimilated by living organisms to
promote growth, development, and reproduction. The types of nutrients required and
the amounts in which they are consumed vary for the different plant and animal species.
In general, nutrients include proteins, carbohydrates, fats, inorganic salts and minerals,
and water. However, the term ‘nutrients’ is more commonly applied to essential elements
such as nitrogen and phosphorus . Plants need at least sixteen essential elements to grow.
They utilise oxygen , carbon, and water from air and soil. The primary macronutrients are
nitrogen (N), phosphorus (P), and potassium (K). The secondary nutrients include calcium
(Ca), magnesium (Mg), and sulphur (S). Micronutrients are absorbed in lesser amounts and
include chloride (Cl), boron (B), iron (Fe), manganese (Mn), molybdenum (Mo), copper
(Cu), and zinc (Zn). Some plants need some other elements, often called beneficial elements,
such as cobalt (Co), nickel (Ni), silicon (Si), sodium (Na), and vanadium (V). Most of these
elements are described in other chapters (Chapters 5 and 7); this chapter focuses mainly on
nitrogen and phosphorus.
The reserves of nutrients in soil and water often deviate from those needed by plants;
often, therefore, optimal plant growth is hampered because not enough of one or more of
these nutrients is available. Because the primary nutrients N, P, and K are so important for
plant growth, they are most often the limiting nutrients and are the main constituents of
fertilisers . In most cases, the most limiting nutrient is N or P. The ‘optimal’ molar N:P ratio
for plants, in which N and P are in balance and neither N nor P is limiting, is about 16:1
(Redfield et al., 1963). This corresponds to a mass ratio of 7.2:1, although some experts use
a mass N:P ratio of 10:1 (Thomann and Mueller, 1987). This implies that a mass N:P ratio
greater than 10 indicates limitation of P. In this case, plant growth is largely controlled by the
concentration of P. An N:P ratio of less than 7.2 indicates N limitation , which implies that
N controls plant growth. Deficiencies of the secondary nutrients Ca, Mg, Fe, and S are less
common, although plants may absorb large amounts of these elements.
An important property of nutrients is that they undergo a relatively rapid cycling within
ecosystems. Nutrient cycling includes all the processes by which nutrients are transferred
from one organism to another (see Figure 6.1). Nutrients are taken up by primary producers
(e.g. green plants, algae , and mosses) from the soil solution or surface water, whereby they
enter the food web and become part of living organic materials. The nutrients are further
transferred to higher trophic level s (i.e. the position of organisms in the food chain or food
web) by grazing and predation, until the organic material (the whole or parts of an organism,
litter, or faeces) dies and becomes detritus . The dead organic materials are decomposed by
microorganisms (bacteria and fungi), which use the organic matter for nutrition. Eventually,
the organic matter is mineralised, which makes the nutrients available again for primary
producers. Additional local inputs of nutrients come from atmospheric deposition , fertilisers ,
and human wastes. In addition, N in soil may come from the fixation of atmospheric N by
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