Page 25 - Nutrient Management Planning Guide

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Chapter 2.1

Sixteen mineral and non-mineral nutrients are essential for plant growth. The non-mineral nutrients—carbon (C), hydrogen (H) and oxygen (O)—account for approximately 96% of dry plant weight, mostly in the form of carbohydrates. The sources of C, H, and O in plant materials are carbon dioxide (CO

2

) in air and water (H

2

O). The energy that drives their conversion into plant material is derived from sunlight.

Mineral nutrients, classified as macro- or micronutrients, are usually obtained from the soil. The macronutrients— nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca) and magnesium (Mg)—account for approximately 3.5% of dry plant weight. Accounting for about 0.04% of dry plant weight are the micronutrients— chlorine (Cl), iron (Fe), manganese (Mn), zinc (Zn), boron (B), copper (Cu) and molybdenum (Mo). While this is widely regarded as the traditional list of essential micronutrients, some experts argue that elements such as nickel (Ni), silicon (Si) and cobalt (Co) should be included.

For the majority of prairie soils, N is typically the most deficient (e.g., first limiting) nutrient, followed by P, K, and S. Micronutrient deficiencies in western Canadian soils are rare compared to the size, extent and financial importance of N, P, K, and S deficiencies.

The Principle of the First-Limiting Nutrient

Plant growth will take place normally until it is restricted by the supply of an essential nutrient. A deficiency of any essential nutrient cannot be corrected by the addition of other crop inputs. This forms the basis of Liebig’s “Law of the Minimum”, which says that the level of crop production is limited by the nutrient in shortest supply.

Crop yield is determined by the supply of individual nutrients relative to their required levels for optimal yield. In Figure 2.1.1, the capacity of the barrel represents crop yield, which is limited by the height of the shortest stave of the barrel (e.g., the first limiting nutrient, which is N in this example).

Ion Absorption by Plant Roots

Generally, plants absorb essential nutrients from the soil in soluble, inorganic forms. Nutrients in organic form must be converted to inorganic forms prior to plant

Adapted from Brady and Weil, 2002

Figure 2.1.1 Conceptual Illustration of the Law of the Minimum, Showing Nitrogen as the Most Limiting Nutrient for Crop Growth