Page 30 - Nutrient Management Planning Guide

26

Soil Nutrient Cycling

Phosphorus

Less than 20% of the total P content of surface soils (0 to 15 cm) is plant-available. Consequently, P is regarded as the second most limiting nutrient in western Canadian soils.

One of the main roles of P in plants is the storage and transfer of energy. The high-energy phosphate bonds in molecules such as adenosine triphosphate (ATP) and adenosine diphosphate (ADP) drive virtually every biochemical reaction in plants (Figure 2.1.5). Phosphorus is also part of important structural components of plants, such as nucleic acids, phospholipids and coenzymes.

Figure 2.1.5 Summary of Processes and Functions Driven by High-Energy Phosphate Bonds in ATP and ADP

Why Fertilizer P and K Content is Expressed as Percent P

2

O

5

and K

2

O

Years ago, geochemists described mineral contents as the oxide forms of elements that form upon heating of a substance. When laws governing fertilizer usage were being developed, this standard was adopted (although these are not the actual forms that are in commercial fertilizers). To convert between P

2

O

5

and K

2

O and their elemental P and K forms, the conversion factors below can be used:

% P

2

O

5

× 0.44 = % P % P x 2.29 = % P

2

O

5

% K

2

O × 0.83 = % K % K x 1.2 = % K

2

O