Page 33 - Nutrient Management Planning Guide

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Chapter 2.1

Figure 2.1.8 Phytic Acid

Phosphorus availability for plant uptake can be affected in a number of ways. For example, dissolved P in soil solution will react with soil constituents to form less soluble compounds. Sorption, or the retention of P on soil particles, makes P unavailable to plants for uptake. Sorption is thought to be the major mechanism responsible for decreases in soluble P. Desorption refers to P leaving soil particle surfaces and going back into solution.

Soil pH can also influence P availability. At high pH values (alkaline soils) Ca and Mg phosphates develop and at low pH values (acidic soils) aluminum and iron phosphates develop. These low-solubility products remove P from the soil solution since they are hundreds to millions of times less soluble than P fertilizers.

Water or wind erosion of topsoil is a major route of P loss. Soluble P can also be lost in surface runoff or through deep leaching under some circumstances. For example, when the P fixation capacity of the soil is exceeded and precipitation is greater than soil water

holding capacity, the result is a downward movement or surface runoff of orthophosphate and soluble organic P. Unlike N, there is no mechanism for gaseous P loss from soils.

Potassium

A small percentage of the total arable land on the prairies is K deficient. The highest proportion of these soils is in Alberta. Typically, the most severe deficiencies exist on the coarse textured (sandier) soils located within the Black, Dark Gray and Gray Luvisolic soil zones. The highest levels of plant-available K exist in the Brown and Dark Brown soil zones.

Unlike most essential nutrients, K is not an integral part of any plant structural component. However, it does play a role in many processes vital to plant growth. Among the functions of K in plants are enzyme activation, transport of sugars, plant water balance and regulation of stomata.

Potassium “activates” many different enzymes involved in plant growth. For example, photosynthesis as well as starch and protein synthesis are key pathways that rely on K-influenced enzyme systems.

The transport of sugars produced during photosynthesis also depends on K. Inadequate K can result in a build-up of photosynthetic products in the leaves, which can adversely affect the rate of photosynthesis and plant growth.

Potassium, along with sugars and other inorganic ions, influences the water balance within plants and helps to maintain an inward concentration gradient between roots and the soil solution. Potassium also influences the transpiration rate by controlling the size of the stomatal openings in response to environmental and internal plant conditions.