In this third article published in Groundswell magazine on potting mix properties, Geoff Cresswell explains how the physical, chemical and biological properties of a potting mix influence plant nutrition.
Plants require 16 elements for healthy growth and to complete their reproductive cycle (Table 1). These essential elements, are divided into major and minor elements based on the amounts required. In addition, there are other elements that are considered beneficial to plant health including silicon, aluminium, cobalt, selenium and sodium. Of these, silicon is possibly the most helpful for plants grown in soilless media because they are generally low in plant available silicon. Silicon increases plant resistance to a range of pests, diseases and environmental stresses including moisture, temperature, wind and nutrient disorders.
Table 1 Elements essential to plant health
No one essential element is more important to plant health than another. However, the supply of nitrogen normally limits growth most in horticultural systems. This is because plants require more of this element and because it is readily leached from potting mixes and consumed by microorganisms that decompose organic matter.
Plants absorb most nutrients from the soil solution through the roots. This means that the fertilizer must be either water soluble or readily broken down into ions by microbes to be plant available.
Solubility is important because nutrients must pass through a cell membrane in roots to enter the plant. This membrane is permeable to water but not to nutrient ions and most large organic molecules. If it was, the cell contents would leak out causing the cell to die.
The low cell membrane permeability prevents nutrient ions from freely moving into the cell at normal soil solution strengths. This property is essential because it allows plants to regulate the nutrients it absorbs through roots.
Uptake of nutrients across root cell membranes is controlled by electric pumps that are tuned to individual nutrient ions (Figure 1). These transporters are like key holes in the cell membrane formed by folded proteins. The holes have a shape and electrical charge that only allow ions with a specific size and charge (key shape) to pass through.
This micromachinery allows the plant to control the uptake of essential elements and to exclude elements that are potentially damaging such as sodium and chloride. It also enables nutrients to move into the plant against a concentration gradient (uphill).
Figure 1 Transport mechanisms for nutrients across the root cell membrane
Absorption through a transporter uses energy derived from respiration. This chemical reaction is fuelled by carbohydrate and requires oxygen. Roots get oxygen from the root zone which is why the air-filled porosity (AFP) of a potting mix is important.
Nutrient uptake is impaired when roots do not receive enough oxygen to fuel respiration. This is why plants in water-logged media develop nutrient deficiency symptoms. The main signs are pale green foliage and yellowing of the oldest leaves which are caused by nitrogen deficiency. Yellowing of new leaves which is caused by a trace element deficiency such as iron. Scorching and spotting of older leaves caused by the unregulated uptake of ions such as manganese and chloride.
All nutrient ions have either a +ve or a –ve charge (Table 1) and so plants must excrete a counter ion (H+ or OH-) when the nutrient is absorbed to maintain a charge balance.
Table 1 Electrical charge on nutrient ions absorbed by plant roots
This balancing response alters the pH of the soil solution and this, in turn, influences nutrient availability (Figure 2). The optimum pH range for most plants (5.5-6.5) is where the trace elements are most available for uptake (except for molybdenum).
Figure 2 Influence of pH on the availability of plant nutrients
Because plants generally absorb more cations than anions, the pH of a potting mix usually falls over time. This change is accelerated when urea or ammonium is present in the fertilizer. Urea does not have a significant charge but is converted by soil microorganisms into ammonium and so behaves like a cation.
Not all plant species lower the pH (Figure 3).
Figure 3 pH of seedling mix after growing different plants under the same conditions https://www.ces.ncsu.edu/depts/hort/floriculture/plugs/ghsubfert.pdf
Nutrient supply is a function of the concentration of an element in the soil solution and how well the concentration can be maintained or buffered in the face of root uptake, leaching and competition from microorganisms.
Supply is also related to the available water content of a potting mix. Mixes that hold more water also ho