Growing media – nutrient and pH interaction

1 July 2016
  • Whastsapp

Growing media – nutrient and pH interaction

Having looked at the physical characteristics of growing media, in the second in the series Dr Jim Smith − ICL’s technical manager for growing media − explores the science behind how nutrients are held in growing media.

This topic can appear complicated so a little simplification may help.   Basically, particles within growing media can be seen as negative magnets, which attract positive ions such as Potassium K+ and Calcium Ca2+.  

The link process is called cation exchange capacity (CEC), or is sometimes referred to as buffering.  Peat and fine bark tend to have a high inherent CEC so are capable of holding lots of nutrients.  In contrast wood fibre and perlite have a low CEC and tend to be nutrient poor.

The advantage of a high CEC is that these positively charged nutrients are held on the surface of the growing media particles and are readily available to the plant.  In contrast most negatively charged ions − such as sulphate, chlorine and nitrate − are always in solution and can be washed out of a pot, should there be excessive rainfall or irrigation.

The actual nutrients in the growing media depend on the amount and type of fertilizer. It is usual to add a small amount of balanced starter fertilizer, to help plants establish after planting, and then rely on liquid feeding or controlled release fertilizer.

pH correction

pH is a measure of how acid or alkaline a material is which is important to growing media as it impacts on nutrient availability.  

Different growing media raw materials have different inherent pH values, but actually plants are more tolerant than you might expect.

In scientific terms, pH is a measure of the concentrations of hydrogen (H+) and hydroxyl (OH-) ions in solution. Higher numbers of H+ ions make the material more acidic, higher numbers of OH- ions more alkaline while equal numbers mean the material is neutral.

The ratio of H+ to OH- is important, as is the actual number of ions in total. Peat creates a high number while wood fibre produces few.

The optimum pH for trace element uptake in peat is about 5.5 while in agricultural soil 6.8 is ideal. Other materials fall inbetween.

At low pH fertilizer cations become more readily available and while ericaceous plants − such as Rhododendrons and Hydrangeas − appear to tolerate this, others can find it toxic. Naturally peat has a pH of 3.8-4.2, generally necessitating dolomitic lime to be added to raise the pH.  

Conversely, high pH locks up cations often leading to deficiencies.  For example at a high pH Calcium links with Phosphate, which is not soluble in water and therefore not available to the plant roots.

Coir and bark have a pH of around 6.0, which may be optimum for trace element uptake, but both materials still require a small amount of lime to provide the calcium and magnesium essential for growth. 

Conclusions

  • Incorporate raw materials like peat and bark to provide best nutrient holding
  • Optimize pH using dolomitic lime to suit the species of plant being grown
  • ICL works closely with growers to optimise the growing media for the circumstances.
  • For further advice and recommendations please contact your ICL Technical Area Sales Manager or distributor.