Phosphorus, with the symbol P and atomic number 15, is the sixth chemical element in the biochemical sequence of essential plant nutrients. In the previous article in this series, I likened the role of magnesium in the biochemical sequence required for healthy plant growth to the ‘engine room’.  To continue the analogy, phosphorus can be likened to the fuel of life, the energiser. Phosphorus is the energy bridge that transfers electrons between high and low energy states of calcium and silicon. It is not enough for chlorophyll to simply gather the energy of the sun – that energy has to be transferred into producing sugars out of carbon dioxide and water.  That can’t happen without phosphorus.

As a mineral, phosphorus is most often present in its most oxidised state as inorganic phosphate rock.  In phosphate form it is a component of Adenosine Triphosphate (ATP) which transfers chemical energy within cells to enable metabolism.  Due to its vital role in plant growth, phosphorus is one of the major components of the inorganic NPK fertilisers used by conventional farmers.  As with the nitrogen component of those synthetic fertilisers, there are major consequences to the on-going use of phosphorus in this form.

Phosphorus is a stable, lazy element that doesn’t move easily through the soil.  It reacts with calcium, aluminium, iron and manganese to create insoluble phosphates – it becomes rock that is unavailable to plants, however it is biologically available.  Soil balance is key to phosphorus management because in acidic soils your “tie-up” can happen within hours.  As an indication, there are a couple of things that tend to happen to phosphorus at various pH values:

  • pH >8 = fixed by Ca
  • pH ≤ = fixed by Al
  • pH 3 – 4 = greatest fixation (A lot of imbalanced agricultural land sits in this range)
  • Ideal pH is between 5 and 6 on the KCL pH test (6 to 7 on H20 test).

P2O5 is the plant available form of phosphorus, but once again we rely on nature and soil life to digest the soft rock phosphate and make this available to the plant in a constant stable form. One of the key things for this is a healthy living soil which promotes Mycorrhizal fungi.

Like nitrogen, the refined soluble input contributes to the burning up of organic matter and micro-organisms, killing off the community of life in the soil that does support healthy plant growth.  So, in essence the more chemical phosphorus you use, the more you will have to use! The nitrates and phosphates used on agricultural land are major pollutants of our natural water systems.

Phosphorus is responsible for, and plays key roles in:

  • Vigorous early season root growth
  • Vigorous shoot growth
  • Effective pollination
  • Seed formation and fertility
  • All DNA

Good phosphorus levels result in great photosynthesis.

The biodynamic system of farming we use as part of Avondale’s holistic BioLOGIC® approach supports the use of natural soft rock phosphates, and a more quick release source like guano.  What we aim for is a constant supply of slow release phosphorus which gets released by the micro-life in the soil. There are many different practices which fit into the whole BioLOGIC® approach which contribute to great mineral management. These include increasing humus in soil (which we write more about soon), the use of humates in the system, cover cropping and increasing soil micro life.  Organic matter is the key!

We are looking for luxury levels of natural plant-available phosphates in the soil, we aiming for a range of 50 to 70ppm on the Mehlick III test in the soil. Phosphorus is a synergist for magnesium uptake and therefore works hand in hand with it.  Another interesting aspect to bear in mind is that the K:Mg ratio in the soil should be around 1:1 in ppm for optimum phosphorus uptake.

Good levels of phosphorus increase the BRIX levels in our vines as it fosters the production of complex sugars resulting in high nutrient density.  This positively impacts on the taste, colour, size and shelf life of our crops.  The other great advantage of achieving high BRIX levels is that vines become more disease resistant.  If you can maintain levels above 14 BRIX, there will be no disease in the vineyards.