Potassium, with the symbol K and the atomic number 19, is the 8th and last chemical element completing the biochemical sequence required for healthy plant growth. You can read my previous articles on the fascinating interplay of boron, silicon, calcium, sulphur, nitrogen, magnesium, phosphorus and carbon, which all precede potassium in the biochemical sequence.

Elemental potassium is a silver-white, soft, alkali metal that oxidizes rapidly in air and is highly reactive with water.  In nature, potassium occurs in the form of ionic salts, and those positively charged ions are necessary for the functioning of all living cells.  Previously I likened magnesium to an engine room and phosphorus to fuel.  To continue the analogy, potassium is akin to the spark plug in the system.

Potassium is a major electrolyte and allies with silicon for the transfer of materials in and out of cells as well as throughout the plant – from nutrient uptake by the roots through to fibrous transfer via stems.

Potassium is involved in regulating many enzymatic reactions within the plant and is therefore the element that really makes things happen. Some of the key roles of potassium are:

  • Aids disease resistance
  • Assists with seed and root development
  • Is required to converting nitrogen into protein, and therefore reduces the amount of non-protein nitrogen in the system
  • Influences stomata regulation which improves gas exchange for photosynthesis, and achieves a greater water efficiency and drought resistance
  • Facilitates the sizing up of fruit and preventing the build-up of sugars in the chloroplasts

Factors Affecting Potassium Availability

There are many different factors that influence potassium availability.  It depends on what types of soils you have. Here are a couple of pointers:

Soil pH: As the soil pH is reduced (increasing soil acidity) the availability of potassium is often reduced.

High Mg Soils: Excessive magnesium leads to tighter closed soils which traps potassium in the clay colloids. Improving Ca:Mg levels alleviates this problem

High Ca and Mg Soils: If the sum of calcium and magnesuim levels are greater than 90% it becomes very difficult to build potassium levels as all spaces are taken by existing cations with stronger bonds

High Na Soils: Excess sodium will mean that the plant will take up the sodium rather than potassium. Sodium should never exceed potassium in base saturation percentage

The ration between K and Mg is very important: Ideal ratios in the K:Mg ratios in the soil should be 1:1 in ppm and 1:3 in base saturation

High K soils: When the base saturation level exceeds 7.5% it can lead to excessive weed pressure and competition. Soil structure also tends to decline as potassium ions tend to leach and this results in poor soil structure and eventual collapse of soil particles

Due to the promise of getting larger fruits, the use of chemical forms of potassium such as the cheapest potassium chloride as a major fertiliser is prevalent in conventional farming.    Unfortunately, the chlorine component kills off the micro-life in the soil, and should be completely avoided.  Potassium sulphate is a very soluble, better source of potassium that has worked well for us in the process of balancing our soils.  90% of potassium found in soils is insoluble potassium silicates! However, if you have healthy communities of micro-life then there should be lots of available potassium in the soil.

Just like our conventional counterparts, the natural farmer wants luxury levels of plant available potassium in the soils.  However, our focus is on achieving that luxury level through the creation and management of humus in our soils.  Due to potassium’s major role in fruit formation, it stands to reason that with the removal of the crop comes a potassium deficiency.  One of the ways we counteract this at Avondale is by creating a vermi-compost from the grape skins after fermentation.  By using worms to digest the potassium-rich grape skins, we create beautifully balanced compost with high humus content and decent levels of plant available potassium and even better source of microbes.

Throughout this series of articles on the biochemical sequence it is clear that an abundant, thriving community of life in the soil is vital to ensure the availability of all the important nutrients for the vines.  At the core of this community of life is the organic component of the soil – the humus.  In the next series of articles, I will be sharing my knowledge and experience in creating and managing humus.