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Archive for September, 2012

There are many myths about potassium. The biggest one being you need it for flowering.  WRONG!  You need potassium no more or less than any other nutrient for flowering.  Sure, some commercial growers might change the ratio of nitrogen to potassium as the plant moves from vegetative to fruiting but it has nothing to do with the formation of flowers and fruit, it’s more about flavour.  Too much nitrogen can produce fruit that is big but watery, tasteless and low in sugar and other flavour components

Potassium is important in that it helps plants tolerate stressors such as cold/hot temperatures, drought, and pests.  It is a catalyst for many plant enzymes and helps regulate water use in the plant by affecting the opening and closing of the stomata in the leaves and water movement in and out of cells.

Potassium is a funny nutrient in some respects because we often see little response to it in trials.  But palms and other plants that clump, may respond to it by increased clumping and branching.  We’ve seen that response in some native plants as well, such as Stirlingia.  Too much potassium can make stems very brittle so they snap easily.

Signs of potassium deficiency can be quite dramatic and also species specific.  Sweet corn gets a sort of burning – a band around the leaf margins which is dried out and dead looking.  Carnations also get necrotic spotting at the tips of older leaves.  Hoyas end up with necrotic spots all around the leaf edges.  The symptoms are always on the older leaves because potassium is mobile and will move to the place of greatest need – we call that a sink and is often the fruit or flowers, or at the very least a growing point.

Potassium is supplied in many products as potassium nitrate – 36-38% potassium and 12-13% nitrate depending on formulation and purity.  Potassium sulphate can also be used but is more acidifying – an effect we often wish to avoid in our soils.  The cheapest source of all is muriate of potash or potassium chloride which we don’t really recommend at all because of the high chloride (salt) content.

Good organic forms of potassium are wood ash, dried seaweed and blood and bone, though the phosphorus content of blood and bone is rather high relative to the nitrogen (N) and potassium (K) and plants fed with blood and bone will need both N and K supplemented.

Potassium is readily leachable in our sands but is held on slightly better than nitrate in the presence of clay or compost as it is a positively charged ion.  Nevertheless, we see high rates of leaching of potassium from newly applied compost.

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