Now a little about Potassium

After discussing Nitrogen and Phosphorus, we come to the third essential element for plant nutrition, Potassium (K). Potassium exists in most soils and minerals that are naturally occurring. It is rarely deficient in plants at normal pH levels. It is used by all cells in the sodium/ potassium balance across cell walls; just one of its many uses. Like Nitrogen and Phosphorus, it is essential in many processes in all living things.

Chemical fertilizers contain Potassium as a soluble salt usually in the form of potassium chloride (KCL), potassium sulfate (KSO4) or potassium nitrate (KNO3). It is usually mined out of the earth and purified from high potassium containing minerals. I learned this about potassium from Wikipedia and just had to share it.

“Into the 19th century, niter-beds were prepared by mixing manure with either mortar or wood ashes, common earth and organic materials such as straw to give porosity to a compost pile typically 1.5×2×5 meters in size.[3] The heap was usually under a cover from the rain, kept moist with urine, turned often to accelerate the decomposition and leached with water after approximately one year. Dung-heaps were a particularly common source: ammonia from the decomposition of urea and other nitrogenous materials would undergo bacterial oxidation to produce various nitrates, primarily calcium nitrate, which could be converted to potassium nitrate by the addition of potash from wood ashes.

A variation on this process, using only urine, straw and wood ash, is described by LeConte: Stale urine is placed in a container of straw hay and is allowed to sour for many months, after which water is used to wash the resulting chemical salts from the straw. The process is completed by filtering the liquid through wood ashes and air-drying in the sun.[3]”

The old name “Potash” comes from the ashes cooked in a pot with some form of Calcium nitrate could yield potassium nitrate and could be used as fertilizer. Nowadays we do not need to save our horse’s stale urine as potassium is commonly available in any complete fertilizer inexpensively.

A note about the old system of saving ash from a fire is due here. Many gardeners still believe that wood ashes (KCO3) are good to put in the garden straight from the fireplace. The form of potassium right from the fire is potassium carbonate and is a soluble salt which acts very similarly to sodium chloride which is table salt. Imagine table salt put on your plants in the garden. It would not take too much to get a salty solution floating around the roots in the soil moisture. When the concentration of salts is more intense outside the root, than inside the root, the soil draws water out of the root to try to equalize the concentrations. This leads to dehydration of the root as water is drawn out through the cell membrane. Plants will look wilted and droopy even though the soil appears moist. You do not want to use wood ashes much around plants for this reason. When the wood ashes (KCO3) are mixed with a form of Nitrate (ie. CaNO3) , such as the animal urines noted above, the soluble salts switch cations and become potassium nitrate and Calcium carbonate.
If you do not want to take my advice and skip the wood ashes just be aware that it can be very strong and at times harm the plants.

As we touched on already, one of the characteristics of potassium is that it comes in the form of a soluble salt. As such, it dissolves well in water and may leach away. Potassium is needed when the plant is making fruits especially, so be sure to use a complete fertilizer in the vegetable garden.

Now going back to the first post about plant nutrition, this whole topic was suggested by an alert reader who had a friend who was watering her flowers with Propel brand sports drink to give them potassium. I think by now we are ready to do the math on how much potassium she was getting in the drink and whether that was a kooky but not ill-advised thing to do.

Propel water contains potassium citrate which is about 38%K. Each bottle contains 100 mg of potassium citrate which equals 38 mg of potassium. A bottle of Propel water costs $.75. The Potassium in the sports drink costs .00000837 pounds for $.75. Hmmm. I think I’ll go buy some Rapid-Gro.


A Brief Tale of Phosphorus

A brief post about Phosphorus continues our series of Plant Nutrient posts. Yesterday, we spoke about Nitrogen as the primary macronutrient needed by plants. Today we pick up the theme with the second most important nutrient, Phosphorus.

Phosphorus is another element on the periodic table of chemicals. It is second only to Nitrogen as a very important nutrient needed by plants. In fact, all organisms need it as it is the base structure of cell membranes, and the energy carrying molecule in living things. It is usually found in commercial fertilizer in the form of P2O5. naturally occurring rocks such as apatite contain large amounts of phosphorus and are the source, in soil of the element in nature.

The thing to remember about Phosphorus is that it is never water-soluble. As opposed to Nitrogen which is always leaching away with water, once Phosphorus is in the soil, it stays where it is put and binds tightly to soil particles. It is therefore commonly abundant to most soils and additional fertilizers containing it are seldom needed. A soil test of your garden soil will tell you whether the native soil is already well supplied with Phosphorus and if it is, you will not need to worry about the concentration in your fertilizer.

If you add Phosphorus that is not needed, it can bind to silt and run off into natural waterways causing troublesome extraneous levels of fertility in lakes and streams. Foaming along the edges of waterways is usually from Phosphorus runoff. This can cause an overgrowth of algae and seaweed that cause an imbalance in the body of water leading to depletion of Oxygen for fish, etc. Extra phosphorus is not only a source of pollution but is an unnecessary expense. In limestone based soil, which is most of the cultivable United States, you usually do not need to use anymore than 5% Phosphorus in your fertilizer. If your soil tests low in Phosphorus, use of up to 12% may be warranted but seldom more than that. Remember, pouring fertilizer on the top of the soil leaves a heavy layer of insoluble Phosphorus at the very top and not much penetration down into the rest of the root zone. A good way to apply it is when you are turning over the soil with a rototiller or plow and can incorporate Phosphorus deep into the field.

Phosphorus is the second number in the three number sequence on the fertilizer bag. It is an expensive component and makes up the majority of the cost of production of a fertilizer. The most expensive sources of Phosphorus are water-soluble forms specially created to be watered in. An example of this is Rapid Gro or Miracle Gro where the product is applied to individual plants by a watering can. These are only economical when used in very small batches applied to specialty crops such as vegetable gardens of flowers. They have been advertised over the years as extra good for flowers and fruit. Although plants do use a little more to make these structures, not a whole lot more are used versus a plant you are growing for foliage such as grass. The main response you see from applying water-soluble fertilizers to flowers and such is from the Nitrogen, not the Phosphorus.

While we are on growth responses from fertilizers, I’ll throw in another old wives’ tale. There is one stating that if your flowers do not flower and grow only green leaves, your fertilizer is out of balance and you have given too much Nitrogen and not enough Phosphorus. Usually the reason that your flowers do not flower is lack of sunlight. Both Nitrogen and Phosphorus are need for flowering and seldom is Phosphorus deficiency to blame for lack of flowers. At the most, the flowers may be stunted and smallish, but if it is going to have flowers, it will. More tomorrow on the third macronutrient, Potassium.