More on Minerals

by Don Trotter

Hello fellow Earthlings, and welcome back to our discussion on minerals that your precious plants require for sustained health and happiness. We'll be discussing the two mineral constituents in the infamous N-P-K and my all time favorite mineral for plants, calcium. Phosphorus Potassium and Calcium are the talk of the day, so let's have a chat about a topic that is very dear to the vigor of your garden.


Phosphorus is used by plants in different forms and availability of phosphorus to plants is dependent on the solubility of this mineral in soil. However the availability of phosphorus is often tied up in compounds of limited solubility. The minerals that phosphorus links to depend on soil pH. In neutral to alkaline soils phosphorus will link up to calcium forming a compound known as calcium phosphate rendering the phosphorus unavailable to plants and insoluble. In soils with an acid pH phosphorus will often link to iron or aluminum to form phosphate compounds that also bind phosphorus as insoluble and mostly unavailable to plants. These relatively insoluble forms of phosphorus are called "solid-phase" phosphates and can function as a phosphorus savings account in the soil. Isn't that clever? The amount of solid phase phosphorus (phosphate) in a particular soil may actually account for 99% of the total phosphorus that appears on an analysis of soil. This means that a little as one percent of the total phosphorus that shows up on a typical soil test may actually be readily available to plants. Solubility of phosphorus is controlled by several factors including the amount of solid-phase phosphorus present in the soil. The greater the total amounts present in the soil, the better the chance of having more phosphorus in solution. Another important factor is the extent of contact between solid-phase phosphate and the soil solution. Greater exposure of solid-phase phosphate to the soil solution and to plant roots increases the ability to maintain replacement supplies. During periods of rapid plant growth, phosphorus in the soil solution may be replaced ten times or more per day from solid-phase phosphorus. Soil temperature and pH also affect the solubility of phosphorus. Maximum availability of soil phosphorus occurs at pH levels of 6.5 to 7.5.

Phosphorus is present in all living cells. It is used by plants to form nucleic acids like DNA and RNA and is also utilized in the storage and transfer of energy through energy-rich linkages (ATP and ADP).

Some natural sources for phosphorus in the natural garden are soft-rock phosphate, hard-rock phosphate, and good old bone meal.

Phosphorus stimulates early growth and root formation in plants. It speeds up maturity and promotes flowering and seed production as well. Symptoms of phosphorus deficiency in plants include:

  • Slow growth; stunted plants
  • Purplish coloration on the foliage of some plants
  • Dark green coloration with the tips of leaves dying
  • Delayed maturity
  • Poor fruit, flower, and seed production


Potassium is used by plants in the form of positively charged ions or "cations"(pronounced CAT-eye-uns). It is not synthesized into compounds like phosphorus is, but tends to remain ionic within plant cells and tissues. Potassium is essential for the transport of sugars and for starch formation as plants convert sunshine to food (photosynthesis). The pores in the leaves of plants (stomata) require the presence of potassium to open and close their guard cells in order to breathe. Potassium produces higher function of vascular plant tissue for better transport of nutrients. It increases plant resistance to disease. It also increases the size and quality of fruits and vegetables.

Soils may contain 40,000 to 60,000 pounds of potassium per acre. However only 1 or 2 percent of this amount may actually be available in the soil solution to plants. The rest is either tied up on particularly stingy clays known as expanding lattice clays or it occurs in primary mineral forms that are unavailable to plants.

Supplemental potassium is often applied to natural gardens from powerful mineral sources such as potassium sulfate, which also adds some sulfur to soil. Other materials that supply potassium are kelp products, wood ash, an amazing material called "Greensand", and a mineral called Sul-Po-Mag (sulfate of potash magnesia. Sul-Po-Mag is a rich source of potassium, sulfur, and magnesium. Who says natural products can only do one thing at a time?

Potassium is used heavily by plants that have very high carbohydrate production rates like fruit trees. Potatoes use gobs of potassium due their need to produce high levels of carbohydrates as starch in the potatoes themselves. Symptoms of potassium deficiency in plants include:

  • Tip and marginal (leaf edges) burn starting on more mature foliage
  • Weak stalks and stems
  • Small fruit and shriveled seeds
  • Slow growth


Calcium is considered a secondary plant nutrient but is grossly underrated. It is the feeling of this natural gardener that calcium is just as important as nitrogen, phosphorus, and potassium in quantity for plant health and vigor. Calcium is absorbed by plants as a positively charged ion or cation. It is an essential part of the structure of the cell wall in plants and must also be present in order for new cells to form. This means if there is no available calcium, plants don't grow.

Calcium is used in acidic soils to raise pH values more toward neutral. The materials most often used to do this are calcium carbonate (Lime) or calcium magnesium materials called dolomite and dolomitic lime. These materials supplement calcium to the soil for increased plant vigor while they alter the pH in acidic soils. Calcium also affects clay soils by loosening the electrochemical bond between clay particles. This will allow for better water and air penetration into otherwise "tight" clay soils. In the western part of the country where soils are more alkaline and in areas where little summer rains fall, a calcium and sulfur material called "gypsum" is often used. Gypsum does not alter soil pH nearly as much as lime products and supplies an ample amount of calcium to increase plant health. Gypsum is also fairly effective at loosening clay and compacted silt soils that are often encountered on the newly constructed suburban lot.

In the last few years a new material that supplies superior amounts of calcium has come to the attention of natural gardeners and farmers. This material is found in a fossil kelp (marine macroalgae) deposit that is actually found where an ancient ocean once was and what is presently called the state of Nevada in North America. This fossilized kelp material provides concentrated calcium and every other known plant growth nutrient, along with plant growth hormones as well. This material is known as Kelzyme.

A good thing to remember about calcium is that it is present in all living cells and is not mobile in plant tissue. What this means to a natural gardener is that composted plant matter is also a supply of calcium. So it is good to remember that a well-composted garden plot is also getting some calcium from that organic matter. Calcium deficiency is identified by some of these symptoms:

  • Death of growing points (terminal buds) on plants.
  • Death of root tips
  • Abnormal dark green appearance of foliage
  • Premature shedding of blossoms and buds
  • Weakened stems
  • Blossom-end rot
  • Crinkling of new growth

Got Questions? Email the Doc at
Check out Donald Trotter's books Natural Gardening A-Z, The Complete Natural Gardener and Rose Gardening A-Z at

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