Fertilizer

by Louis Kilbert, PhD

Reprinted from the Newsletter of the Michigan Cactus & Succulent Society

It will be spring soon! The best time to do most of your potted-plant chores; the best time to fertilize them. "Natural" fertilizers contain the chemicals that provide nourishment to our plants. Manufactured (artificial?) fertilizers contain many of these same chemicals. There are some things not added to the latter that are present in the former. The three chemicals most likely to be deficient in any soil are nitrogen (N), phosphorous (P) and potassium (K)(also called "potash"). The three numbers you will see listed on a box of feritlizer are the concentrations of NPK in that order. Nitrogen is for leaves and the green epidermis of succulents, Phosphorous is for flowers and seeds, and Potash is for strong stems including the interior structure of cacti. It is important to have enough calcium and magnesium as well.

Horse manure contains about 0.7:0.3:0.6::N:P:K and 0.3% calcium and 0.1% magnesium. (Caution: if you plan on using manure, fresh manure is chemically very "hot" or reactive; it will damage your plants! Chicken manure is especially hot. Manures should be composted for up to one year before being used as fertilizer.) There is a woman, "The Turtle Lady", on the Winter Hardy Cactus group on the internet who has trained her horses to urinate in a different spot from where they defecate. Urine adds salt to the mix. Therefore, she is creating low-salt natural fertilizer.

Salt poisoning is the one bane of fertilizing. All plants, including cactus and succulents, require fertilizer. Plants grown in pots are especially susceptible to salt poisoning, sometimes called salt build-up. You can see the salt building up on the top edges of an old clay pot and various places on a plastic pot. Potassium in the form of potassium chloride (Potash of chlorine) is one of the primary culprits responsible for this problem. Wood ashes from a fireplace or fire pit contain another form of potash; I'm not sure if it would be any better. Caution: wood ashes must be aged or composted before use! In the open-garden, fertilizing with potash in the fall is advisable, because potassium binds to the tiny particles of clay and stays there; whereas, chloride is washed through by winter snow-melt and rain. It's the chloride that most causes salt poisoning. If you grow your plants in saucers on windowsills, the chloride cannot wash out of the soil and builds up. Dry potassium chloride will cause "burning" ( osmotic damage) to the roots, so it's always best to use water-soluble potash when fertilizing pot-plants.

Nitrogen in commercial fertilizers comes mostly from urea and ammonia, natural components of urine. Nitrogen is also sometimes supplied as "nitrate" (NO3-), which is the form that is actually taken-up by plants. On the other hand, nitrate is washed out of the soil, because it is highly soluble; and therefore, must be added regularly. Urea-formaldehyde is a slow-release form of nitrogen. In the garden, nitrogen is most likely to be lost in winter (don't fertilize with nitrogen in winter), because micro-organisms which would tie-up and hold the nitrogen in the soil are inactive. Whereas in a potted plant, nitrogen is most likely to be lost during the heavy watering days in the heat of summer and most effective in the cooler days of spring and early fall.

Phosphorous is a major component of bone and therefore of bonemeal. (The form actually taken-up by plant roots is dihydrogen phosphate {H2PO4-}). At relatively high pH's (6.0 or more) phosphate combines with calcium or magnesium in the soil; and in that form, the phosphorous is available to the roots. At lower pH's (less than 6.0) or strongly acidic soils, the phosphate combines with aluminum or iron making it unavailable to plant roots. This is also why it is necessary to add extra iron to acid soils (e.g. azaleas and rhododendrons).

Important factors of fertilizing: 1) Timing; When is the best time to fertilize? and Which fertilizer should be used at the various times of the year? 2) Relatedness; The three major components (NPK) must be balanced, too much of one may inhibit use of the others. and 3) Leaching: You can make leaching work for you by washing salts through and out of the soil and by using slow-release pellets to keep leaching from washing all the fertilizer out of the soil.