HI Dave
Miracle gro would have to be the worst fertilizer available for potted plants. I know it is widely recommended by the bonsai fraternity, but miracle gro is absolute rubbish.
The difficulty with this product is in finding out what is actually in it. If you look at the side of the pack it will list the elements NPK trace elements etc. Which is fine. But Miracle gro is a mineral fertilizer; what are the actual components? To find this out we need to look at the MSDS form. For Scotts America, you have to request this. However in Australia it is required by law to be on their website. OK that’s fine. However, I suspect the fertilizer you are using is Miracle-Gro® Nursery Select All Purpose Water Soluble Plant Food. They don’t sell that here in Australia. It is possible they use potassium phosphate to get the P levels up. However we may make the assumption that it is simply a variation on the amounts of components, as the MSDS form applies to at least four different miracle gro products, and all miracle gro MSDS forms have the same basic ingredients.
So Miracle Gro All Purpose contains the following chemicals, at undisclosed amounts,
Urea
Urea phosphate
Potassium chloride
Ammonium phosphate
Iron EDTA
Let’s look at the elemental line up.
Firstly, there is no Magnesium, or Calcium or Nitrate. Calcium and magnesium are essential macro elements. These need to be present in appreciable amounts. Magnesium and Calcium react with phosphate to produce an insoluble salt, so generally ferts high in P have no Ca and may have very low Mg, if any. Compare with Miracle Gro tomato food which has 5% Ca, but NPK of 15:2.2:9 (aus numbers)
In bonsai growing there are only three sources for your plant to gain these elements; your initial potting mix, your irrigation water, and your fertilizer. The amount contained in potting mixes is finite, lasting a couple of months at best. If the plants are not getting them from your irrigation water, which may have a degree of hardness associated with it, they’re not going to get it from miracle gro. A “hard” water can provide all the calcium a plant needs, but it needs to be proportional to the amount of magnesium present in the water, to little magnesium and the calcium will push the plant towards magnesium deficiency.
The bad in miracle gro.
Urea is a very poor source of nitrogen for plants, but is cheap to make. Plants prefer nitrate first, ammonium second and urea as a last resort. Why is this? Plants use nitrates preferentially because it takes minimal energy to convert it into usable forms. Plant cells are slightly alkali, so when they absorb ammonium, it quickly converts to ammonia, which is toxic to plants, so plant cells have to use carbohydrate to convert ammonia to nitrate, and fast. Urea is difficult to breakdown, using a lot of energy, and requires the presence of Nickel for the enzymes to work. Who adds nickel to their fertilizer program? How long has your tree been in a pot? Is it nickel deficient? Probably! That’s one problem. Another problem is that plant roots don’t work at low soil temperatures, so ammonium ferts can be toxic to roots because it quickly depletes the cells of important nutrients to neutralize them. Urea is worse as it uses a lot more. Miracle gro only has Urea and ammonium, not good.
In the ground urea is broken down by bacteria in the soil to produce ammonium and then onto nitrate. Which is great for plants in the ground. In pots these bacteria will take two days at optimum conditions to convert urea to nitrate. However, during low oxygen conditions the process stops and can reverse, at adverse pH’s the process stops. The bacteria also have to be present in large enough numbers to be effective, they are easily killed by high soil temps. Fortunately urea is highly soluble, doesn’t attach to CEC sites and leaches quickly. Urea and ammonium both lower the pH of soils so that manganese present in the potting mix may become toxic, so these are not the best chemicals to add because they cannot be controlled.
The other bad thing in miracle gro is it contains potassium chloride, KCL. Scotts give no information on how much chloride is in their products. Chloride is toxic to plants at high enough concentrations. Chloride toxicity causes marginal leaf burn, among other things. Since KCl is the major source of potassium in miracle gro, the chloride levels may be very high (it’s possible they are using potassium phosphate to lift the K levels to get a 20:20:20 fert but would still need KCL to get to 20). Normally chloride is held at bay by nitrate and sulfate ions, but nitrate is missing and so chloride can be taken up by roots to balance electrical charges in cells. Chloride should never, under any circumstances be added to a bonsai fertilizer.
What I think happens with this fert is that most of the urea is leached out in the next watering, some will be taken up by the plant, some by bacteria. The chloride is excessive and can cause toxicity, there is no magnesium or calcium and so the plant can become deficient in these elements if not supplied by another source, such as irrigation water.
Having a tree in full sun exacerbates these problems as the tree struggles with transpiration demands. A tree in the shade will show much less symptoms or none at all.
I would recommend moving the tree to the shade, change fertilizers or simply stick with the organic fert (check for MG and Ca) If the org fert is from Scotts ditch it because they dose it with urea), supplement trees with Epsom salts or better, with magnesium nitrate (if you can find it). At a rate of 1 level teaspoon per 27 liters of water. Water with this weekly, in addition to you new fertilizer which should be used at half recommended strength. Perhaps consider finding out what your irrigation water is like, local councils usually can provide this.
Hope you find this informative. It would be handy to have the American MSDS of the product you are using because there are some big assumptions with my assessment. Though I believe the major components are the same.
Paul
