Question
I live in central Fl (Orlando area) - soil is sandy and "weak" as far as natural nutrients go.  I was wondering about using Epsom salts to turn the hydrangeas blue; read your response to Deb on this question -- apparently not a good idea.  Can you recommend an easy, inexpensive way of making my soil more acidic.  Thanks in advance.

Answer
Some people use Epsom Salts (MgSO4 aka 'Magnesium Sulfate') to make sure their plants are getting enough Magnesium.

This element is considered a secondary nutrient, and a vital part of the Chlorophyll molecule, C55H70MgN4O6.

Take a good look at that molecule.  How many atoms of Carbon?  55!  Hydrogen?  70!  Magnesium?  ONE!

You don't need much.  But try growing a green plant without it, and you have a plant with zero Chlorophyll.

Master Gardener Linda Chalker-Scott, PhD, works at Washington State University Extension Center.  Dr Chalker-Scott once posted a bold anti-Epsom Salts blast in a list entitled 'Horticultural Myths' where she noted: 'An automatic application of Epsom Salts to plants or Soils that are not Magnesium deficient is a poor management strategy that can injure the plants and contaminate the Soil.'  Here's the URL:

www.puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/Myths/Epsom%20salts.pdf

Now for a little Soil Science.  In your Soil, Calcium, Phosphorus, Manganese, Iron and Potassium have something that chemists call an 'inverse relationship' with Magnesium.  When you fertilize with any of them -- Organic or Chemical Ca, P, Mn, Fe or K -- you actually LOWER the Mg that the plant gets.

Very bad.

Fertilize too much with just ONE of these and you affect the Mg.  Same thing happens when you overdose with other elements.  Too much Calcium will shut out P, K, Na or Fe.  Excess Sodium will shut out Mg or K.  Excess Copper or Phosphorus will shut out Iron or Zinc or Aluminum.  All plants need a BALANCE of nutrients.  You have to make that happen.  Since most Soil has PLENTY of Mg, the best thing you can do for any of your plants is to make sure nothing keeps them from getting it.  And that goes for every nutrient on the menu.

Soils in North and Central Florida are usually Acid (pH testing between 5.0 and 6.5).  Even these must be MORE Acid.  If the Soil is high in Phosphorus, a Hydrangea will bloom with Pink flowers EVEN THOUGH THE SOIL IS ACIDIC AND HAS PLENTY OF ALUMINUM.

Note that Magnesium deficiency DOES happen.  It's quite common in Florida.  Symptoms show up on Trees and Shrubs on the oldest leaves, sometimes as ruddy flushes, sometimes as Chlorosis, followed by loss of lower leaves.  Growers with Calcareous Soils, like those in parts of Florida, that find their plants are Magnesium deficient must treat them with foliar sprays; the Soil pH is too high for the Mg to be absorbed.

Very complex, these elements.

Now, there are many claims about Epsom Salts.  Yours is the first I've come across that proposes it will lower the Soil pH.

Let's about Blue blooms.

To develop their intense, saturated Blue flowers, Hydrangeas -- H. macrophylla -- are treated with one or two doses of Aluminum Sulfate.  Typically, a grower will mix 15 grams per 1 Liter of Water.  In your case, 1 Tablespoon per Gallon of Water is probably sufficient.  Don't overdose.  Don't splash any on the leaves; it causes damage.  Don't apply this treatment ALL the time, just when you see buds beginning to form.  You have to do this EARLY enough to affect the color, but not TOO early that you cause a nutrient imbalance.

To look Blue, these shrubs need two things:

1.  A VERY low Soil pH Optimum -- as close to 4.5 as you can get it; and

2.  Aluminum to manufacture Blue pigment (delphinidin 3-monoglucoside, an anthocyanin also found in Concord Grapes) for the flowers.

If one is missing, i.e. you have too little Al OR the pH is up around 6.0, your Hydrangea's hues will be pale, Baby Blues, rather than intense Sky Blues.  The problem: When your Soil pH reaches 5.5, or lower, Phosphorus (usually H2PO4) in the Soil is blocked from being used by plants.  It's even worse when you're growing things in Clay Soil.  But too much Phosphorus will block your Hydrangea from getting enough Aluminum to bloom in Blue.  And because Al is NOT even a micronutrient, it will eventually cause stunting, or death, if you use it on a regular basis.  It is nevertheless necessary for those anthocyanin molecules to turn those petals Blue.

Therefore, in this particular case, do not fertilize AT ALL with Phosphorus until AFTER the plant has flowered.  Phosphorus, as stated above, will interfere with absorption of the very Aluminum this plant needs to make the Blue petal pigment.

O.K.  How do you lower the pH of your Soil?

Let's start with one potential problem that will make it completely impossible to lower the pH.

Free Lime.

A Soil containing Free Lime (usually in the form of Calcium) CAN NOT BE FIXED to grow Acid-loving plants.  I'll spare you the chemistry lesson on that.  But if you want me to go into it let me know.

Does your Soil sample have Free Lime?

Test for it.

You can actually do this at home.  No complicated, boring, professional testing needed.

Get a Spoon, and scoop up some Soil.  Place in a container.  Get some Vinegar out -- any kind, it doesn't matter so long as it's Vinegar.  Drizzle over the Soil.  And watch closely.

Any bubbles?

Take a good look.  Be absolutely certain.

No bubbles is GOOD.  Bubbles means there's a reaction taking place and you are NOT going to grow Blue Hydrangeas in that Soil no matter WHAT you do.  Because you can't change the pH.  It's got free Lime.  This is non-negotiable.

If you see no bubbles, order some litmus paper on the internet for pH testing, and try these pH-lowering techniques:

1.  SULPHUR.  Mix into Soil around the plant.  This takes time, at least one year to affect Soil pH effectively.  Re-apply every few months.

2.  IRON SULFATE.  Mix into Soil around the plant.  This works faster than Sulphur.  9 lbs per 100 sq ft of Soil is effective.

3.  SPHAGNUM PEATMOSS.  Mix into 1 or 2 cu ft per plant.  The pH of the Soil will slowly decrease.  Peatmoss has a pH around 5.0.

4.  ACIDIC IRRIGATION.  Watering with Lemon Juice (pH around 2.3), Orange Juice (3.4 to 4.0), or kitchen Vinegar (4.0 to 4.5) will lower pH temporarily.  Dilute with Water, in a ratio of 2 parts 'ACID' to 1 part 'WATER'.

Break for lunch.  Please remember to rate me when you are done.

Any questions?

THE LONG ISLAND GARDENER