I've been wondering about all of that. Thanks for the insight.

Just a note to clarify for those interested. The pH of magnesium sulfate in water is above 7 (alkaline) because of hydrolysis of the sulfate anions, not the magnesium cations. Magnesium cations are virtually neutral in their reaction in soils. It is the sulfate that causes the alkalinity. Similarly in magnesium hydroxide it is the hydroxide that produces the alkaline reaction, not the magnesium.

The pH of materials when you test them in the kitchen isn't the same as their pH reaction on the soil. Elemental sulfur has no pH (contains no hydrogen). But in the soil, bacteria gradually oxidize the sulfur and the oxide combines with water to make a strong acid. Pure epsom salt should be pH neutral, and the alkaline pH reading probably came from your tapwater. Normally treated municipal water has alkaline pH, but there is not enough material in it to influence soil pH unless the water is noticeably hard or salty.

The link below about P and roses is helpful. Note that your Austrailian source is about cereal grain production. Harvesting cereal grains removes a lot of P from farmlands every year, so farmers have to add P every year or so. That is not the case for most ornamental gardens. However, if you have alkaline soil, it would be a good idea to alter the pH with sulfur. The effect of high pH will probably show up as deficiency of iron before there is any problem with phosphorus.

Here is a link that might be useful: myth of phosphate for flowering

Newtie, sulfate ions don't react with water to make an alkaline solution. Magnesium sulfate- Epsom Salts- forms mildly acidic solutions when dissolved in water (pH around 6) because of reaction of the magnesium ions with water. Strawberryhill, if you measured an alkaline pH for Epsom Salts, either something's wrong with your pH indicator or your water is very alkaline.

Adding another phosphate link.

Mike_Rivers is a professional chemist, so we should all listen to him.

Here is a link that might be useful: Myth of Phosphate, part I

Mike, I also thought that way the first 2 times I tested epsom salt (magnesium sulfate) solution. I expect it to be acidic with the sulfate - I was completely surprised by the alkaline result. I tested it the 3rd time yesterday, again alkaline. I tested it the 4th time today with a higher concentration of epsom salt, the result is clearly alkaline. Try it yourself by boiling red cabbage for 10 minutes, and test it with vinegar (pH at 2.4 to 3.4), peat moss (pH at 3 to 4), baking soda (pH at 8.2), then compare the greenish/purple baking soda with epsom salt solution, you can see how greenish and alkaline epsom salt is.

I have on my kitchen counter 5 test tubes of only purple cabbage juice, bright pink/red solution (only a few drops of vinegar), calcium citrate (same color as the control red cabbage), baking soda (greenish purple), and epsom salt (greenish purple).

When the litmus paper is wet, it's hard to differentiate the ranks of color. Red cabbage juice is easier to read, and you can detect the color better. Before doing the experiment, I spent a long time googling what others report regarding pH of epsom salt: some reported neutral, some reported alkaline, but none reported acidic - I tested epsom salt to prove that it's acidic with the sulphate, but I was surprised by the 4 results (twice with litmus paper, and twice with red cabbage juice).

I also crushed 250 mg. Magnesium supplement (Magnesium oxide and magnesium stearate) into red cabbage juice. It took a while for it to dissolve. The result is very greenish, MORE alkaline than baking soda.

I still have the solution of reb cabbage juice in MiracleGro potting soil (turned slight pink) as a slightly acidic control.

"Mike_Rivers is a professional chemist, so we should all listen to him."

Michaelg, I read the above and I'm still laughing.

Strawberryhill, the dye in red cabbage juice changes color when it reacts with hydrogen ions. I wonder if it might also change color by reacting with magnesium ions and thus mislead you on the pH. All I can suggest is buying a good pH meter.

Mike, many thanks for your feed back - I went to Walmart to buy distill water and "Jungle" pH test strips (freshwater aquarium test) by the fish tanks. It ought to be accurate since people can really sue them for dead fish! I tested distill water, epsom salt in distill water, my tap water, and magnesium supplement (250 mg of magnesium oxide)in distill water. Here the result: distill water neutral at 7, epsom salt slightly more alkaline at 7.3 to 7.5, my tap water at 8, and magnesium solution is WAY ALKALINE PAST the 8.4 mark.

My conclusion: since epsom salt is magnesium sulfate, the magnesium part is strongly alkaline (as Milk of Magnesium is to neutralize an upset acid stomach), and the sulfate part is acidic - it balances out to be neutral. Depending on the ratio of magnesium to sulfate, some epsom salt can be more alkaline, rather than neutral.

It's good to know that the sulphate part is acidic, since there's a government website that recommends people using composted leaves (rich in sulfur) to bring the pH down in their garden.

Strawberryyhill. These are the simple facts: Magnesium cation is slightly acidic in water, Milk of Magnesia is alkaline because of the hydroxide ion, and sulfate ion is neutral. Elemental sulfur - the yellow stuff - makes soils acidic because elemental sulfur is oxidized by soil organisms to produce sulfuric acid. Sulfate ion is already oxidized and adding it to soil does nothing at all to the soil pH.

Composting leaves initially produces organic acids which can lower soil pH, but this is only temporary. Eventually, the organic acids get oxidized to carbon dioxide and water and the main things that persist in the soil are metal carbonates and phosphates, which are alkaline.

Here limestone underlies the subsoil and infuses the well water. The soil is 6.5-7 pH. My soil has tested slightly deficient in phosphorous. It has been benefited by applications of green sand and one application of triple phosphate. The triple phosphate improved both the roses and the bulbs. I would not add triple phosphate on an annual basis as it is very high in phosphorous and repeated applications would throw the mineral balance too far in one direction. The green sand provides both phosphorus and an acid reaction. From reading here I understand that the acid improves the availability of phosphorus.

Different parts of the country have different mineral profiles as well as climates. To state flatly that phosphorous is not needed is unhelpful. Maybe in most places it is not needed. Here it is, whether made available by direct application or by acidifying the soil.

The best advice given here is to test the soil. Then we know
what to add, how much, and how often.

Cath

Oh yes, and retest at reasonable intervals which I must confess I have not done.

I'm resigned to the fact that except in raised beds, I can never alter my soil's pH. I'm on a hillside and different parts of my garden sit on slightly different rock bases, but for a good part of it, the soil is only ten inches above limestone. Two hills over and it is a completely different limestone formation, less chalky but still alkaline. This is prime wine grape country, though mostly here we grow housing developments.

Rosefolly