Hello, kc_in_wa. Both are used to lower soil Ph in alkaline gardens but aluminum sulfate adds aluminum to the mix, something required to turn hydrangeas blue. The process that turns hydrangeas blue requires two things: aluminum in the soil and an acidic soil Ph Level.

Aluminum naturally occurs in many soils; a soil test can tell you that. If it occurs in the required quantities where you live, then it is not necessary to ammend your soil with aluminum sulfate; ammonium sulfate, iron sulfate, soil sulphur, elemental sulphur or flowers of sulphur can all be used to lower the soil Ph.

The biggest problem with aluminum compounds is that they cannot be used near certain plants like azaleas, rhododendrons and magnolias. A little does no harm but it is toxic in large quantities.

Lilacs are not acid loving plants. You need to keep their soil alkaline instead. So do not add sulphur to lower Ph Levels near these plants. Instead, you need to add lime, which raises the Ph Level.

Luis

Thanks Luis,
If I understand this, the ammonium sulfate won't do anything to make my flowers more blue. If I locate the Aluminum sulfate will it likely harm the ground cover (reptans) around the hydrangeas.
Thanks for the info on Lilacs. That would explain why the acid fertiziler wasn't doing anything! Fortunately they are hardy.

Aluminum sulphate will have no ill effect on your Ajuga (I guess reptans is an Ajuga reptans).
When you'll found it, use strictly as directed, do not overdose!

Hello, Wendy. I have not seen recommended amounts of aluminum specified in parts per million. That is because the actual process of turning hydrangeas blue is far more complicated and requires monitoring additional things.

For example, PH Levels above 6 to 6.2 will inhibit absorption of aluminum regardless of how much aluminum your soil has. So you could have large amounts of aluminum and a plant in very alkaline soil would have a low aluminum absorption rate.

Phosphorus in large amounts also makes it difficult to absorb aluminum, which is why people who want to get pink hydrangeas also apply triple superphosphate to maintain the color. If you want blues then you want to minimize phosphorus. Oh, you also want to maximize pottasium levels.

Some of the aluminum recommendations that I have seen for commercial hydrangea wholesellers ignore aluminum measurements of ppm and instead suggest adding 8oz of a drench to a 6" pot every two weeks. Of course, the drench should be added to soil that is already moist and I stress that it is for commercial use. The drench is composed of 1lb of aluminum sulfate in 10g of water. It is used with 35% sulphuric acid source to keep the soil Ph at 5.8 and a low phosphorus and high pottasium fertilizer.

Wow... nobody said life was simple, hu? Ha!
Luis

Luis,
very interesting. Thanks for add'l info.

Especially the triple superphosphate part. I have been feeding heavily the past couple of years with high phosphate fertilizers to increase bloom production. Sounds like that contributed to my blooms becoming less blue.

waaaah... I want it all... blue and bountiful ...

Wendy,
Dirr recommendation (from memory):
1.5oZ of alluminum sulphate per 3G of water, weekly application starting from the time when buds are just start forming till first flower to open. Water thoroughly the night before the application. Don't overdoze and don't do more frequently than once a week.

George, isn't alum sulphate granular? is 1.5 oz a liquid measurement? if granular, I assume it dissolves easily in water. Above amt for a single established plant?

Glad to hear its relatively quick acting, unlike lime which can take several months to increase ph.

wendy,

Phosphorus fertilizers inhibit the uptake of the aluminun ions needed to develop blue "flowers."