Concentrated fertiziler - how long is it good for?
 

I have been using an MSU dry fertilizer and mix it up for each round of watering. With the few plants I have, I may be measuring eights of teaspoons. Then I have to wait for it to dissolve.

I've read that a good approach is to mix up a batch of concentrated liquid fertilizer and then dilute that as needed to make your watering solution. You only have to measure the powder once, it gets dissolved, and you are much less likely to make errors due to measuring small amounts.

My question is, do I have to worry about that liquid fertilizer concentrate spoiling or going bad in some way?

I’m growing mainly Phals, indoors, at the present time, and they are doing well... Consistently growing new leaves, spikes, roots, flowers. Very little in the way of pests or disease.

Thanks in advance for helping a beginner.

It doesn't "spoil", per se, as it is mostly made up of inorganic compounds, and the iron EDTA is VERY stable.

What you DO have to watch for is the precipitation of calcium compounds, but that can be avoided by doing two things:

1) Use very hot water when making the concentrated solution in the first place, and let it cool slowly. Do not refrigerate it.

2) Don't make too strong of a concentrate.

For example, I do that with K-Lite, which has more calcium in it. For weekly watering I use about 75-100 ppm N. 75 ppm N is about 1/2 teaspoon/gallon for that formula. So that is stays stable, I make up my concentrate that has the 1/2 teaspoon in 2 ounces of solution - I use 3/4 cup powder (36 teaspoons) per gallon for convenience, so my 2 ounces per gallon of final solution yields about 85 ppm N.

The manufacturer says that the maximum solubility is about 2 pounds per gallon, but the Ca will drop out readily if the concentrate is that strong.

That's just what I was looking for.

Thanks for the detailed response and thanks for the info on your website. I find your particular style of presenting information very well suited to the way I learn and approach problems.

Why should you dissolve it in very hot water? Just curious as to how that will effect the storage life. Are you trying to sterilize it?

Hi

I think what Ray is saying, and if i remember from chemistry 101 correctly, that water solubility rises with temperature.

In some online documents - there's mention of hot water causing calcium compound of some sort precipitation.

True, though that was my concern. Wouldn't dissolving at high temps and letting it cool down again lead to potential precipitation? I know he said keep the concentration down and I'm guessing that might be why.
Maybe it's just to speed up dissolution. Though my experience with powdered fertilizers is they tend to dissolve very quickly even in room temp water.
It just seemed pretty specific in his instructions to use hot water and I was curious why as I can't really think of any immediate reason why hot water would be better than room temp.

Sometimes the above statement is correct, but not always. Search "calcium sulfate solubility temperature" on the web and one of the top results is a paper that includes solubility curves for various forms of calcium sulfate. Gypsum has its highest solubility roughly between 20 and 50 C, less soluble above that range. The anhydrite form of calcium sulfate is much less soluble at high temperature. There are more than a few compounds that behave this way.

Dissolution and the relationship with temperature depends on what you are trying to dissolve.

OW is right.

Most fertilizers containing calcium add it in the form of calcium nitrate. At 40°C (104°F) it's more than twice as soluble than it is as 20°C.

The issue, however, isn't solubility as much as it is "combinability". It likes to combine with other ions when in aqueous solutions, and they like to precipitate. Just shake the solution well before dispensing it.

So dissolving in hot water reduces the precipitation caused by recombination?