Deliquescent fertilisers

[rodsmith]

I'm getting a bit concerned about some of my smaller cacti, now that we are getting cooler and damper weather. During the summer and early autumn I feed with fertiliser once a month (Miracle Gro, which is low nitrogen and contains the necessary trace elements that cacti need). I have noticed that the soil in a few of my cactus pots is showing damp, even though I have done no watering for well over a month.

I have notice over the years that if left uncovered the raw fertiliser crystals become progressively wetter and, doing a bit of research on Google, it is a known fact that many artificial nitrogenous and phosphoric fertilisers are deliquescent (attract moisture).

It appears to me that in a damp atmosphere, even residues of applied fertiliser might attract moisture into the compost. This certainly seems to be happening to some of my cacti this year and I am concerned that now that the plants are no longer growing, the damp soil will have an adverse effect on the plants and roots.

Has anyone experienced this phenomenon and, if so, how can it be avoided?

It occurs to me that this could be partly weather related, eg if the weather remains cool after the last application of fertiliser (in September) the plants may become dormant earlier and there could be residual fertiliser in the soil; or perhaps I have been feeding too much fertiliser. The plants appear to be healthy though.

Will I need to repot those that are damp with fresh "fertiliser free" compost now, to avoid problems with rot over the winter?

Advice of fellow growers would be much appreciated.

Rod Smith

[DaveW]

I have only used the dissolvable Miracle Grow in a hose end feeder for my plants. If you used the Miracle Grow you dissolved first and then watered on I would not have thought it would then be hygroscopic in the soil. However I presume you are therefore using Miracle Grow slow release granules in the compost in order to be worried about water retention?

http://www.lovethegarden.com/product-de ... plant-food

A comment on slow release fertilisers from the following link.

"Well, while strictly speaking not an end-all to this problem of neglect, controlled-release fertilizers can help make up for our horticultural shortfalls by automatically delivering measured doses of plant nutrients over the space of two or three months... sometimes longer.

Such a product is a balanced combination of primary plant nutrients (nitrogen, phosphorus and potash... sometimes with trace elements included) which have been enclosed in a slightly porous polymer coating that allows its contents to escape into soil at predetermined rates instead of being poured out all at once. Some rely upon soil biology to assist in that release while others must wait for sufficient soil moisture and increased soil temperature to get the job done. Temperature and soil moisture are chief controlling factors in timing fertilizer release.

In a recent issue of Greenhouse Grower magazine, Goris Passchier of Northern Star Minerals reminds that, because the fertilizer formulation is actually a salt, it is hygroscopic and therefore attracts water through the coating which then turns the contents to liquid.

He tells us to think of M&M chocolate candies. Something very desirable covered in a thin candy shell. Stored in the chill of a refrigerator, the chocolate remains a solid; subjected to warmer temperatures, it becomes softer and a little can be squeezed out of the shell; but left on a hot surface, the contents become fluid and create a gooey mess even if gently handled."

http://www.hillgardens.com/ctl-rel-ferts.htm

Browsing the web for information though many Americans don't like Miracle Grow:-

"We get tons of email from people who are killing their plants with liquid chemical fertilizers, especially Miracle Grow (it's heavily advertised so many people buy). We guess they believe the ads or use too much, but rapid decline is the common complaint.

Equally bad as using liquid chemical fertilizers improperly is using commercial potting soils that have fertilizers, hormones or other stuff INCLUDED into the bag. You would be shocked by the amount of email we get about soils that kill plants, e.g. "I re-potted my plant and almost immediately the plant started turning yellow, dropping leaves and looks really sick and dying." is super common in our email questions

For whatever the reason, the story is true. We can say that EVERY experiment we have tried using such soils with potted plants has lead to quick death. As you might expect, most "my plant is dying" emails specified Miracle Grow soil probably because Miracle Grow advertises heavily and sells lots of bags. But other brands seem to be the same; that's what email says here

Must be too strong; too fast a release. Sadly, MOST store shelf space is now devoted to the soils injected with crystals of water soluble chemical fertilizers, so you have to shop even harder to find "clean" soil. You might consider mixing the chemically-injected soils with, say, plain Canadian peat moss or Perlite or coarse sand to dilute the chemical action ...we'd start with 50% dilution."

http://mgonline.com/articles/fertilize.aspx

http://davesgarden.com/guides/terms/go/2307/

Whether those comments apply to the Miracle Grow available in the UK, or are even correct I do not know, and cacti anyway tend to prefer a more acid soil.

As to your point whether Miracle Grow granules retain water in the compost when you are trying to dry it off I do not know. However it seems to require water and heat to work, something not usually occurring in the close season for our plants, so whether that alone would switch off it's water retaining capacity over winter or even if it would rob the rest of the compost of what little water was left in it and so dry that off I do not know. No doubt our tame chemists will tell both of us?

DaveW

[rodsmith]

Thanks for your comments Dave, and the copied quotess. It looks as though Miracle Grow has caused problems to others. I am not using the slow release version, just the dissolvable crystals which I water onto the soil. I should also have mentioned that I keep a cool fan heater on all year to maintain a buoyant atmosphere and I have another fan heater set at 6 degrees C to maintain a reasonable temperature in the winter. I will be interested to hear other members' views.

Rod

[anders]

I have fed my cacti with liquid fertilizer at every watering (1–2 weeks interval) since spring, but I have not seen anything similar to what you describe. Not MiracleGro, and less than the recommended dose, though. If what you see really is a solution of deliquescent salts I think you have been too generous.
Have you measured the humidity?

[iann]

The amount of fertiliser required to attract enough moisture to make your soil visibly damp would be very damaging to the plants. This is doubly true if you have watered in a solution.

It may be that your soil itself is somewhat "deliquescent" at least to the extent that it rapidly absorbs any dew that forms. The humidity in your greenhouse at this time of year is likely to be around 90% and condensing conditions will occur frequently unless you heat or ventilate strongly.

There may also have been more moisture than you realised still in your "dry" pots.

[Adrian]

F

[rodsmith]

From my own experience I've noticed when a pot stays wet for longer than it should there may already be a problem with the plants root system and nothing to do with the fertiliser used. If the plants roots are healthy the soil will usually dry out reasonably quickly even at this time of the year, if there is a problem the soil stays damp for much longer, it's especially noticable in plastic pots. My own treatment is to carefully tap the plant from the pot, even at this time of the year, if the roots appear white and healthy I can simply place it back in it's pot with rootball intact and minimum disturbance. If there are no signs of roots on the outside of the rootball or the roots are brown and soft/rotting the plant is stripped of soil, any dead or dying roots removed and then allowed to dry off for a few days before being placed in a small pot with pure, dry grit sand to overwinter.

Thanks for the helpful tips and thanks Adrian for your suggestion, which I followed, checking one of the small cacti that seems to be worst affected. To my relief the compost was only slightly damp and there were plenty of white roots showing. The compost is quite gritty and free draining and I'm going to leave "as is".

Regards

Rod

[Lindsey]

I had same problem this year, with Phostrogen bought from local independent bits-and-bobs shop that sells a few gardening items. When about a third of the packet remained, I decided to bin it.

Wish we could still get that Chempak fertiliser in the little red plastic tubs. There's another similar product around, Vitax? It doesn't deliquesce but I don't like the smell

[DaveW]

"It doesn't deliquesce but I don't like the smell."

Probably to discourage fertiliser sniffers trying to get high on it Lindsey!

DaveW

[Jim_Mercer]

Wish we could still get that Chempak fertiliser in the little red plastic tubs.

I think it is still available but the tub has changed colour...

chempak.jpg (7.31 KiB) Viewed 4278 times