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Trichocereus soil pH and lime application Options
 
Ahubaba
#1 Posted : 5/4/2020 4:15:33 PM

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Hi all,

We came to the end of yet another growing season where I am from and I want to repot some of the larger cacti I have. I also want to use this chance to introduce the plants to some fresh soil and hopefully optimize growth for some problematic specimen in old soil. I had some strange things happenning to 2 of my larger pachanois. 1 pachanoi went a bit yellow under full sun. This plant was planted on the ground, then transplanted into a very soggy location with a lot of clay 6 months later before getting potted up. It has been in a 30cm pot for a year now but it always had that dissapointing growth rate and I noticed it tended to go yellow significantly under full and intense sun. I did a colorimetric pH test (where you mix some soil with pH indicator liquid) and found that the soil has a pH of around 6-6.5. Another spineless pachanoi was frequently going yellower under intense sun as well and the pot soil it was planted in had a pH of 6-5.5. I should add that both of these plants are completely healthy in colder months and show lush green growth except mid-summer. Now it is also worth noting that there was some soil compaction in the potting mixes for both plants, espeically in the case of the spineless pachanoi where I can see the small rootlets on the surface and the soil is basically rock hard but strangely does not include much clay. Also it seems that I have been applying liquid compost way excessively. The instructions suggest using it every 3-5 months but I accidentally applied the full dose every 2 weeks. The long story short, I might have concentrated the older soild with a lot of nutrients which is why I want to repot the plants into much larger containers with a lot of fresh soil in them, and preferably make a slightly more alkaline mix to counteract the somewhat lower pH in the old soil. This brings me to my final questions: how much powdered dolomite lime or powdered limestone (gardening lime) I should add per volume of potting mix to achieve a pH of 6.5-7? Also should I break the compacted soil while repotting the cacti? The main ingredients of the potting mix will be as follows:

2 part sieved potting mix (all purpose)
1 part perlite
1 part sand

I know you might think there are posts out there addressing this question but believe me no one tells the EXACT amount of lime they use on trichocereus.

Also these are the fertilizers I have been using if that gives you a clue about compaction as I heard too much cal/mag may cause soil compaction which is one the suspects as well:

-Synthetic all purpose garden fertilizer at 1/9 strength 1gr/L (N: P:K 25:5: 8. 8 ) + S 4.6 / Mg 0.5 / Fe 0.18 / B 0.005 / Cu 0.005 / Zn 0.004 / Mo 0.001

-Osmocote slow release all purpose fertilizers with trace elements was sprinkled onto the potting mix at half recommended dose.

-Liquid Cal/Mag (5% / 1.4% respectively) at 1 ml/L

- Trace element mixture in powder form. 25g/9L was used since the recommended dose (25g/L=225g/9L !!) seemed unrealistically high for fortnightly applications. Ingredients: (% W/W) sulphur 6.29, Ca carbonate 10.00, Mg sulphate 3.62, manganese sulphate 2.88, iron chelate 2.73, copper sulphate 1.25, zinc sulphate 1.00, boron 0.09 and molybdenum 0.09

-Liquid potash (P 2.0: K 14.0) at 20 ml/9 L

-Liquid compost mixture containing fish and seaweed solution, humate and trace elements 50ml/9L

-Great White micorrhizae mixture in powder form (1 teaspoon/9L). These bacteria were used much less frequently, around 3 times per growing season.

-Organic phosphorus granules (N: P:K 0:12:0). This one was eyeballed unfortunately. Sprinkled over the soil sparingly each year.

Only 2 types of fungicides were used against black and orange spots with little success. Those were a solution of copper ammonium complex (93g/L) at 5ml/L and myclobutanyl at 0.05g/L.

I would appreciate every answer! All the best and keep safe Thumbs up
The fool who picked this pen will always be wiser than the fool who dropped it.
-Excrept from Chasing Shadows
 

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Grey Fox
#2 Posted : 5/5/2020 1:13:20 AM

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Ahubaba the yellowing is probably caused by too much sun during periods of hot weather. These cacti benefit from afternoon shade once the temps get into the 90's and above Fahrenheit. In much of their native range the high temps rarely go above the 70's F. High temps combined with full sun all day will cause the cacti to turn yellow and stall out new growth until the temps moderate. But with shade in the afternoon they remain green and continue growing, even when the temps are over 100 F.

Hopefully someone can answer your questions about potting soil. Generally speaking, most cacti are able to thrive in soil that is slightly alkaline. It is common for desert soils and ground water to be alkaline. I can tell you from personal experience that Trichos do great in soil that is slightly alkaline. Although they will probably be fine in neutral soil as well. I'm not sure about slightly acidic soil and how it affects them.

All the best with your cacti. Thanks for sharing your experience with us.
IT WAS ALL A DREAM
 
Ahubaba
#3 Posted : 5/5/2020 8:18:23 AM

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Thanks for you input Grey Fox. Yellowing and a bit of skinny growth is what bothers me the most. Though looking back at it I think I have bigger suspects like hot weather and full sun than soil chemisty. Also kept my plants in a shed for a month too which caused obvious etiolation. Too much going on to blame one thing really. I sure used a lot of organic fertilizer too in this growing season and in the next one I will try fortnightly application with much more dilute ferts. I will try 1/2 tablespoon gardening lime and 1/2 dolomite lime per gallon of potting mix for this next round of repotting and see how it goes. Also thinking of sprinkling half a tablespoon on the compacted soil and mixing it in. Probably will break the hard soil around the rootball too if the larger roots have not invaded the mix already. I always thought that messing up the ideal soil composition is easy while making it fertile again is not so simple so I see these repottings as a chance of revitalization.

Again thanks for chiming in an all the best Smile
The fool who picked this pen will always be wiser than the fool who dropped it.
-Excrept from Chasing Shadows
 
Grey Fox
#4 Posted : 5/5/2020 2:42:49 PM

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I wish I knew more about how best to fertilize and amend the potting soil. But all my experience with Trichos comes from growing them directly in the ground. My trichos get some organic compost worked into the soil at planting, and then some additional compost worked in every spring. But growing in containers is more complicated than that. I hope you get feedback from some other people. Take care Ahubaba.
IT WAS ALL A DREAM
 
 
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