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My tomatoes are developing purple spots at the bottom leaves. This is supposed to be a phosphorus deficiency. Tomatoes are growing in pots.

Small ones are in around 4 gallons (~14 L) and big ones are in 8 gallons (~28 L) pots.

I bought 100mL of 30% P2O5 and 20% K2O solution. So 30g of P2O5 and 20g of K2O It comes without instructions for tomatoes. How much should I add it to tomatoes? Or if you like chemistry, how concentrated solution should I prepare?

I am planning to water tomatoes with it. Should I spray it on the leaves too?

Update: enter image description here

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UPDATE:

Images taken 10h after adding 1g of p2o5 to 30kg of soil.enter image description hereenter image description here

Images taken 58h after adding 1g of p2o5 to 30kg of soil.enter image description here enter image description here

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Progression of slightly damaged leafs stopped completely or almost comletely, while the heavily damaged leaves still tend to get purpler. Just much slower. Like the plant isn't even trying to preserve them.

sanjihan
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  • This sounds straight forward, coming from you, but is there a way for you to send pictures? Please. These are your indoor tomatoes? Patio? What have you added in terms of Nitrogen? That is kind of a big deal to get the percentages correct. What mulches have you added? Anything you've added. I once gave instructions without enough information and the plant died. Why? Because the soil he used had fertilizer already in it in abundance. I felt so bad...more info please! And pictures... – stormy May 24 '17 at 17:56
  • https://gardening.stackexchange.com/questions/32993/do-purple-leafs-of-tomato-return-to-normal-state. Basically I am getting the same symptoms, just on other plants. I think the new 'infected' plants werent fertilized at all. Just garden soil with manure, coir and forest soil. Yesterday I've added small amount ~10g maybe, of granular NPK 7,3,10. It didn't disolve so I bought some liquid stuff. Oh and they were sprayed with algae a couple of days before. – sanjihan May 24 '17 at 18:49
  • You better write a book Sanijahan! Sprayed with algae? That is a new one for me. Where did you get that idea from? Was it live or dead? Where were you able to get that stuff from or did you make your own. You have already added fertilizer! Do not add any more! Doesn't have to be liquid for the plants to use. I would have preferred more phosphorous to be higher than the nitrogen but this is enough! Manure has Nitrogen in it. If that nitrogen raises the formulation 7 -3- 10 from 7 % to above the 10 of potassium it will reduce your reproductive growth and you will get fewer tomatoes. – stormy May 24 '17 at 19:00
  • Could you please send a picture? Answering questions on this site using pictures and back and forth asking questions to get enough information with which to answer responsibly is so incredibly tough. Verging on irresponsible. Please send a picture. Do not mess with adding any more fertilizer until we can see what you are dealing with. You are most likely right but have you noticed no one else is answering? Send me that site with the algae spraying info. If you want to try something that would be safe, find equisetum, Horse Tail. Dry and grind up and give that to your soil! Silicon. – stormy May 24 '17 at 19:13
  • algae are called ascophyllum nodosum. Popular solution where I leave. My parents love the stuff, but honestly I have no idea what they are and if they are necessary. i've added images. I've took them in the middle of the night, so flash made them look even greener as they are. That scary purple dots were covering only third of the leaf yesterday. I've also found some topic on the concentration of P in soil. The trick is to add ppm in your search query. They say 20-30ppm is what I am looking for. 1ppm converts to 1mg/kg soil. – sanjihan May 24 '17 at 21:21
  • I just love you Sanjihan! I'll play with this a bit. I'll be back soon. – stormy May 24 '17 at 21:29
  • Did you ever look up or purchase Jorge Cervantes 'The Cannabis Encyclopedia'? Gees, that is what I am looking into right now. The internet can be so lame. He does go into different plants as heck, Cannabis, is a plant and so he uses other plants when discussing...stuff like this in nitty gritty detail. And by your pictures I am 75% sure your diagnosis is correct. Buttt...hang on! – stormy May 24 '17 at 21:39
  • I've looked it up. None of the libraries in my area have it. Based on heard, I really need to order it from amazon – sanjihan May 24 '17 at 21:53
  • You HAVE to get this book, Sanjihan! Every time I dig into this book lately I actually think of you! This information is soooo well done and, I just know you will eat this book up! Need to have some idea about pH, think we just talked about this somewhere. Too high or too low the phosphorus already in your soil will not be available to your tomatoes. If there are deficiencies in zinc. When the pH gets too high there changes in its ion form making phosphorus unavailable. Temperature also dictates the uptake of phosphorus...cold soil below 50 degrees F inhibit phosphorus uptake... – stormy May 24 '17 at 21:58
  • Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/59257/discussion-between-stormy-and-sanjihan). – stormy May 24 '17 at 21:58
  • Do you actually mean Phosphorous pentoxide, or phosphorous, measured as pentoxide? The Pentoxide itself is **nasty** stuff, and liable to eat holes in your leaves. A hundred ppm (milligrams per litre) sodium diphosphate or similar is likely more than enough enough to perk up your tomatoes. Just get one of the many premix garden fertilizers, and use it. – Wayfaring Stranger May 27 '17 at 15:45

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