Confused about fertilizing indoor plants

danesnpits(2a)September 23, 2013

I have about 55 species of indoor plants. I am still learning how to look after them and have made a list of each plant and their care from water, fertilization, light etc. I am confused. Some of the plants I have say to use 10-10-10 ratio of fertilizer. For the life of me I don't see any indoor 10-10-10, just outdoor 10-10-10..So what would a person do? Should I just take my 20-20-20 and half dose?

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A ratio stays the same when you change all the numbers by the same factor. It's like putting a fraction in lowest terms: it's still the same fraction whether you call it 1/2 or call it 8/16. So yes, a 10-10-10 ratio is the same as a 1-1-1 ratio or a 20-20-20 ratio, and you would just use different amounts.

However, from what I've read here, no plant wants a 1-1-1 ratio.

I don't actually know anything from experience about what fertilizer plants want. I have about 2 species of indoor plants, myself.

    Bookmark   September 23, 2013 at 7:27AM
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Tiffany, purpleinopp GardenWeb, Z8b Opp, AL(8B AL)

Too little fertilizer = no problem. Too much = likely toxic.

Spreadsheets are great for those kinds of lists, BTW...

    Bookmark   September 23, 2013 at 9:02AM
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if you feed with every watering 1-1-1 ratio is very good for lots of plants. especially those with colorful foliage with pink/red in it. i give it to my caladiums (which i grow strictly indoors), crotons (half a year indoors), coleus (outdoors), begonias when they are growing leaves only. also calathea/stromante and African violets.
it could be used for succulents too or slow growing plants that dislike hi N.

    Bookmark   September 23, 2013 at 10:27AM
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Al (tapla) has a long post explaining fertilizer.

Key points:

1. Excess of any nutrient makes it harder to take up water and other nutrients, and in extreme cases can cause "fertilizer burn", or even kill the plant outright.
2. Plants use nutrients in a ratio of about 3:1:2 (as measured by the NPK numbers).
3. A plant can grow until it either reaches its full genetic potential, or until its growth is limited by scarcity of something. If growth is limited by scarcity of one element, there's no benefit from having more of some other element. So it's good to have the fertilizer be pretty close to 3:1:2.
4. Factors like pH and temperature can limit an element's availability, even if it's present in the right amount. But normally it's better to fix the conditions than to try to compensate by providing excess of the unavailable nutrient.

Here is a link that might be useful: Fertilizering Containerized Plants IV

    Bookmark   September 23, 2013 at 11:25AM
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tapla (mid-MI z5b-6a)

First, you're correct in thinking that half as much 20-20-20 will yield a solution the same as 10-10-10 (if it was soluble). Another way of saying it is, 1 tsp of 20-20-20 in a gallon of water yields a solution of the same strength as 2 tsp of 10-10-10 in a gallon of water. Generally though, 10-10-10 formulations are slow release, which is different from controlled release, and 20-20-20 formulas are usually soluble in water. While you wouldn't normally mix 10-10-10 in water, it does contain half the N, P, and K that 20-20-20 contains.

A lot of growers are quick to recommend a fertilizer with a specific NPK %, but most are unable to tell you precisely why they recommend it. Often, they will offer the idea that fertilizers with a higher first number (nitrogen) are too high in nitrogen to be suitable for houseplants; but they overlook the fact that the amount of nitrogen supplied has nothing to do with the fertilizer ratio, and everything to do with how much N is supplied by the grower. It's as easy to oversupply nitrogen with 10-10-10 as it is with 24-8-16. Actually though, the consequences of oversupplying nitrogen when using 1:1:1 ratio fertilizers like 10-10-10 or 20-20-20 are considerably more severe because you can't even supply an ADEQUATE measure of nitrogen (using 1:1:1 ratios) with out oversupplying both phosphorous and potassium.

The perfect fertilizer program would find the grower providing all the nutrients essential to normal growth in the exact ratio at which the plant uses the nutrients, and at a concentration neither so dilute that deficiencies are limiting nor so concentrated that toxicity or antagonistic deficiencies are limiting. It's actually impossible to come even close to achieving that end when using a 1:1:1 ratio fertilizer, or fertilizers actually providing more than 1/6 as much P as N. After the factoring is done for the fact that P and K are actually reported as P2O5 and K2O in a fertilizers NPK %s, you'll find that fertilizers with 3:1:2 ratios (24-8-16, 12-4-8, 9-3-6 are examples of 3:1:2 ratio fertilizers) supply nutrients in almost exactly the same ratio as the average at which plants use therm.

There is a significant variance in the overall volume of nutrients taken up by plants, but only a small variance in the ratio the plants actually use. I grow hundreds of species of plants, indoors and outdoors. I formerly used Miracle-Gro 24-8-16 or 12-4-8, to which I added other nutrients. I now use Foliage-Pro 9-3-6 for every plant I grow. It contains all the essential nutrients in a very favorable ratio. It also gets none of its N from urea, which helps to minimize the impact of low light and helps keep houseplants compact.

I know that all sounds complicated to a beginner, but if you have questions, don't be afraid to voice them.


This post was edited by tapla on Tue, Sep 24, 13 at 9:08

    Bookmark   September 23, 2013 at 3:12PM
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I just did a quick search for old forum posts on the subject, but I'm not at all clear about the different forms of nitrogen.

So urea is better than ammonium, because too much ammonium is toxic. But urea decomposes into ammonium, so does that mean nitrate is best?

At the high end of the soil pH range, a little of the ammonium should get deprotonated, turning into ammonia. Once it's inside the plant, it's at the plant's normal pH, regardless of what it was outside. So does soil pH matter when you have ammonium fertilizer, or are the two forms both absorbed equally?

Why does no urea keep houseplants compact?

Then there's calcium and magnesium, but let's leave them and stick with the nitrogen for now.

Here is a link that might be useful: the original version of Al's fertilizer post, with some nitrogen discussion in later posts

    Bookmark   September 24, 2013 at 11:02AM
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After reading all the info, and especially Tapla's, thanks Al! I have bought myself that dyno gro stuff, 9 3 6 I believe? I had to have it ordered in, as no one carries it where I live. But I got a big jug of it, and so far lovin' it!! I can say goodbye to the stupid fertilizers I have..which is about 10 different ones!

    Bookmark   December 4, 2013 at 6:54AM
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rhizo_1 (North AL) zone 7

I hope you don't toss your old stuff...just use it more wisely.

    Bookmark   December 4, 2013 at 3:56PM
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