If your issue is lack of fertility then there is no such thing as a potting mix that will work for you. You need to fertilze regularly.

There are rules of thumb to guide you in terms of how much and how often such as 1/4-1/2 label strength every 3rd watering.

Start there and observe the results over the weeks, you can always increase if necessary. Granular fertilzer isn't all that great in pots as much of it requires bacterial/fungal action to make it available to the plant roots and 'soilless' mixes don't have any and pots which get real hot have it all killed or forced dormant. Use a liquid plant fertilizer that contains macro, micro and trace elements. Mixing some compost into the mix can also provide benefits in terms of micro nutrients and water retention.

Generally speaking potting mix with fertilizer is no better than potting mix without it as it simply doesn't last long at all. Plants in pots MUST be fertilized regularly.

I agree with username_5, except that 'soilless' mixes do have a great deal of microbial action, which will break down fertilizers to make them available to plants. That being said, granular fertilizers are not a good choice for containers.

The fertilizer that came with the potting soil was probably a slow release N-P-K fertilizer such as Osmocote. Good stuff, except it has no minor nutrients. Soluble fertilizers and compost can provide micronutrients if you can't get a micronutrient fertilizer like Micromax.

Thanks so much for those responses. For years I labored under the delusion that all you needed to do was put a plant in soil and the plant would do the rest.

Very exciting!

On to trying this and that to see what works.

There is however the fact that plants vary a great deal in their tolerance of poor soil. Of course weeds being the best example. Somehow they manage without any fertilizer forever. Anyone know why that is???

You are quite right that not all plants require the same amount of fertility. Wildflowers, aka whatever is native to your region (called weeds if we don't like where they are growing ;-) grows with no supplemental fertilization.

Some plants are able to take nutrients from the air, legumes do this with nitrogen. Other plants don't expend much energy so they don't require much food, sepervivums and most desert/alpine plants fit into this category.

Roses, however, most certainly do not.

I am sure there are many reasons for varying nutrient requirements that I haven't mentioned (and don't know about) as well. That is why the general rule for potted plants is to fertilize with a complete fertilizer at 1/4 to 1/2 strength every 3rd watering and observe the results. At this rate very few plants will be over fertilized and very few so under fertilized that they die quickly. This is a good rate to start with when you don't really know where to begin and works well for most things. It is just a generality though, there are, of course, exceptions.

Interestingly I was reading up on container water gardening last night and repeatedly saw the claim from multiple sources that plants grown in water do not require as much fertilization as the exact same plant grown in potting soil even though the plants in water often outgrow those in potting soil. I am not yet sure if this is true or why it is true, if it is. Hmmmm...

Mind if I chime in here? Back in the 1970s I read an article about growing houseplants in nothing but perlite and 1/4 strength fertilizer/water mix. I am quite sure it was good old Miracle Grow. Anyway, it worked very well for foliage plants. I don't recall that I grew any blooming type plants. Funny part was that you were to fill the no-drainage container with fertilizer water all the way to the top of the perlite. Apparently the perlite held enough air to keep them from drowning. Guess the point I am trying to make is that 1/4 strength mix won't burn the roots. I rooted mine in it, so I know there were very tender roots in contact with it.

Most plants are pretty tolerant of a wide range of fertilizer application rates. At the extremes they won't make it but the area in between is pretty big.

A couple of you made points about wildflowers and weeds growing with "no supplemental fertilization". That's not really the case for plants in the ground as there's a constant process in the soil of organic material being broken down into nutrients that can be absorbed by plants. In addition, roots can expand over a wide territory in search of additional nutrient-rich soil. Plants in containers can't do that, so when all the nutrients in a pot have been used up we have to replace them with fertilizers.

Hi, Jim. I've been thinking about this post since it came up more than a week ago. Every time I thought about beginning a reply, what I had to say would lead from thought to thought, changing subject along the way, until the reply seemed disjointed in my mind. Trying to distill my thinking into a short reply is difficult.

First, plants really aren't particular about what soil is made of. As long as you're willing to stand over your plant & water every 10 minutes, you can grow most plants perfectly well in a bucket of marbles. Mix a little of the proper fertilizers in the water & you're good to go. The plant has all it needs - water, nutrients, air in the root zone, and something to hold it in place. So, if we can grow in marbles, how can a soil fail?

Our growing skills fail us more often than our soils fail. We often lack the experience or knowledge to recognize the shortcomings of our soils and to adjust for them. The lower our experience/knowledge levels are, the more nearly perfect should be the soils we grow in, but this is a catch 22 situation because hidden in the inexperience is the inability to even recognize differences between good and bad soil(s).

Container soils fail when their structure fails. When we select soils with components that break down quickly or that are so small they find their way into and clog macro-pores, we begin our growing attempts under a handicap. I see anecdotes about reusing soils, even recommendations to do it all over these forums. I don't argue with the practice, but I (very) rarely do it, even when growing flowery annuals, meant only for a single season.

Soils don't break down at an even rate. If you assign a soil a life of two years and imagine that the soil goes from perfect to unusable in that time, it's likely it would be fine for the first year, lose about 25% of its suitability in the first half of the second year, and lose the other 75% in the last half of the second year. This is an approximation & is only meant to illustrate the exponential rate at which soils collapse. Soils that are suitable for only a growing season show a similar rate of decline, but at an accelerated rate. When a used soil is mixed with fresh soil after a growing season, the old soil particles are in or about to begin a period of accelerated decay. I choose to turn them into the garden or they find their way to a compost pile.

Unless the reasons are economical, I find it difficult to imagine why anyone would add garden soils to container soils. It destroys aeration and usually causes soils to retain too much water for too long. Sand (unless approaching the size of BB's), has the same effect. I don't use compost in soils because of the negative effect on aeration/drainage. The small amount of micro-nutrients provided by compost can be more efficiently added, organically or inorganically, via other vehicles.

To boil this all down, a container soil fails when the inverse relationship between aeration/drainage goes awry. When aeration is reduced, soggy soil is the result, and trouble is in the making.

This can even be carried into the realm of fertilizers, which the soil has great effect on. How can it be suggested to fertilize at this strength and at that interval without knowing the soil or plant material. It's impossible. Plants sharing the same genus often have extremely widely varying nutrient needs. Even the age of the plant causes a wide variation in nutrient needs. Fast soils require higher concentrations and more frequent applications than soils that drain poorly & hold onto water for long periods. Fertilizing on a schedule is always something of a crap-shoot and more often than not will find you under-fertilizing during peak growth & over-fertilizing during the heat of summer. If lucky, the plant won't complain too much, but leaf color is a better method of gaging nutrition needs than adhering to a schedule.

Millie - Plants grown in water actually grow a root system that is physiologically different from those grown in soil, so it wasn't that the perlite held more air, rather that plants grown in aquatic mediums develop tissues called aerenchyma in roots rather than the parenchyma usually seen in mediums with more aeration. This type of tissue has more air-holding ability and is much more brittle than roots formed in soil. Also, recommended full dose concentrations of fertilizer will not "burn" roots, even new roots, when applied when the plant is well hydrated. Problems occur when repeat dosages cause N and/or salt levels to go higher than those in full dose concentrations.

Al