Yes, exactly. It's misleading, when compost is labelled with NPK. That makes it sound like (crappy) fertilizer. But it isn't fertilizer. Compost is an amendment that benefits soil structure and, at least in the short term, doesn't fertilize. I guess they could sell mulch or bottled water with an NPK listed.

Agree. Compost and composted manures are soil amendments, not fertilizers. The small amount of nutrients they contain is only available to the plant IF there is a highly active bacterial herd i the soil to pre-digest them. That herd doesn't exist in a container and simply loading more of the manures into the container solves nothing.

If you want to grow using only organics, which is difficult to do in a container even with an in-depth understanding of how they work, then use the many pre-digested, liquid organic supplements available and follow the label directions for application rate and frequency.

Trying to grow most variety of tomato plants in only a 12" container is nigh on to impossible to begin with unless it is one of the tiny dwarf or basket varieties. So it is vital that regularly and frequently supply the needed nutrients.

I'd suggest some in-depth reading over on the Container Gardening forum here as it has its own set of rules and regulations. The most common one being using much larger containers than 12" and choosing varieties bred specifically for container growing.

Dave

Examples of liquid organic fertilizers

Good recommendations to follow but when you do the math it isn't NPK levels in the fertilizer. It's N, P2O5 and K2O.

Further, when you try to do calculations comparing inorganic fertilizers to organic materials the nutrient availability also becomes an issue so it becomes more like comparing sugar in processed foods to that of fruits and vegetables.

Well, the P and K in NPK explicitly stand for phosphate and potassium oxide. Not phosphorus and potassium. That's just how it's defined. When you do the math, if you want fractional amounts of potassium and phosphorus, you need to scale the P and K accordingly.

Thanks for these helpful comments! Yeah, I guess I was assuming if something has 0.5% of nitrogen but you put in 5 lbs. of it per square foot, that's still a really large amount of nitrogen. However, it sounds like the consensus is that the 0.5-0.5-0.5 label is misleading and it's really more like 0-0-0 in my case. (And maybe the N, P, and K become available only "in the long term" or "with a highly active bacterial herd"?) It also sounds like I may have to transplant at some point into something bigger than a 12" container.

If you can tell us the varietal names of your plants we can tell you how big a container you will need.

As a very general guideline 5 gallon containers is the smallest recommended for the dwarf varieties, 5- 7 gallons (20" diameter) for determinate varieties, and 10 gallon+ for many of the indeterminates. Some of the cherry types are the biggest plants there are and often need even larger containers for best results.

Also keep in mind that containers need frequent nutrient supplements as many of the nutrients will leach out each time you water.

Dave

Thanks, I will look into it. It was given to me indirectly, so I don't know the varietal name at the moment. For the size, I used this list: http://www.hardytropicals.org/blog/entry/pot_sizes_inches_to_gallon_conversion

which said that 5 gallons is 12 inches. Can you recommend a more correct source for the inches / gallons conversion?

I'm not sure, but I think at least the N in compost relates to the nitrogen that can eventually get extracted once soil bacteria start to slowly chew on it. That's very different than the N in a nitrate-based fertilizer, where you get N amount of usable nitrogen like, right now. As Dave said, that's why compost isn't a fertilizer. A fertilizer is "right now" nutrition. There is very little plant-usable nitrogen in compost, but there is some "latent" plant-usable nitrogen that can eventually get released as the firmly attached proteins are digested.

I think the NPK ratings are required to represent the readily available nutrients in the compost. 0.5% is a very small percentage; I find it easy to believe that this compost contains that much available nitrogen, in form of ammonium or nitrates.

The tomato is a "celebrity hybrid" from burpee.

http://www.burpee.com/vegetables/tomatoes/tomato-celebrity-hybrid-prod001175.html

It is a determinate with a spread of 24-36". Let me know if this gives you a sense of how big the container should be? Or I can take this to the container forum.

By the way, I found a website that talks about the idea that the nitrogen in compost is "slow release":

http://www.gardenmyths.com/compost-fertilizer-numbers/

Supposedly only 3% of the nitrogen is released right away, which in my case is 0.005 x 0.03 = 0.00015 = 0.015% of the mass of the fertilizer.

Any nitrogen that is "released right away" is into the air, not the plant. Nitrogen is very volatile, easily vaporizes, into the air.

Celebrity s a semi-determinate variety that averages approximately 5' tall and 3'+ wide. It needs a MINIMUM container size of 5 gallons but will perform much better in 7-10 gallons. The larger the container the more stable the nutrients and soil moisture levels. The smaller the container the more they must be micro-managed for success.

Dave

The N in NPK is defined as the elemental weight of nitrogen in the fertilizer. So tightly bound N in proteins counts as much as nitrates. I agree there are some nitrates in compost, but not much. I'm guessing that most of the nitrogen is inaccessible to plants.

Ah, thank you Josh. Robert Pavlis calls it out explicitly. 97% of the nitrogen content is long-term release, and 3% is immediately available to plants. And that's out of a total of 1% nitrogen content.

Nitrogen in nitrates is not volatile. Nitrogen in ammonia is, and nitrification ideally converts ammonia into nitrates before it escapes. That's how urea works.