what do you mean by "20cm"?

Also, there are two types of fertilizers.. synthetic and organic. Organic is cheaper in the long run( >3 years), but in the short run synthetic is cheaper. I just use free homemade compost (which is organic) as fertlizer and my veges love it.

-Mac

It takes 4 bags of 5-10-5 to give you the same N as one bag of 19-19-19.

The 5% is the N and 95% is the filler.

I hate paying for filler, it's like paying for ice in your soda pop. The higher the #, the less ice they are giving you.

Say one fertilizer costed 1$ and gave you 20cm
and another costed 2$ and gave you 20cm.

Assuming you mean 20 cm of plant growth, correct? If so then I would say there are other factors I'd consider about the 2 choices in addition to price if both gave me the same amount of growth.

For example:
duration of the benefit
ease of application
frequency of application
potential negative side effects or environmental impact
broad spectrum use vs. plant specific use
even source - some companies I just won't deal with

They can all affect the "cost efficency" of a product.

If you buy a truck load of melons and sell them for 25 cents the only to make money at 25 cents is to buy a bigger truck.

Very funny, Paul! I like your logic. I buy the highest number fertilizer equally listed in all three nutrients since I use it on everything...just more or less depending on how delicate the plant is. I believe they take what they need and leave the rest...assuming you did not overload them and harm the root system.

I broadcast 12-12-12 on my entire garden. I use 46% urea for sidedress on corn. I think the best all around fertilizer for all plants is 10-52-17. It's primarily for transplants but I think 1 cupfull on a pencil planted, it would take root. Large produce farms use this when transplanting strawberries and vegies.

Andrew,

You've obviously given some thought to this as you are already at the stage of using math to calculate the best deal. However, as some have pointed out there are all sorts of options in fertilizer that can affect ultimate cost that do not have to do with filler/weight/price of product. If labor costs are high, a time release fertilizer might be worth the added expense so that several applications are not necessary. If this is a long term planting, organic additions of compost might be good.
Without additional info, a meaningful choice can't be made here.

Yes I am taking everything into account. I am using hard and solid fertilizers.

The thing is I'm trying to measure the cost effectiveness in a graph to paste on to my board for a science fair by knowning cm of growth and fertilizer cost.

Do I measur the cost for each centimeter of growth or..
the number of centimeters you get out of the plant per cent?

Thanks a lot.

Andrew

One size (or ratio) does not fit all. Get your soil tested and use a fertilizer that gives your garden what it needs. Some of these suggested ratios would overdose my garden in one or more components. I also will never in my life need to add lime, but some people here suggest it for everyone.

The only way to show "cost efficiency" for fert is to have a control group that gets no fert at all, otherwise you don't have anything to measure against. For the difference to be meaningful, you'd also want to measure the harvest and compare taste, not just look at early incremental growth, unless what you're selling is seedlings.

But as others have pointed out, more variables than you could measure are in play, and the side effects can cancel out the benefits.

For instance, if the growth is too fast, the plant becomes more attractive to aphids and other pests, so I'd consider that a negative efficiency. Too much NPK, without paying attention to the soil microbes (which are negatively affected by megadoses) or to trace minerals that plants also need for optimum growth, can also make plants unhealthy and reduce their nutritive value.

So I save $$ and go for homemade compost, which feeds the soil microbes that feed the plants. After a couple years of using only homemade compost, a little bit of seaweed, and some fermented comfrey tea, I have few pest problems. My vegetables even taste better -- a neighbor who grew the same varieties but used an organic fert was complaining that her snow peas were not sweet, but mine were (same variety, only difference was no fert on mine). Last year I noticed the snail/slug population had decreased and I used hardly any Sluggo (the nontoxic iron compound), and the only time I had squash bugs was when I "imported" some from a neighbor whose spent squash plants I was composting.

Hey Andrew - excuse my cohorts here.. they think to much!

Here's your answer.

$1.00 for 20 CM of growth = $0.05 per CM. A nickel per CM.
$2.00 for 20 CM of growth = $0.10 per CM. A dime per CM.

your math is: Cost divided by growth.

So, if you paid $10.00 and your plant grew 5 CM then your cost per CM is $2.00 per CM.

If you paid $3.00 and had 43 CM of growth then your cost per CM is $0.07 per CM.

got it?

Now, after you've done that.. take into consideration what everyone has mentioned - you can't really judge the value of a fertilizer based on cost/growth alone. Actually, they way fertilizers are compared is be weight. Not just the weigth of the harvest, but the entire weight of plants, roots, fruit, etc.

so, say you wanted to compare 2 fertilizers;

Grab three plants (something easy and fast growing). plant them all in the exact same growing media (dirt). Your first plant is your control plant, do not fertilize this. Then, use your two fertilizers on the other plants. Make sure they have the same lighting/heat/water, and then stand back and watch.

At the end of the test, shake out all the dirt from the plant - roots and all, and weigh all three. At that point your can use a calculation based on weight and cost to give you a value indication.

Instead of cost per CM, you then have cost per KG, G, LB etc.

Hope that helps!! :)