Are You Paying Too Much for Fertilizer?

Fertilizer prices have gone up quite a bit in the last few years and that is especially true for commercial gardening products. There is also a wide range of prices for similar looking products.

Are the higher prices worth it? Are you paying too much?

In this post, I’ll show you a simple way to compare the real cost of fertilizer so that you can make an informed buying decision.

This post uses affiliate links

The Real Value of Fertilizer

Bags of synthetic and organic fertilizer are covered with all kinds of claims but the real value is in the nutrients that are made available to plants. Forget about claims that one source is better than another. There is almost no scientific evidence to support statements like that.

Fish emulsion is not superior to worm castings, or animal manure compost. Proponents of a particular source of material swear that theirs is best and manufacturers also make false claims, but the reality is that they all have the same value to plants, microbes, and soil. I have explained this in detail in the video: How Many Different Fertilizers Should You Use?

The real value of fertilizer rests on two important characteristics: nutrients and carbon. Products can be easily evaluated for these two factors.

Cost of Plant Nutrients

As far as the plant is concerned it is all about nutrients. When they are provided at a good level, plants grow at their maximum potential for a given environment. If even one nutrient is low, plants don’t grow as well.

The nutrient that is most likely deficient is nitrogen and this goes for plants growing outdoors in soil as well as houseplants growing in a soilless media. The reason nitrogen is commonly low is that it is very mobile, dissolving easily in water and moving with it. It is also in high demand for every living organism including microbes and insects. They all horde nitrogen.

The relative cost of nitrogen is also higher than other nutrients. In the last few years, the cost of nitrogen has risen faster than phosphate and potassium.

Given these facts, nitrogen is a good way to measure the value of different fertilizers and that is fairly easy to do because most bags of fertilizer display the NPK value right on the bag. The “N” number is the % nitrogen in the bag.

As I was writing this article I looked at a lot of bags of compost and many in North America are no longer providing the NPK. They just tell you the product is good for the garden. Not showing the NPK value is also common in Australia.

Personally, I want to see the NPK number because it is an easy way to compare products. A bag with no values for nitrogen, phosphate and potassium on the label is suspect and may contain a lot of wood or peat moss, which adds little in the way of nutrients.

Relative Cost of Fertilizer

I have calculated the cost of various types of fertilizer in the table below. Costs are in US dollars, based on the list price for products found on the internet. The ones with links can be purchased from Amazon.

The value in the second column is the cost of one pound of nitrogen. This column makes it easy to understand the relative cost of each product.

Fertilizer	Cost per pound of nitrogen	Amount of carbon
Alaska fish fertilizer, 5-1-1, 1 gal, liquid	$74	very low
Neptune organic fish, 2-4-1, 36 oz, liquid	$435	very low
Superthrive, 0.5-0-0, 1 pint, liquid	$6,261	none
Espoma plant-tone, organic plant food, 5-3-3, solid	$26 (36 lb bag) $65 (9×4 lb bags)	high
Down to Earth organic fertilizer, 4-4-4, 1 lb, solid	$225	high
Wiggle-worm worm casts, 1-0-0, 30 lb, solid	$113	high
EarthSafe bat guano, 8-3-1, 1.25 lb, solid	$260	high
Black Kow compost, 0.5-0.5-0.5, 40 lb, solid	$33	high
Earth Science worm castings, 0.5-0-0, 12 lb, solid	$383	high
No-name cattle compost, 0.6-0.4-0.4, 28 lb, solid	$33	high
Miracle-Gro, 24-8-16, 1 lb, solid	$63	none

The loser in this list is Superthrive with a cost of $6,261 for a pound of nitrogen. The rediculous high price is due to a low nitrogen level in a very expensive bottle. Worse than the price, they still promote vitamin B1 as being good for plants after research has shown it is less effective than water.

The bat guano by EarthSafe was a surprise. The collection of bat guano is not “earth-friendly”.

In general, solid products are a more cost-effective than liquids. Larger containers also keep costs down. For example, you can buy a 36 lb bag of Espoma Plant-tone from Amazon for $46, or you can buy 9 x 4 lb bags (also 36 lb) for $117.

Specialty fertilizers such as fish, kelp, and vermicompost are much more expensive than animal composts or green composts. Contrary to popular belief, science says they do not add extra value to your plants.

I have also added a synthetic fertilizer, Miracle-Gro, to the list for comparison.

Calculate Your Own Cost

You can calculate the nitrogen cost of any fertilizer using this simple formula.

Cost of one pound of nitrogen = Cost of the product/(Weight of fertilizer x % nitrogen/100)

The formula works with any weight unit, e.g. Kg, grams, etc.

Example

A bag of fertilizer costs $25 and contains 3 pounds. The NPK is 5-4-3.

The cost of one pound of nitrogen = $25/(3 x 5/100) = $166.67

The Cost of Carbon

Why is carbon important? Carbon is the organic matter in fertilizer and it is the key component that feeds microbes, which in turn produces healthy soil. Microbes also use the same nutrients as plants, but for long-term soil building, nothing beats carbon.

If you are gardening with soilless media in pots, containers, or some raised beds, then carbon has no real value to you or the plants because you are not trying to build soil – you do not have any soil.

However, if you are gardening in real soil, and that could be in raised beds and containers, carbon has value long term.

We can define carbon as being the solid material in any organic fertilizer. That is not 100% correct but it is an easy way to measure it and this definition works for our purposes. Synthetic fertilizer has zero carbon.

Calculating the amount of carbon in a product is a bit complicated. You need to know the dry weight in a product and few manufacturers give you that. A few compost bags provide the % moisture value which can be used to calculate the amount of dry material.

For gardeners, we can simplify this with a few generalized rules:

• Synthetic fertilizers have zero carbon.
• Liquid organic fertilizers have very little (suspensions) or no carbon (clear liquid).
• Solid organic fertilizer has about 50% moisture plus 50% carbon.

These rules give you a rough idea of the amount of carbon in a product.

Suppose you are gardening in real soil and interested in building soil structure and increasing the number of microbes. In that case, you should always select a product that adds high levels of carbon along with your fertilizer. I prefer animal compost both for price and because it is readily available where I live.

Compost and Soil Calculator

How much compost or soil do you need to buy for a given project? I have created a handy online calculator that you can use for this.

Soil Calculator