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How to quantify nutrient content in liquid fertilizer

In a recent post, I discussed how it was actually cheaper to spray soluble vs granular fertilizer. What about if we use pre-mixed liquid fertilizer? How do we even figure out how much nutrient we are applying with pre-mixed liquid fertilizer? 

Before I learned that you could simply dissolve soluble fertilizer in water and apply it in a sprayer, I was a big user of pre-mixed liquid fertilizers. One of the issues I initially had was figuring out exactly how much of each nutrient I was applying. The math wasn't as straightforward is it was with granular fertilizers. It turns out, it's actually not that difficult but requires an extra step. 

First, we need to convert the liquid volume into a mass. Many products will have the product density displayed on the label or you can look in the Safety Data Sheet (SDS) for that information as well. No SDS? Should you be using products without an SDS?

Even if this information isn't included on the label it is very easy to figure out. All you need is a measuring cup and a kitchen scale. Simply measure the mass of 1 liter of the liquid fertilizer product and you now have the density. 

Let's pretend you have some super fancy liquid fertilizer that has an analysis of 12-0-0. You weigh it out and find that each liter of product weighs 1.3 kg. Remember, 1 liter of water weighs 1 kg but that's not important because the analysis should represent the total amount of the entire solution.

So we have 1.3 kg per liter and 12% of that is nitrogen. That means that in each liter of this liquid fertilizer we have 156 grams of nitrogen ( 1300g/kg * 0.12 N = 156 grams N). If you have a 10 liter jug that means that each jug contains 1.56 kg of nitrogen.

To determine the urea equivalent you can divide this figure by 0.46 or the analysis of nitrogen in urea. For the example above, this works out to the same amount of nitrogen as is found in 3.39 kg of urea. As of writing this post, Ag grade urea costs me $1.23/kg. That at the super inflated price gouging post covid prices too. So the total urea equivalent of nitrogen found in this example product costs $4.17 per jug of liquid fertilizer ( 1.23 x 3.39).

I have absolutely no clue how much liquid fertilizer costs these days but lets just pretend that each 10L jug costs us $50. That means that we are paying $45.83 for "other stuff" found in the jug that isn't fertilizer. That is about 10x as much money for stuff that isn't fertilizer, as is stuff that is fertilizer.

This make me wonder a few things because I currently spend very little on fertilizer and plant health products like fungicide. It actually works out to that I spend about 10x as much on fungicide as I do on fertilizer. Maybe if you use these products you won't need fungicide? Is that anyone's experience?

Is the cost of liquid fertilizer a good deal? We haven't used any here in years.

Anyway, it's important to know what rates of each nutrient you are applying and it's actually not the difficult to figure that out. I hope this helps you improve your fertilizer record keeping and maybe saves you a few dollars along the way.


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