I've been trying to figure this out myself, and I gather the answer is more complicated than your approach because the various ingredients in the liquid fertilizer all likely have densities different from water and, presumably, one another. And even if we took the time to figure out the density of whatever sources of N, P and K are listed on the label, we don't really know what other ingredients (surfactants? stabilizers? whatever?) are in there.

So I think it is practically impossible for us consumers to convert a given volume of liquid fertilizer into weight equivalents of N, P and K, and we need to actually weigh the liquid itself instead.

The volume isn't what matters. The manufacturer has to tell us how much to apply per area. A percentage of liquid isn't the same as a percentage of dry fertilizer. The application rate is what is important. In most cases it has to be diluted with water for application and that is taken into account by the manufacturer. I've applied a couple doses of Neptune's Harvest and really haven't noticed anything other than it reacted terribly with some commercial plant supplement I tried to mix it with. Do you have a known liquid fertilizer without application instructions?

Hi Soccerdad,

The problem with using the application rates is that if we are trying to hit specific amounts of NPK over a year, based on a soil test perhaps, the application rates don't tell us how much of it was put down by following the recommendation. Guaranteed analysis is based on percentage by weight, and AFAICT this is true of liquids as well as granulars.

I would guess that there are good reasons to put down liquids at a reduced rate, and that the suggested application rates are likely good to go by, but still we just don't know how much NPK actually went down without doing a little more figuring.

Not sure if I understand, when a bottle of liquid says 10-X-X it means 10% of it is nitrogen right (don't know by volume or by weight) what's the difference ?

So if I weight up 10 lbs of this liquid fertilizer I should 1 lb of nitrogen. It maybe seem like a large volume like 4-5 quarts. But then to get 1 lb of Nitrogen using dry pellets it would take somewhat large bag also, and to think that 1 lb of nitrogen(or something close to it) is dissolved in a few quarts of liquid makes some sense to me.

Here is an example. Maybe we all can figure this out.

Product X
NPK: 18-3-6

18-3-6 may be applied as a spray application on all turf
grasses: cool, transitional and warm season.

COOL, TRANSITIONAL AND WARM SEASON GRASSES: Rates
may vary from ¼ to 1 pound of nitrogen (17 to 68 fluid ounces) per 1000 sq ft in 4 to 6 applications at 8 to 12 week intervals.

Apply in 1½ to 4 gallons of spray solution per 1000 sq ft.

Pounds N per gallon .................1.90
Pounds per gallon...................10.60

The above I copied from the label pdf for this product. It is clear that the N is a percentage of the product weight. It is also clear that there is some leeway in how strong you can mix the fertilizer. You clearly don't apply straight from the bottle. My problem has always been understanding if and how much does the nitrogen get diluted when mixed with water. Maybe I've always had the wrong impression.

My interpretation of the mix would be a half gallon (+ 4 oz) of fertilizer mixed with 4 gallons of water would equal one pound of nitrogen per 1000.

But if I didn't have this product label then this may not be a consistent way to measure. I still think there is something fishy when you have to mix it with water, but perhaps I am wrong.

What do you guys think?

Hey Soccer Dad,

That's really interesting to me, since the only liquid fertilizer I have (inherited from a neighbor) does not have that type of analysis, though I just noticed that it includes a weight indication next to the volume indication, which is very unusual to see in other liquid products. For example I have bought gallons of milk, juice, paint thinner, and beer, and I have no idea how much any of them weigh, say per ounce.

With a label that good, I agree with you, that is all of the information one would need.

On the dilution question, I hear you but suspect that that *mostly* isn't relevant: the water is just the medium used to meter and deliver the fertilizer. Just like the "other stuff" in a bag of granular fertilizer which, after you add up the N, P and K amounts, never equals 100% or even anything close to it.

I am thinking that if you apply so many ounces of the stuff over so many thousand feet, and have that label to refer to, you will know how much nitrogen you put down. Whether it all gets used by the plants (and I think we all know that all of it does not) is another matter. And since liquid forms of fertilizer - or at least those sold to homeowners - are mostly designed to be taken in by the leaves (so-called "foliar feedings") I can see how mixing the product with more water than the manufacturer recommends could "dilute" it, reducing its immediate potency, even if you have theoretically put down the same amount of fertilizer per thousand feet, but only because the leaves can't take it in if it has run off due to too much water in the mixture.

Then again I can't imagine that the nitrogen that has run off like that has just disappeared; it must be in the soil now, and maybe slower-acting, but still there. Whether it is in a form that can be easily used by the plants I don't know.

Thank you very much for posting that label, great discussion, I hope others can help out. I mean, the reason I still have that inherited liquid fert sitting in my garage is that I have been at a loss to really understand it.

Paul

Hi Poorowner,

Not sure if I understand, when a bottle of liquid says 10-X-X it means 10% of it is nitrogen right (don't know by volume or by weight) what's the difference ?

Yes, it's by weight. Weight is the standard because these elements all have a certain mass, and the recommendations for how much of this or that to put down over a given area are all pegged to that. Volume is too elastic a measurement given how it can be manipulated with useless additives. I understand if that last bit may not make immediate sense, so think of it as weight being an unadulteratable truth, and volume being something for the fertilizer company marketing departments to manipulate.

So if I weight up 10 lbs of this liquid fertilizer I should 1 lb of nitrogen. It maybe seem like a large volume like 4-5 quarts. But then to get 1 lb of Nitrogen using dry pellets it would take somewhat large bag also, and to think that 1 lb of nitrogen(or something close to it) is dissolved in a few quarts of liquid makes some sense to me.

It is my sense that if you use liquid ferts it is more important that you go with the manufacturer's recommended rates than with any other kinds of ferts. For homeowner products, liquid ferts are virtually all designed to be foliar feedings, which means that you are going to apply as much fertilizer as one could expect the leaves to take up virtually immediately, and no more. And do that frequently. Meaning that if you take the time to calculate what you are putting down, with foliar feedings at the recommended rates you will find that you are putting down much less than 1 lb. N per thousand feet at every feeding, but you are doing it much more frequently. The result is more or less the same either way.

Soil feedings with products that linger can be done at higher rates, but only because the grass can continue to feed on them for a time, as opposed to foliar feedings, the value of which are gone right after they are done (if they are not overdone).

Hope that helps, and keep the questions coming!
Paul

poorowner, liquid fertilizer mixtures should not be equated with granular fertilizer. The former is applied over the top of the plant while the latter is for uptake by roots. Leaves are not the primary organ for nutrient uptake. Roots are. That is why we apply some water after a granular application to wash off the leaves. Liquid fertilizers are useful to 'jump start' a lawn at the beginning of the growing season or to bypass the soil when a particular nutrient is 'fixed' by the soil, e.g iron.
It is critical that the dilution rate is accurate when using liquid fertilizers or leaf 'burn' can be caused.

Back to the original question. If you have a generic 10-10-10 liquid fertilizer I'd be about half confident that you'd get 10% nitrogen by weight without knowing the manufacturer's instructions. I don't know of any industry standard in liquid fert. ronalawn's point about leaf burn is well taken. It's my impression that a liquid fert is about small frequent applications. I would not want to spray at a rate of 1# nitrogen per 1000 because I assume it is all quick release. If you try it let us know how it works out.