Dissolved urea fertilizer

16 years ago

I sowed a couple of flats with lettuce and will try raising them under lights (mostly to have something to do this time of year.)

Soil is 1/2 peat moss, 1/4 perlite, 1/4 sterile compost + 1/2 cup wood ash. Now they're up, I think I should feed them a bit of nitrogen but am not sure how much to use.

My thought is to raise the N by 0.05, i.e. go from N:P:K of X:Y:Z to X+0.05:Y:Z. I have ~20 lbs (10 kg) of (dry) soil in each flat so 0.05% is approx 5 grams of N. Since urea is half nitrogen, that would be 10 gr of urea.

So my inkling is to dissolve 10 grams of urea in enough water (1/2 gal) to evenly water the flat and water the lettuce with it.

Before I go off experimenting, I'd much appreciate a sanity scrub. A couple questions pop up:

Is the NPK calculated as a percentage of dry soil weight? I was kind of surprised just how much a thoroughly watered flat weighs compared to dry weight of the ingredients.

Is increasing N by 0.05 a reasonable amount?

I understand urea volatilizes in ~ 3-4 weeks. How often should I repeat this watering to maintain this nitrogen level? (It would seem that weekly watering with 1/3 of the urea amount would make sense, but I'm not at all sure my 'common sense makes sense' here.

I'm assuming the compost in the soil supplies enough P, and the 1/2 cup of wood ash both buffered peat acidity and supplied some K. If there's anything else I should add to these flats, I'd love to hear.

Thanks in advance.

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Comments (3)

justaguy2
16 years ago
Before I go off experimenting, I'd much appreciate a sanity scrub.

Stand by for the 'sanity scrub' ;) (you asked for it ;) Is the NPK calculated as a percentage of dry soil weight?

No, the NPK numbers have nothing whatsoever to do with soil weight. Those numbers are based upon the weight of the fertilizer product alone. If you purchase a bag of fertilizer labeled 10-10-10 then 10% of the weight is nitrogen, 10 percent phosphorus and 10% potassium, the rest is primarily filler. Is increasing N by 0.05 a reasonable amount?

I have no idea what you mean by increasing N by .05 I'm assuming the compost in the soil supplies enough P, and the 1/2 cup of wood ash both buffered peat acidity and supplied some K.

That is a very unsafe assumption. The organic matter provides no nutrients plants can use without biological processing. Your seed starting endeavor is not likely to play host to the biological organisms needed to do the processing in a reliable manner.

I would recommend adding Miracle Grow 24-8-16 at 1/4th strength with every watering after true leaves have formed as long as you water the flats to the point you have drainage water every single time you water. If you are watering in 'sips' then you need to switch the growing media to one that allows you to water to run off every time without risk of over watering. MG 24-8-16 is mostly urea so no need to dissolve the granular fert in water and it also contains most of the other essential nutrients in a form not dependent upon biological processing to be of benefit to the plants.
josko021
Original Author
16 years ago
I do appreciate the comments - thank you. As you can tell, I'm new at this.
The reason I bring up NPK of potting soil is because I've seen it on packages of potting soil sold at my local Home Depot. The bags I looked at seem to have about 0.1:0.1:0.1 for the entire contents.
Now, if I take a bag of Bovung (1:1:1) and mix it with an equal amount (by weight or volume, I'm not sure) of sand, does it make sense to think of the resulting mixture as 0.5:0.5:0.5 ? Taking that a step further, can one think of NPK for a shovelful of garden soil? And then, what is the ideal NPK for my lettuce flat soil? Does it make sense to think of soil this way?
I use urea 'cause I can get it cheaply from a farming friend, and I've been using it for years as a lawn fertilizer. It does work there. I've got a couple hundred pounds in the basement right now, and would like to use it if it makes sense.
So maybe a much simpler question is how much urea to dissolve in a 1/2 gal of water with which to water/saturatea 18" x 18" x 4", and how often should I feed my lettuce flats this way.
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justaguy2
16 years ago
Ah, I understand where you are coming from now. What you see on the bag is the result of CRF (controlled release fertilizer) having been added to the mix.

Trying to maintain a set amount of nutrients in the mix is going to prove close to impossible without precision testing. Using a dilute water soluble with each watering and ensuring each watering is to run off is about the closest one can come to maintaining a consistent level of nutrients in a practical sense. Now, if I take a bag of Bovung (1:1:1) and mix it with an equal amount (by weight or volume, I'm not sure) of sand, does it make sense to think of the resulting mixture as 0.5:0.5:0.5

Yes, it does, but it doesn't really mean anything when those nutrients are not immediately plant available, but instead will be released over time. To know the relative concentration of each nutrient (lets say in parts per million) would still require precision testing. And then, what is the ideal NPK for my lettuce flat soil? Does it make sense to think of soil this way?

In my opinion it only makes sense to think of it the way you are if you are doing hydroponics and have the relevant test equipment to ensure each parameter is within the specs you set. So maybe a much simpler question is how much urea to dissolve in a 1/2 gal of water with which to water/saturatea 18" x 18" x 4", and how often should I feed my lettuce flats this way.

Sorry, but I am going to have to decline to answer this as I really don't know. I am not familiar with how effectively granular urea dissolves in water. Is the product coated? If you wish to persue this approach you still need a reliable source for all other nutrients major and minor. If you wanted to do some research and math I am sure you could work it out. Look at hydro sources for recommendations in parts per million for N and then figure out the actual amount of N in your product and do the math to figure out the amount you have to add to the water to get the ppm in range.