Figuring out fertilization requirements

It’s probably applies to fruits as well, but for vegetables in particular, most of the literature will say you need XXX lbs of nitrogen per acre, for example.

Yet, I’m assuming that assumes a particular spacing both within rows, and between rows. So how do you know you’re actually using the right amount?

Even this guide for onions from Dixondale farms doesn’t really specify how many plants per square foot are being assumed here. Any advice?

Plus, how “wide” is my row as far as the plant is concerned?

Well, an acre is 43,560 sq ft. When the recommendation is say, 20lb of N per acre, you have to do some cipherin’.

(You may be aware of all this, so if so, maybe someone else could benefit as well.)

Say your plot is 1000 sq. ft, so you divide 1000 by 43560, which is 0.023. To get the amount of N for your plot it’s 0.023*20, or 0.46lb. If you’re using 10-10-10 fertilizer you’ll need about 5lb of it.

As far as application goes, I guess you can put it in a furrow along the row of your plants, the 5lb split up evenly along the rows. I think the furrow has to be within 6in or so of the row, and maybe an inch or two deep.

If it’s just a general recc, you could broadcast it out with a spreader before planting.

Saying all this, you will probably need to get a soil sample of your plot and send it in to your local ag extension office. It costs us $3 per sample where we’re at, but yours will probably be different.

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Typically its better to under fertilize than over. You can always add more but once its in the soil, it takes time to go away. Are u using chem ferts or organic? Organic is typically more forgiving…

Yes, but the entire plot is not full of plants, you have pathways. Feeding those pathways seems useless.

Furthermore, I might be looking up recommendations for onions but if I space my onions at 6 inches and someone else spaces them at 8 inches, the person who spaces them at 8 inches probably needs slightly less fertilizer than I do. None of the recommendations seem to take any of that into account or tell you how to adjust for it.

OK, here’s my exact situation. Let’s take onions again as the example.

I have four 20’ rows. Two sets of “double” rows 8” apart and 20’ long. Three feet between each “set” of double rows. Plants are 6” apart.

How much do I need?

i gave up years ago following fertilizer ratios. i let the plants tell me. alot has to go into figuring this out. i keep a journal of what i give each year and what time of the year so i can adjust accordingly. last year i double fertilized my raspberries because i fertilized in late fall, didn’t log it, and fertilized again early spring. luckily they are fall bearing so they kind of bounced back some and fruited but i had dead zones in the row where shoots were burnt.

So I have four rows. How “wide” are the rows? A foot? I can’t imagine the roots have grown any more than 6" away (on both sides) from the plants yet. So 80 X 1’ wide = 80 square feet.

Using the formula in the link - 80 X 0.0037 = 0.296 lb of N for the entire season (this seems really low) - since 160 lbs of N per acre is the seasonal recommendation.

At the bottom of the page, it says to basically apply N at intervals of 20 ‘units’ every two weeks, more or less, for a total of 160 ‘units’ of N. To put it another way, you apply 1/8 of the amount above each time.

So, 1/8 of the amount above is 0.037 lbs of N, or 0.37 lbs of 10-10-10, each time, which again, seems low!

Well, the nutrient per acre rec is to rectify a certain deficiency in that particular plot. In my opinion, it doesn’t matter how much is planted.

According to your math, that is the correct amount of N for 80 sqft for a season. I came up with a total of 0.29lb of N as well. And, when it’s split up into 8 doses, then it is 0.36lb or 6oz of triple 10 fert. So each 1x20 ft row would get only about 1.5oz per dose.

The rec from the website says 160-80-140 of N-P-K per acre. So if you did use triple 10, you’d be using twice as much P as needed. You’d be better off using something like 10-5-10, at the amounts you calculated.

No it doesn’t seem a lot, but fertilizer, especially non-organic, can be pretty potent. I agree with with the other posters in that too much can do more harm than good.

My soil tests reveal low P, however.

What are your other nutrient levels, like K, Ca, Mg, and the pH? My report didn’t show N levels, I think because it’s a nute that tends to be quite volatile and leaches out more than the others.

That was from December, 2017. After Compost, lime, and other amendments. The P was very low before and the pH was 5.3.

From 9/30/2016, before I did anything.

For kicks, my textural analysis.

Looking at that report the high potassium is an issue. usually a nutrient that is not needed in most soils. Too much potassium disrupts the uptake of other important nutrients, such as calcium, nitrogen and magnesium, creating deficiencies. Ways to solve the problem is to keep the soil moist and some can be flushed out. Other ways is to avoid chemical fertilizers, use organic which have less potassium. Some have no potassium. I have found that using organic and supplementing the missing nitrogen with urea or AN would help too.
I have been turning more and more organic as basically as Bob mentioned your feeding the soil, not the plants, so it doesn’t matter how many plants are there. Using organics supplies a continuous source of nitrogen over time. Nitrogen is very soluble and the only soils that test high in nitrogen are ones with organic material breaking down. What I don’t like about urea is it is like crack nitrogen. It works, but so does having a healthy soil all the time.
Many or all people here who have orchards have stated using urea. I stopped using any chemical fertilizer for my trees and my yields have been unreal. I have posted many photos of the fruit I’m getting. The stuff isn’t needed. And maintaining the soil organically is so easy. You don’t have to worry about exact amounts, just be in the general suggested range.
I’m not an organic nut, i spray my trees with synthetic fungicides. I just want what works best and easiest to do.
Onions are fairly forgiving plants, but nitrogen is important I would use urea because you don’t have time to build the soil. It’s hard to burn them, they like nitrogen for sure. But the potassium problem is a real issue. Using urea and an organic for the other nutrients should give you excellent results. Alfalfa, Garden -Tone or other organics, Millorganite is 6-4-0 so has no potassium. Now approved for vegetable gardens. Is super cheap too. If you can stand putting human waste on your plants. As far as it containing heavy metals that is now a myth. It actually has less than all chemical fertilizers for the lawn. Probably has less than 10-10-10 has. I must say it works so well on my grass, it never looked better.

Build your soil over time by using mulch, and if possible cover crops etc. Any mulch is better than no mulch. Add compost to it every year an inch at least. If you do this you will see a huge difference come harvest time. It takes a few years to get where you want.

Again I’m not an organic nut, I just want what works best and building the soil is working really really well. I was surprised and pleased with how my plants are performing. You can see a huge difference.
Also a note on high Ca and Mg. Not an issue, you soil is acidic and you will need to add more in time, the Ca and Mg will quickly diminish, you want it high as your acidic soil with bond that stuff up over time, fairly quickly.

Alfalfa pellets locally go for about $11 / 50lb bag. They are a great organic addition to most soils and there are no seeds cuz the pelletization process effectively sterilizes them…


Hmmm, not sure why you say this as the most recent test shows P levels in the optimum range.
The soil test tells you exactly how much to use. So say you have a 1000 sq ft of onions, and they recommend 2.5# urea (say), BUT your question is what about the pathways, etc. So if 50 % of your 1000 sqft are pathways, then only 50% are onions, then measure out 50% of your recommended amount (2.5) leaving 1.25 # to spread throughout your onion area.

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Guess I should say my native soil is low in phosphorus. All I added last year was compost and bone meal, plus some of the 10 10 10 that I discussed above. That’s what took the potassium from optimum to excessive, and I think bumped up the other nutrients as well. I also added lime which I know will add to the calcium in addition to raising the pH.

Drew - good info, I’ll keep all that in mind and try to find some urea and maybe alfalfa and milorganite.

I need to get a soil test myself, been a long time.

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