Suggested NPK Levels For Plants?

[Flowers]

Hey everyone! I just fixed up my soil today (I pretty much just removed all of it and put in some nice potting soil), and did a cheap, at home soil test. My pH is at about 7.0, Nitrogen about 160 lbs/acre(medium), Phosphorous about 20 lbs/acre(medium), and Potassium about 40 lbs/acre(low).

I was able to find the ideal pH levels for all the plants on my list, but I cannot seem to find any suggestions for ideal Nitrogen, Phosphorous, and Potassium levels for different plants. For example, what is a proper range of Nitrogen levels to grow tomatoes in? Then, for example, if tomatoes require 300 lbs/acre of Nitrogen I know that I need to find a high Nitrogen level fertilizer. I know that this changes throughout the growing season, so any information regarding this would also be appreciated.

Anyways, here is my list of plants that I'm planning to grow this year:
Carrots, chamomile, celery, cilantro, cucumber, peas, pumpkins, spinach, tomatoes

Any advice would be super appreciated! Thank you so much.

[rainbowgardener]

You couldn't find that information, because it isn't quite that simple. Tomatoes and other fruiting plants use a lot of nitrogen in the beginning while the plant is in leafy growth stage. For blooming, plants need a lot of phosphorus and too much nitrogen produces lots of green growth at the expense of flowering. For fruiting, potassium is especially important. And this is still a very simplified version. How much of what nutrients plants need/ can access from the soil varies with soil pH, water levels, temperature, etc.

Personally, I don't worry about all that stuff and just try to make sure I have good rich soil with all the macro, micro, trace nutrients and minerals the plant needs (which is a lot more than just NPK). Then the plant can just take what it needs, when it needs it.

It's like if I were to try to give you all the nutrients you need, one at a time. Today you need oranges for vitamin C and spinach for folate and iron, etc. Or I can just put a big buffet in front of you and let you pick what you want/need and eat a diversity of stuff all the time.

All I ever add to my soil is compost, compost tea, mulch.

[Flowers]

So would you recommend I ammend the soil so that it has high levels of all three macros and just let the plant take what it needs? Or should I just leave the soil as it is?

[rainbowgardener]

In the long run, best thing you can do for your garden is start a compost pile (if you aren't familiar with composting, see our Compost Forum). But that won't help you right now. Since you know you are low in potassium, you probably do want to add some. Wood ash is a good source, but is alkaline and will raise your soil pH. People often add lime to their soil, deliberately to do that. If your soil needs or can handle raising the pH, that would be a good source. Otherwise kelp meal or greensand are good sources, that will also add other minerals and micro-nutrients.

Nitrogen is not very long lasting in soil and always needs to be renewed.

[Susan W]

We all have different ways to approach the garden, so I'll throw out my 2 bags full. I use a fair amount of bagged stuff, being in town and all. I am also working on the soil as a living mass. To add to your mix you put in, I would add bagged poo (black kow from Lowes my preference), and a bag of compost -soil helper. I like cotton burr, but also use Natures Helper from HDep. I am trying to use more of the Epsoma -tone product to build the soil. It has low numbers, is organic, has extra stuff in it. It's found at the garden center and box stores. Then plant!

By next season can add home done compost, and freshen the bed, add more garden tone and plant again!

[ElizabethB]

Consumer beware of home test kits.

When I went through the Master Gardener program part of the curriculum was an individual project. My project was to compare the results of home test kit results to lab test results.

My findings:

pH

Tablet test kit
pH paper test kit

Finding: unreliable - way off the mark - not worth buying

pH Meter

Finding: IF the directions are followed the test are reliable and the results are very close to lab test results. If the directions are NOT followed the results are garbage.

Soil Nutrients

I did comparisons using numerous home test kits.

Finding: ALL were garbage. Not even close. Do not waste your time or money.

Conclusion: Have your soil lab tested. $10/$20 dollars. Well worth the money and much less expensive than randomly amending your soil.

Question: How confident would you be with a Physician who prescribed medication without an evaluation or diagnosis?

Why not treat your soil with the respect of a competent evaluation and diagnosis?

Good luck

[imafan26]

I would do as rainbow said and just add more organic matter and compost to the garden. Different plants will use nutrients in different amounts at different times in their life cycle.

Nitrogen is a volatile nutrient. Even on a lab soil test, usually nitrogen is recommended but not tested. That is because nitrogen levels are changing all the time. Too much nitrogen at the start and your seeds may fail to germinate. While plants are actively growing they use the most nitrogen, but if you want more fruit, you want to avoid putting in too much or your will have lots of leaves and very little fruit or bulbs if you are growing roots. Usually the safest way to apply nitrogen and the way it will be recommended on a soil test would be to divide the total nitrogen requirement into two or three applications.

You can always supplement with fish emulsion or AACT or add some wood ash (do not add too much, it will alter the pH), greensand, kelp meal, or throw in more banana peels, potatoes and other high nitrogen materials into your compost pile.

There will be a lot of variation in soil tests depending on the controls. The soil tests from the stores are not very accurate in absolute numbers and you have to be careful. You should use distilled water, otherwise it will be measuring what is in the water and may skew the results. It is recommended to get a soil test before you start a new garden. Most University extensions provide a low cost service and if you request organic amendments, they will tell you exactly how much and what kind to add. After that they recommend every couple of years, unless you are changing pH, then you would have to recheck in 6 months. You should only try to change pH no more than a point at a time. It takes about 6 months for the change to happen. Certain things like wood ashes, chicken manure, peat moss, can change pH significantly, so you want to make sure you only put in the amount you need.

My soil tests all showed extremely high levels of phophorus and high levels of calcium and potasssium. I try to use zero phosphorus fertilizers and no chicken manure, bone meal, or lime as the pH in two of three plots are alkaline. I probably never have to add phosphorus intentionally for years. I do end up adding some phosphorus anyway since there will be some in compost and steer manures, that cannot be avoided.

I plant scavenger crops to use up some of the excess nitrogen and calcium.

[Flowers]

Thanks so much for the advice! I think I'll I'll probably just add some bagged compost and start a compost pile for next season. I think I might also try the kelp meal for an extra boost and get a proper soil test next year.