How to measure fertilizer?

[gary350]

If you need to fertilize a certain plant how do you know if 1 teaspoon is too much or too little?

My soil test shows, Nitrogen 0, Phosphorous 100, Potassium 90. This year my garden needs NO P&K. It only needs N nitrogen.

I have already learned that nitrogen does not stay in soil very long and hot dry 100° temperatures makes nitrogen be gone sooner than 4 months. After winter next summer soil will be very near 0 nitrogen.

After having 36" of rain another soil test might show lower P&K numbers. I am shooting in the dark unless I get another $15 soil test after having 36' of rain. Soil test says, rain will lower P&K and remove N.

This year my garden gets NO P&K all summer.

I bet commercial farmers do soil tests once a month and probably know from experience what NPK will be all summer with no soil test.

Plants need P&K but no information can be found on how much P&K plants remove from the soil?

I am trying to learn how much fertilizer each plant really need. At the moment in my garden my guess is plants need NPK = 5-0-0 and corn will need 30-0-0.

FFA online shows corn only needs nitrogen. Many farmers plant 2000 acres of corn, they recommend row spacing = 32" and seed spacing = 5" and a certain number of lbs. of nitrogen per acre. FFA nation wide maps recommends nitrogen according to, geographical location. air temperature and rain fall.

 

[oneeye]

Most fertilizers have recommendations on how much to use. I use half the manufacturer's recommendation because I can always repeat and add the other half later if needed. As far as using nitrogen in the hot heat of the Summer, it will cause problems to your plants in hot Summer. Spring and early Summer would be the best application times.

 

[Oliver Buckle]

I was given to understand that P does not wash out, and can even become a problem, with too much being present because people regularly apply NPK fertiliser without checking.

 

[gary350]

I was given to understand that P does not wash out, and can even become a problem, with too much being present because people regularly apply NPK fertiliser without checking.

P can become toxic with too much in the garden soil. We have had 30"+ rain sense April 1st. Rain will wash P away but not sure how quick? I need to be using less fertilizer but not sure how much. Once soil has enough P it does not need more. I'm not using anymore P&K the rest of this year.

 

[cpp gardener]

That sounds good. P is the hardest nutrient to reduce if it gets too high because it binds so strongly to soil particles.
Adding N in the Summer is only problematic if you use something very strong or immediately available and don’t water sufficiently. If you’re using a nitrate form of N, you need to use it lightly and water it in well.

 

[Helen Wasserstein]

To determine the precise amount of fertilizer needed, it's essential to consider both soil tests and specific plant needs. Since your primary concern is nitrogen, you can focus on fertilizers with higher nitrogen content, like your mentioned 5-0-0 or 30-0-0 for corn. Generally, for corn, about 120 to 160 lbs of nitrogen per acre is often recommended, but this can vary based on growing conditions and previous crop history. Monitoring your plants' growth and condition can also give clues; yellowing leaves, for instance, can indicate nitrogen deficiency. Regular soil testing, even if done annually, helps in making informed fertilizer adjustments.

 

[John miles]

If you need to fertilize a certain plant how do you know if 1 teaspoon is too much or too little?

My soil test shows, Nitrogen 0, Phosphorous 100, Potassium 90. This year my garden needs NO P&K. It only needs N nitrogen.

I have already learned that nitrogen does not stay in soil very long and hot dry 100° temperatures makes nitrogen be gone sooner than 4 months. After winter next summer soil will be very near 0 nitrogen.

After having 36" of rain another soil test might show lower P&K numbers. I am shooting in the dark unless I get another $15 soil test after having 36' of rain. Soil test says, rain will lower P&K and remove N.

This year my garden gets NO P&K all summer.

I bet commercial farmers do soil tests once a month and probably know from experience what NPK will be all summer with no soil test.

Plants need P&K but no information can be found on how much P&K plants remove from the soil?

I am trying to learn how much fertilizer each plant really need. At the moment in my garden my guess is plants need NPK = 5-0-0 and corn will need 30-0-0.

FFA online shows corn only needs nitrogen. Many farmers plant 2000 acres of corn, they recommend row spacing = 32" and seed spacing = 5" and a certain number of lbs. of nitrogen per acre. FFA nation wide maps recommends nitrogen according to, geographical location. air temperature and rain fall.

You're on the right track — soil tests, plant type, and even weather all matter when it comes to figuring out how much fertilizer to use. You're also right that nitrogen leaches out quickly, especially with heavy rain and heat. That’s why nitrogen needs to be applied more often, and in smaller doses, compared to P and K.


Most backyard gardeners don’t need monthly soil tests, but yes, commercial farmers do more frequent monitoring and often apply nitrogen in multiple split applications — especially with crops like corn that are heavy feeders.


You asked about how much P&K plants actually remove from the soil — here’s a ballpark:

• Corn removes about 0.35 lbs of phosphorus (P2O5) and 0.25 lbs of potassium (K2O) per bushel produced.
• Tomatoes might remove around 0.3 lbs of phosphorus and 0.4 lbs of potassium per 100 pounds of fruit.

If your soil test shows high P and K (like yours does), you're totally right to hold off adding more. Too much P especially can build up and actually harm plants or water systems (runoff issues).


And yes — rain can absolutely lower your nutrient levels, especially nitrogen. If you’ve had 36" of rain, I’d say your instincts are correct: nitrogen’s likely gone, and P/K might be a bit lower too, but still probably high enough to skip more for now.

 

[gary350]

We had about 4 ft of rain in tornado alley the month of April, I would love to do another soil test to learn how much that lowered P & K. I don't mind spending $15 for another soil test but I hate to loose another quart of my good garden soil. TN soil is clay and rock good soil is hard to find. I sometimes rent a dump trailer to haul 10 tons of free much from the recycle to my garden. Then a rental front loader tractor to spread the mulch, then mulch needs to be tilled into the garden soil. The older I get the slower I get, I am not sure I can do that again.