How Much Fertilizer to Add to Your Garden
Testing your soil is the best way to find out what is lacking in your garden as far as nutrients are concerned and the earlier you test it, the better. If you have done a soil test you’ve at least gotten the basic results: soil pH and the levels of the big three nutrients - nitrogen, phosphorous, and potassium. if you're deficient in any one of these nutrients, your garden will suffer. So, time to add some amendments or a fertilizer.
But, the plethora of amendments available to the home gardener these days can be overwhelming. And, once you've chosen an amendment or a fertilizer you have to determine how much of it to use. Too little won't do enough and too much can kill your plants where they stand. How do you calculate how much of something to add to the garden?
It boils down to two things: how deficient is your soil and how much nutrient is in that bag?
Let's start with the first one. How deficient is your soil? Most soil tests, even the cheapest off-the-shelf versions, are going to give you some semblance of just HOW deficient your soil is of each macronutrient. Whether it's a university or mail-in test or an off-the-shelf kit, your test is going to tell you your nutrient is either very low, low, medium, high, very high, or excess. The test results may also use terms like depleted, deficient, adequate, sufficient, or surplus. No matter the verbiage, you get the idea. You'll get a separate result for each of the macronutrients.
Now, how much of each nutrient is in that bag of fertilizer or amendment on the shelf at the garden center? Those numbers you see on the side of the bag are telling you exactly that. If the bag says the nutrient ratio is 5-10-10, that means the bag contains 5% nitrogen, 10% phosphorous, and 10% potash or potassium. So, if it's a 20 lb bag, it contains a a pound of nitrogen and two pounds each of phosphorous and potash.
This information is important if you got a university or mailed soil test that came back with recommendations on how to amend your soil. It may say something like, "add two pounds of phosphorous per 1,000 square feet of garden" for a soil depleted in phosphorous. This is where your math skills come in handy. If your garden is 1000 square feet, well, you're in luck. You'll use that whole 20lb bag of fertilizer to get the 2 lbs of phosphorous you need. That's assuming, of course, you also need the pound of nitrogen and the two pound of potash. But what if you don't? And, what if your garden is only 250 square feet?
Let's tackle one at a time. If the ONLY nutrient your soil test shows you deficient in is phosphorous, first off, congratulations. Second, you'll need to find an amendment that contains only phosphorous. You don't want to inundate your soil with more nutrients than it needs. That's almost as detrimental to your garden plants as not enough nutrients. It's also not good for your soil or the water table. Once you choose an amendment, you'll need to determine the percentage of phosphorous in that product. Say you choose a soft rock phosphate with a nutrient composition of 0-3-0. Perfect, it's 3% phosphorous and no other nutrients.
Now, let's tackle the size of your garden. The university recommendation came back for a 1000-square-foot garden. So, just divide your 250-square-feet by 1000 to get the correct ratio, then multiply that by their recommended application rate.
250/1000 = 0.4
0.4 x 2 pounds = 0.5 pounds.
So, you'll need a half a pound of phosphorous for your garden. Now, to see how much rock phosphate that equates to, divide 0.5 by .03 (the decimal equivalent of the 3% phosphorous in the rock phosphate).
0.5/.03 = 16.66.
So, you'll need roughly 16 and 1/3 pounds of rock phosphate to amend your garden according to the recommendations you received. This is all hypothetical, of course, but the math is the same no matter what the recommended application rate is. But, what if you didn't get a recommendation? Then how do you know how much to apply?
This is, I think, where most people get tripped up. You'll need to base the application rate on the bag instructions and your understanding of just how deficient your soil is. So, if the results say adequate, use the lower recommendation on the bag instructions. If the results say depleted use the higher end of the recommendations. And, if the results say deficient, use a midway point. Remember to start slow and work your way back up. It's much better to add too little and need to amend again later than to add too much and have to wait for it to leach out before you're able to plant your garden or, worse, planting anyway and burning your plants.
And always be cognizant of the other ingredients in that amendment or fertilizer. If your soil is not deficient of potassium, don’t use something that has three percent potash just because it has the 15 percent phosphorous you want. There are products on the shelf that have only the macronutrient you need and sometimes includes some of those micronutrients.
Determining how much fertilizer to use in the garden can be tricky but it's worth the time and effort to do the calculations and follow the instructions closely. Adding too much of anything to the soil is not only bad for your plants but it's bad for the environment, so be vigilant and err on the side of caution.
Questions on any of this? Feel free to contact me! You can also listen to the following episodes that talk more about plant nutrients, soil amendments, and fertilizers:
Your friend in the garden,
