As a newer gardener, I have not previously gotten around to using soil test information. I’ve been planning an ornamental overhaul in a small area of my yard and wanted to soil test to make sure the plants added to the area had the best chance of success. The HGIC website has a lot of soil test information, but I’ve never looked at it closely myself until now. Even though our content is well-written and organized, the subject is intimidating with many choices to make, multiple steps, math homework to figure out how much fertilizer to use in your space, and as I learned, many caveats or roadblocks. I can see plenty of people just giving up and not adding any soil amendments, or just going to the hardware store and buying a bag of fertilizer and applying without thinking too much about it.
Even after reading our material thoroughly, I still needed several questions answered from our experts. I’ve got a handle on it now, for the most part. With this post, I hope to detail how I figured out what fertilizers I needed, how to apply it, and then the most important things I learned that I didn’t feel are made clear.
I’ve got this space in my backyard that wasn’t being used for much. In the summer of 2020, mid-pandemic, pre-offspring, I decided to revisit a teenage hobby of mine – remote control cars. I took a shovel and cleared out an area and built ramps and hills out of dirt, for my very-own mini-dirt track. 2 years later, my wife asks me “are you going to keep the track? It’s ugly.” So, my task is to beautify it with improved landscaping and added vegetation. We also have a bunch of hostas in our front yard garden that keep getting eaten by deer. Our backyard is smaller and enclosed, so we hope the hostas will avoid attack in their new backyard, trackside location. We are also adding other plants good for shade conditions.
The location is on a bit of an incline, with the higher elevation area being more sandy and rocky, and the bottom softer and wetter. There is an “infield” of the track that is often stood on that is grass, but has been overtaken with weeds. I am planting nice plants and shrubs around the outside, and re-seeding the infield with grass. Since I know from a lot of digging to make the track that the soil is quite different in different areas, I wanted to soil test so that I could find out what exactly I might need to do to make sure the plants have the soil they need in their different locations.
Soil test basics
If you aren’t familiar, soil testing is when you take samples of soil from an area in your yard or garden and send that off to a lab that will reply with data and recommendations for what you need to add the the soil for the crop or plants you have or will have growing in that area. You can send multiple samples (labeled) from different areas and get specific data for specific areas of your property.
Amending the soil based on these results can help your ornamental plants, lawn, and food gardens.
We have a list of 6 recommended labs that will give you soil test results. You’ll have to choose which type of tests you want. You’ll want to choose the ones labeled for homeowners. Beyond that, the test facility may ask what type of “crop” you are growing; lawn, ornamentals, trees, etc. This is nice to have because they will then send you recommendations tailored to that specific crop. My coworkers at the HGIC told me that lawn and ornamentals will have similar recommendations, so I decided to just mark all locations as “lawn” for crops.
I took samples from the high up sandy area, the infield grass area, and the wetter, lower area, and prepared to send to a lab. I labeled them A, B, and C, and took a picture of each bag in the test location so I could remember where each one was.
I then spent $10 shipping dirt through the mail!
I got my test results
….and whewee! – it was time to dive in and figure out what to do with this information.
There is a LOT of information presented on these results. HGIC has a great infographic explaining all parts of soil test results at this link. In your results, you’ll be presented with lists of data of how much of certain nutrients or pH levels you have, graphs of your levels, and a lot of what I think is pretty extraneous for what myself and most homeowners need. I found the boiled-down information in the “recommendations” section of my test to be what I needed to use to take action. The lab has interpreted all my levels they measured and produced a list of recommended amounts of nutrients to add that would help the basic “crops” I selected grow.
Below are my recommendations for my three locations.
What to do with this data?
Comparing my results, it looks like all three locations can use similar levels of N, P2O5, and K20, – around 3-4 lbs per 1000 square feet of land. My A location requires 5.3 lbs of S (sulfur), additionally. All three locations suggest some low amount of Mg, but it is so low, that I am not going to take action on that. So, what are these?
Through my research on our fertilizer page, a basic thing I have learned is that :
The three numbers on fertilizer products (e.g., 3-4-3) represent the percentage, by weight, of N (nitrogen), P (phosphorus) expressed as P2O5 (phosphate), and K (potassium) expressed as K20(potash). For example, a 5 lb. bag of 10-5-5 fertilizer contains 0.5 lbs. of N, 0.25 lb. of P2O5, and 0.25 lb. of K20.
The N, P, and K numbers on the packaging refer to the percentage of each nutrient found in the bag. I’m assuming that if that does not add up to 100%, the rest is filler.
Hey – look at that: N, P, and K are right next to each other and in the same order on the results I received. So, it looks like I need to find a fertilizer product or products that will let me add roughly 3.5 lbs per 1000 square feet of each N, P, and K to all my locations.
Location A up on the hill additionally needs 5.3 lbs/1000sqft of sulfur, which I learned is an acidifier that raises soil pH.
They suggest “5D” of CaCO3, in my location B results, which confused me for a minute until I saw the note below the chart saying “lime expressed in pure CaCO3. D= Dolomitic.” However, since I know lime is used to adjust pH, and that area was listed as inside the optimal pH range, I’m not even going to bother with lime.
Shopping for fertilizer
Instead of going to the hardware store and browsing immediately since I’ve never really shopped for fertilizer, I decide to shop around online to see what types of products are generally available, what their nutrient numbers are, and do math to see what I would need.
After some confusing math and shopping, I decided to run a question by the expert consultants at the HGIC to check my work and offer advice. I had asked if a 10-10-10 product I found was right for me.
They came back with a bit of a surprise:
The N rec of 4 lbs. should be disregarded. People should always follow the UME N fertilizer guidelines for turf which comply with state law (max. 2.7 lbs. of N/1,000 sq. ft. per year and maximum 0.9 lbs. of N/1,000 sq. ft. per single application).
If P is low in the soil, it is added separately and not as part of the N fertilization that occurs mostly in fall for lawns. Using 10-10-10 could deliver the total correct amount of P & K but would add more N than is allowed by law. So, you would apply N with a separate turf fertilizer. You could reduce the N fert amount by half if the turf is well-established and you leave your clippings in place.
There are regulations in the state of Maryland about the amount of N fertilizer you can add at a time. See: MD Fertilizer law in a nutshell. Over-fertilizing is bad for the environment – excess can wash into waterways and affect wildlife. I just need to find P and K to add and I’ll leave the N alone I need a product with “0” as the first of the three numbers.
I head to the store
I find a bag that says it’s got sulfur – it’s soil acidifier. Check that one.
I find a liquid bottle of “Morbloom Concentrate” with numbers 0-10-10. Great! No nitrogen, and equal numbers P and K!
But wait – I get home and start doing the math about how much of this morbloom I need, and it becomes confusing because this is a concentrate, it’s measured in ounces, not pounds, and this is a liquid.
I run this by our experts, and apparently liquids are not great for increasing P and K levels in the soil amendment like this. They are ideal for quick nutrient boosts for potted plants. The liquids absorb quickly and wash away in regular soil, so this is not a longer-term solution. I’ll be returning this to the store.
HGIC recommends to me to get specific products. It sounds like there isn’t a fertilizer product that will have the equal P and K levels I am looking for. I’ll have to buy, measure, and apply P and K separately. I am recommended to get Muriate of Potash (K) which I found at 0-0-60 levels, and after scrounging a bit, I found bone meal (N and P) at 4-12-0 levels in my garage. The bone meal has nitrogen in it, which I was somewhat avoiding, but it is at a relatively low level compared to the P, so I believe this will be fine.
Time for math
I’ve got my fertilizers – time to do the math to figure out how much of each to apply in my specific spaces. I know how many lbs per 1000 square feet are recommended to add from the soil test results, but I need to figure out:
- How many lbs of each nutrient I need for each of my specific spaces based on their area.
- What actual amount of the product I bought (volume or weight) do I actually need to scoop out and sprinkle around my locations, calculated based off the percentage of the nutrients listed on the bag.
First, I needed the areas in square feet of my locations. For this article, I’ll just stick with my location B to which I will be adding grass. Refer to your middle school geometry text books on calculating area. I am not going very accurate on this, so with tape measure and some rough math, I came to 50 square feet (not a large space).
Since my recommendations are for 3.5 lbs of phosphate and 3.8 lbs of potash per 1000 square feet, I need to figure out the relative amounts of each for a my smaller 50 sqft area.
So: 50 sq. ft./1000 sq. ft. = .05 or 5%. I need 5% of 3.5 and 3.8. That is 0.175 lbs. of phosphate and 0.19 lbs. of potash.
Second, I need to figure out how much of the bone meal (10% phosphate) and muriate of potash (60% of potash. )I need to get 0.175 lbs. and 0.19 lbs., respectively.
The N-P-K numbers represent what percentage of each nutrient the product is comprised of. So, this means that the 4-10-0 bone meal is 4% nitrogen and 10% phosphate (P2O5) and the 0-0-60 muriate of potash is 60% potash (K20) and.
So, I need to divide the amount of each nutrient I want to add to my garden by the percentage of those nutrients in the two fertilizers:
0.175 lbs. of phosphate needed divided by the percentage of phosphate in the bone meal: 0.175 lbs. /0.10 = 1.75 lbs. of bone meal for the 50 sq. ft. area..
0.19 lbs. of potash needed divided by the percentage of potash in the muriate of potash: 0.19 lbs./0.60 = 0.31 lbs. of muriate of potash for the 50 sq. ft. area.
After calculations, unfortunately the required amount of bone meal fertilizer would deliver more N (1.4 lbs./1,000 sq. ft.) than is allowable in a single application under Maryland Law,but I’ve decided to apply it anyways. This is a small area that is pretty closed off and isolated from roadways and ways for things to wash into the water supply, so I think any environmental affects will be minimized.
Mass or volume
I’ve got my amounts calculated by weight, but I wasn’t planning on taking a scale out to the garden and weighing out the correct portions. Luckily, our page has links to documents with charts showing mass and volume conversions for common fertilizers. According to this document:
- Bone meal: 1/3 lb. = 1 cup
- Muriate of potash: 1/2 lb. = 1 cup
So, I need
- 5.7 cups bone meal
- 0.58 cups muriate of potash
Applying the fertilizer
I’ll scoop the amount needed of each and sprinkle it as evenly as I can around the areas. Certain spots have been mulched already which isn’t ideal, but I’ll try to sweep some mulch areas away temporarily to sprinkle the fertilizers directly onto the soil, then sweep the mulch back.
If it is not forecasted to rain soon, I’ll give the areas a bit of a water to start the fertilizer dissolving and dissipating into the soil.
I needed something to scoop and measure with, though, so I grabbed a plastic solo cup. I got out a measuring cup and poured two cups of water into the solo cup which filled it to one of the rings at the top. With that in mind, I scooped 1/4 of a solo cup of murate of potash, and a bit less than 3 cups of bone meal.
Key takeaways from a first time fertilizing job
Certain things stuck out in my mind during this process as things I should remember moving forward:
Liquid vs solid/powder/granular fertilizers.
The liquid stuff is not the best choice for maintaining adequate nutrient levels in your landscape!
Fudging the numbers
There’s so much room for small errors and variations throughout the process, that I don’t think absolute accuracy in deciding how much to apply is very important. I have a feeling that the fertilizer conversion to cups is more of a rule of thumb than a precise conversion. Measuring the area of your landscape is hard to do extremely accurately.
There’s got to be a lot of variability in how much product actually absorbs and stays in the soil due to the make up that soil, weather that may wash away some part of it, and more. You absolutely don’t want to throw double what is needed on your soil (in fact, there are laws about how much nitrogen you can apply to turfgrass in Maryland), but for me, I think getting in the ballpark and sometimes eyeballing it may work well enough. I’m a casual gardener trying to improve my soil, not optimize it.
Focus on recommendations from the soil test
A ton of data is presented to you in your soil test results, but all you need to take action with is what they recommend you add.
Disregard the Nitrogen (N) recommendations from the soil test
I’ve got my compost pile, and my mower mulches grass clippings and leaves that I can scoop up and apply to an area that needs it. Unless the soil test says I am very deficient, the normal, natural ways of adding compostable materials should be enough. And I can always fertilize with some nitrogen fertilizer if plant leaves are small with a pale color.
– Dan Adler
HGIC Web and Communications Manager