How To Calculate Soil Volume and Amendment Amounts

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Amending is easy, but you need to know your soil volume to get it right!

We pride ourselves in providing the most comprehensive Soil Testing Analysis and Formulating Custom Amendment Packages for clients. But often times we receive the soil sample, we receive information about the clients cultivation site, but the soil volume isn’t there or isn’t calculated. We’re here to make this super simple for everyone, let’s take a look.

How to Calculate Total Soil Volume

So, there are two main container types and formulas for potting soil you’ll most likely encounter. Then two formulas for if you’re growing in native soil you’ll want to use. Let us take a look at all four.

• Raised Beds
  • Formula: All measurements are in feet (Bed Length x Bed Width x Soil Depth) / 27 = Total Number of Cubic Yards
  • Explained: By taking the square footage of your bed and multiplying it by your soils depth, you will get an accurate representation of how many cubic feet of soil in your container.
    • Most likely the soil depth will be between 6″-12″, so we’ll need to convert this to feet. We do so by dividing the depth in inches, by 12. For example 8″ soil depth = 8/12 = 0.66 ft.
    • Once this is obtained, divide by 27 (the number of cubic feet in a cubic yard). This will give Total Cubic Yards of soil volume in your container.
  • Example:
    1. (60 ft long x 8 ft wide x 0.83 ft or (10″) deep) = 398.4 cu. ft.
    2. 398.4 cu. ft./ 27 = 14.75 cu. yds.
• Pots
  • Formula: (Gallons per Pot x Number of Pots) / 200 = Total Number of Cubic Yards
  • Explained: By taking the number of gallons in each pot and multiplying it by the total number of pots this will give you an accurate representation of the total number of gallons of soil.
    • Once this is obtained, dividing by 200 (the number of gallons in 1 cubic yard) you will be given Total Cubic Yards of soil volume in your pots.
  • Example:
    1. (45  gallon pots x 500 ea pots) = 22,5000 gallons of soil
    2. 22,500 gallons / 200 = 112.5 cu. yds.
• Native Soil in Rows
  • Formula: All measurements are in feet (Bed Length x Bed Width) = Total Number of Square Feet
  • Explained: We suggest amending native soil rows by square foot rather than cubic yard. To calculate this, measure the desired area of amending, rather than your total canopy square footage.
  • Example:
    • Greenhouse is 66 ft x 20ft wide = 1,320 sq. ft.
    • Inside the greenhouse are two beds – 60 ft long x 8 ft wide x 2 beds = 960 sq. ft.
    • 1,320 sq. ft. vs 960 sq. ft. – it’s to each clients benefit to amend only soil in which plants are planted or to give the correct measurements in order to avoid over amending actual planted square footage.
• Native Soil in Holes
  • Formula: All measurements are in feet (3.14 x Radius Squared x Hole Height x Number of Holes) / 27 = Total Number of Cubic Yards
  • Explained: Calculate the volume of the soil by multiplying Pi by the Radius Square by the Hole Height.
    • Most likely the soil desired amending depth with be between 12″-18″ so we’ll need to convert this to feet. We do so by dividing the depth in inches by 12. For example 18″ soil depth = 18/12 = 1.5 ft.
    • Once this is obtained, dividing by 27 (the number of cubic feet in a cubic yard) you will be given Total Cubic Yards of soil volume in your container.
  • Example:
    1. (3.14 x (1.5 ft radius x 1.5 ft radius) x 1.5 ft deep) = 10.60 cu. ft. per hole
    2. 10.60 cu. ft. x 100 holes = 1,060 cu. ft.
    3. 1,060 / 27 = 39.25 cu. yds.

Volume of Amendment

Now that we have the soil volume calculated, your Soil Testing Analysis has been provided, and you have your amendments, it’s time to apply them as consistently and evenly as possible. Let’s look at how this is most easily done. For the following explanations we will use a standard of 10 pounds of amendment per yard of soil with the exception of Native Soil in Rows which will be 10 pounds per 100 square feet. Actual application rates will vary based on Custom Formulations or between our In House Blends such as (Foundational Formula, Double Depper, or Mothership Mix).

• Raised Beds
  • How Much Amendment to Purchase: Total Yards x Given Application Rate = Total Weight of Amendment to Purchase
  • Example: 14.75 yards of soil x 10 lbs per yard = 140.75 total lbs to purchase
  • How to Apply: There are many ways to approach how to apply amendments evenly. We recommend taking your cubic yardage per bed and applying evenly by weight to the volume of the soil. Then tilling it in with your desired method (rototiller, broad fork, etc.)
    1. Place a 5 gallon bucket on a scale and zero it out or use bagged amendment
    2. Measure out weight of amendment that can fit inside your bucket or use bagged amendment (weight will be on the bag)
    3. Divide your total amendment weight by the weight in each bucket or weight of each bag
    4. Divide your bed by number of buckets or bags you will be using
    5. Apply each bucket or bag evenly into the broken down sections of your bed
    6. Mix amendment into soil
  • Example:  Your bed as we calculated above, has 14.75 cu. yds. of soil. You will have 140.75 lbs of amendment to apply.
    1. Your bucket holds 20 lbs of amendment
    2. 140.75 lbs / 20 lbs = approximately 7 buckets
       1. If using bags divide by bag weight
    3. Divide your bed length by number of buckets or bags, either by visualizing it or physically marking off sections; if the number is not even add the last bucket or bag into the shortest section of bed.
    4. Apply each bucket or bag evenly across the entire surface area of each zone
    5. Mix amendment into soil
• Pots
  • How Much Amendment to Purchase: Total Yards x Given Application Rate = Total Weight of Amendment to Purchase
  • Example: 112.5 yards of soil x 10 lbs of soil = 1120.5 total lbs to purchase
  • How to Apply: There are many ways to approach how to apply amendments evenly. Taking the total weight of amendment purchased and dividing it by the number of pots. Then tilling it in with your desired method (rototiller, broad fork, etc.)
    1. Calculate Amendment Weight per Pot
    2. Place desired application container onto scale and zero it out (we recommend using a measuring cup)
    3. Measure out the weight of amendment needed per pot and mark on the application container the fill level
    4. Apply the contents of the application container filled to the line, to each pot
    5. Mix amendment into soil
  • Example:  Your total weight of amendment is 1,120 and you have 500 pots
    1. 1,120 lbs of amendment / 500 pots = 2.24 lbs per pot
    2. Fill a zeroed out measuring cup with 2.24 lbs and mark fill level
    3. Apply each measuring cup evenly across the entire surface area of each pot
    4. Mix amendment into soil
• Native Soil in Rows
  • How Much Amendment to Purchase: Total Square Footage x Given Application Rate = Total Weight of Amendment to Purchase
  • Example: 960 sq. ft. x 10 lbs of amendment per 100 square feet = 96 total lbs of amendment to purchase
  • How to Apply: There are many ways to approach how to apply amendments evenly. We recommend taking the square footage of the bed and applying amendment by weight evenly across the square footage of the soil. Then tilling it in with your desired method (rototiller, broad fork, etc.)
    1. Place a 5 gallon bucket on a scale and zero it out, or use bagged amendment (weight will be on the bag)
    2. Measure out weight of amendment that can fit inside your bucket if using
    3. Divide your total amendment weight by the weight in each bucket or bag
    4. Divide by number of beds
    5. Divide your beds by number of buckets or bags per bed
    6. Apply each bucket or bag evenly into the broken down sections of your bed
    7. Mix amendment into soil
  • Example:  Your beds as we calculated above, have 960 sq. ft. of soil. You will have 96 lbs of amendment to apply.
    1. Your bucket holds 20 lbs of amendments
    2. 90 lbs / 20 lbs = approximately 4.5 buckets
       1. If using bags divide by bag weight
    3. Divide by 2 for the two beds
    4. Divide your bed length by number of buckets or bags, by either visualizing it or physically marking off sections; if the number is not even add the last bucket into the shortest section of bed
    5. Each bed will get 2.25 buckets, or appropriate bagged amount. Apply each bucket or bag evenly across the entire surface area of each zone
    6. Mix amendment into soil
• Native Soil in Holes
  • How Much Amendment to Purchase: Total Yards x Given Application Rate = Total Weight of Amendment to Purchase
  • Example: 39.25 yards of soil x 10 lbs of amendment = 392.5 total lbs of amendment to purchase
  • How to Apply: There are many ways to approach how to apply amendments evenly. We recommend taking the total weight of amendment purchased and dividing it by the number of holes being filled. Then tilling it in with your desired method (rototiller, auger, etc.)
    1. Calculate Amendment Weight Per Hole
    2. Place desired application container onto scale and zero it out (we recommend using a measuring cup)
    3. Measure out the weight of amendment needed per pot and mark on the application container the fill level
    4. Apply the contents of the application container filled to the line, to each hole
    5. Mix amendment into soil
  • Example:  Your total weight of amendment is 392 lbs and you have 100 pots
    1. 392 lbs of amendment / 100 pots = 3.92 lbs per hole
    2. Fill a zeroed out measuring cup with 3.92 lbs and mark fill level
    3. Apply each measuring cup evenly across the entire surface area of each hole
    4. Mix amendment into soil

Wow! We did it! Soil volumes are correctly calculated and amendments can be made and distributed evenly regardless of your cultivations style or container type!

Once soil is fully amended we suggest watering it in to activate the micro-biology with our Soil Activation Protocol which helps build healthy, organic, living soil!

Questions? For additional information: Call us at (707) 273-8758 or Email us at info@soilscapesolutions.com