This calculator will provide the ratio of ingredients needed for making a DIY soil-based potting mix and will break down the percentages in ratios (for mixing) as well as cubic feet (for buying).
The calculator will help you estimate how much to buy (in cubic feet) and how much to mix your soil to create your own homemade potting mix. The amount of lime (or ash) needed will also be listed if you are using peat moss.
Why does the calculator provide both cubic feet as well as the mixing ratio? Most DIY recipes call for mixing ingredients by the gallon or bucket; but home improvements store don’t sell potting soil ingredients by the gallon or bucket. You need to know how many cubic feet you’ll need to buy before you can even begin mixing your ingredients.
You have the option to enter how many square feet of soil-based potting mix you need and the calculator will break down each of the ingredients for you. Otherwise, the calculator will provide the recipe for 2 cubic feet of soil-based potting mix.
What Is Soil-Based Potting Mix And What Is It Used For?
Most outdoor potted plants and quite a few indoor plants perform better with a soil-based potting mix. A soil-based potting mix is heavier, provides more stability, has a higher microbial activity, and retains water longer. A soil-based potting mix is used in circumstances where a soilless mix is insufficient. A soilless mix is extremely lightweight, and has improved drainage and airflow over traditional soil-based potting mix.
If your outdoor plants dry out too quickly, is a heavy feeder, or blow over easily when its windy, a soil-based potting mix is ideal.
This potting soil recipe is lighter than the type of soil mix you would use for a raised garden bed.
How Much Potting Mix Do You Need
Determining how much potting soil you need is a good starting point before buying your ingredients. If you only need enough soil for one or two small houseplants, you’ll probably save money buying it premixed. But if you are potting up trays of seeds or filling a lot of pots, it makes sense financially to mix your own.
A good point of reference is that a standard-sized wheelbarrow holds 3 cubic feet. You would need 3 cubic feet to fill 6 10″x12″ planters.
If you already have a DIY potting mix recipe that calls for gallons instead of cubic feet, a good point of reference is that 6.2 dry gallons is equal to one cubic foot. If your DIY recipe calls for buckets, the average dollar store bucket holds two gallons; so three buckets would be equal to one cubic foot.
For seed trays:
For a #1206 seed tray insert (1.5×1.5) with 72 cells, you would need 21.36 cubic inches of soil mix. To find the volume of soil mix you need for different seed tray inserts, refer to the chart at GreenhouseMegastore.com.
For flower pots and planters:
For one 12″ round planter, you would need 925 cubic inches of soil
For one 12″ tapered flower pot, you might need only 659 cubic inches of soil.
If we needed to know how many cubic feet, we would divide cubic inches by 1728 (or multiply by 0.000578704).
Visit our How Much Potting Soil For Flower Pots Calculator to find the amounts need for other planter sizes.
The Elements Of A Soil-based Potting Mixture
A soil-based potting mixture typically consists of topsoil, compost-mix (garden soil), peat moss and perlite. Each ingredient is added based on its capacity to hold water, add nutrients, increase weight/stability and improve drainage and provide aeration. Here’s a breakdown of what each ingredient provides:
Topsoil – Topsoil is a nutrient-rich layer of soil that increases density (and weight). Topsoil helps somewhat with water retention, while also draining well. Although it is a nutrient-rich layer of soil, it does not contain enough nutrients on its own to support the root growth of most plants. It also tends to dry out quickly and cake at the surface unless other soil amendments are added.
Garden Soil – Garden soil is a mixture of soil (usually topsoil), compost, shredded bark and other organic materials. This composted soil provides a high nutrient content. Garden soil is not meant to be used alone; it should always be mixed into existing soil. It has excellent water retention, but very poor drainage. When used alone, it can become water-logged and suffocate roots.
Peat moss: Peat moss is considered to be an ideal additive for most soil-based mixes because it can hold both water and improve air. Without peat moss, soil may eventually become too compacted. The fibrous structure of peat moss can hold 15 to 30 times its weight in water. The peat fibers also give it a large amount of pore space (80 to 90 percent of its total volume). It holds nutrients well, and it readily shares them with the roots, thanks to its slightly acidic pH. It has a pH of about 4 so lime needs to be added to get it closer to a pH of 6.
Perlite: Perlite is a volcanic-rock that has been heated to produce lightweight white particles. Perlite also helps prevent soil compaction. It holds air and promotes good drainage but does not hold as much water as vermiculite.
Can you use compost instead of garden soil? Yes, but if you substitute compost you would need to use 50% less and then increase topsoil and peat moss by 25% each. You would also need to keep the compost warm and moist for a week or two before adding it to your soil mix. Warmth and water allows microbial activity to process the compost so that excess ammona and organic acids in the compost can dissipate before they damage germinating seeds, tender roots, and young plants.
Adjusting The pH Of Soil-Based Potting Mix
If you are using sphagnum peat moss, which is slightly acidic, you will want to add lime or wood ash to slightly increasing the pH level of your mix.
However, you should use caution because soil alkalinity of above 7.0 pH causes nutrients such as phosphorus, iron, boron, manganese, copper, zinc and potassium become chemically bound to the soil and become less available for plant use.
You would apply 5 pounds of lime per 100 square feet to raise the pH by one point. UVM recommends an application rate of 8.5 lb. per cubic yard. (The calculator will figure these amounts out for you).
One cup of limestone equals 3/4 pound. 0 .75 lbs goes in to 5 pounds about 6.6 times. 6.6 times 1 cup equals 6.6 cups. 5 lbs of limestone per 100 square feet would be a little over 6 ½ cups. So for each square foot, we would need .067 pounds of limestone.
According to the National Gardening Association, wood ash can also be used to raise pH as a substitute for lime. Wood ash contains up to 70 percent calcium carbonate, as well as potassium, phosphorus, and other trace elements. Because of its very fine particle size wood ash is a fast-acting liming material, which could have negative consequences. Wood ash should be limited to 2 pounds per 100 square feet and should only be applied every other year. You would use 40% of wood ash compared to the ratio used for lime (the calculator will figure that out for you too).
Mixing Potting Soil
When you’re mixing your potting soil, you probably won’t be using the same measurement as you did when you purchased each of the ingredients (which are usually sold by the cubic foot). The exception is if you bought everything in one cubic foot bags; that would make it easier. To make things more confusing, most DIY recipes call for mixing it by the bucket because it’s a much more manageable way to figure out the amounts. So rather than measuring by cubic feet when mixing your potting soil, you would be measuring by ratio.
The calculator will provide mixing ratios (just to make your life easier).
How much potting soil do I need to fill a 5 gallon bucket?
If you are measuring your soil using 5 gallon buckets from home improvement stores, their 12.5 inch wide by 14.5 inch tall 5 gallon buckets hold approximately 1.03 cubic feet of soil. If your bucket is a different dimension, you can use this calculator to find it’s cubic foot capacity: How Much Potting Soil For Flower Pots Calculator
Soil-based Potting Mix Recipe:
The DIY basic recipe for soil-based potting mixes is 1 part topsoil, 1 part garden soil, 1 part peat moss and 1 part perlite.
If you want to substitute the garden soil with pure compost, you would reduce the compost portion from 25% to 12.5% and increase topsoil and peat moss from 25% each to 37.5% each. You will also need to take additional steps (as outlined further up the page) to prep the compost so that ammonia and acid levels are reduced before it is added to the soil mix.
The modified recipe for soil-based potting mix would be 1.25 part topsoil, 1/2 part compost, 1.25 part peat moss and 1 part perlite.
What About Fertilizer And Other Nutrients?
You may want to incorporate other soil supplements to your mix. Add the ingredients that are specific to the plant’s needs or that the growing conditions require.
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How Much Potting Soil For Flower Pots Calculator