How Much Soil Conditioner Do I Need? A Complete Guide

Soil conditioners are materials added to soil to improve its physical properties like water retention permeability, water infiltration drainage, aeration and structure. The question of how much is needed depends on the project. Here is a guide on determining the right amount of soil conditioner for your gardening or landscaping project.

Why Use Soil Conditioners?

Compost, manure, peat moss, coconut coir, leaf mold compost, and other organic matter are good for the soil because they make it more stable. They increase nutrient and moisture retention. Soil conditioners also help loosen up hardened soils so they drain and breathe better. Good soil leads to healthier plants.

Some key benefits of adding soil conditioners:

• Improves moisture retention
• Increases nutrient retention
• Improves drainage and aeration
• Promotes healthier root growth
• Reduces soil compaction
• Increases microbial life and activity

So whether you’re prepping a new garden bed top dressing your lawn or filling a raised bed, soil conditioners are a critical addition for optimal plant growth.

How Much Do I Need?

How much soil conditioner you need will depend on the project. This is a list of common gardening and landscaping uses and how much conditioner they need:

Top Dress Lawns

To lightly cover grass seed, apply a 1/8 inch layer of compost.

• 10 sq ft: 1.5 cubic ft bag (extra will be leftover)
• 50 sq ft: 1.5 cubic ft bag
• 100 sq ft: 2 bags in 1.5 cu ft size or 1 bag in 2-3 cu ft size
• 200 sq ft: 2 bags in 1.5 cu ft size or 2 bags in 2 cu ft or 1 bag in 3 cu ft size
• 500 sq ft: 4 bags in 1.5 cu ft size or 3 bags in 2 cu ft or 2 bags in 3 cu ft size

Apply Mulch

To apply a 1 inch layer of mulch:

• 10 sq ft: 1.5 or 2 cubic ft bags, approx. 1 bag
• 50 sq ft: 3 bags in 1.5 or 2 cu ft OR 2 bags in 3 cu ft
• 100 sq ft: 6 bags in 1.5 cu ft OR 5 bags in 2 cu ft OR 3 bags in 3 cu ft
• 200 sq ft: 12 bags in 1.5 cu ft OR 9 bags in 2 cu ft OR 6 bags in 3 cu ft
• 500 sq ft: 28 bags in 1.5 cu ft OR 21 bags in 2 cu ft OR 14 bags in 3 cu ft

Weed Suppression

For a 3 inch layer of compost or conditioner to suppress weeds:

• 10 sq ft: 2 bags in 1.5 or 2 cu ft size
• 50 sq ft: 10 bags in 1.5 cu ft OR 7 bags in 2 cu ft OR 5 bags in 3 cu ft
• 100 sq ft: 18 bags in 1.5 cu ft OR 13 bags in 2 cu ft OR 8 bags in 3 cu ft
• 200 sq ft: 34 bags in 1.5 cu ft OR 25 bags in 2 cu ft OR 17 bags in 3 cu ft
• Over 500 sq ft: Recommend buying bulk

Raised Beds

For new raised beds, fill with a quality potting or raised bed mix.

• 8×4 bed (32 sq ft): 32 bags in 1.5 cu ft OR 24 bags in 2 cu ft OR 16 bags in 3 cu ft
• 4×4 bed (16 sq ft): 16 bags in 1.5 cu ft OR 12 bags in 2 cu ft OR 8 bags in 3 cu ft

Amending Existing Soil

To amend soil with 20% conditioner to 80% existing soil:

• 32 sq ft area: Need 9.6 cu ft of amendment
• Use 7 bags in 1.5 cu ft OR 5 bags in 2 cu ft OR 3 bags in 3 cu ft

Containers

Potting mix quantities needed:

• 5 gallon pot: 0.5 cubic ft
• 1 gallon pot: 3 dry quarts
• 12” pot: 8 dry quarts
• 18” pot: 28 dry quarts
• 14” shallow container: 8 dry quarts
• 18” shallow container: 20 dry quarts
• 3’ window box: 20 dry quarts
• 4’ window box: 1 cubic ft
• 23” high half barrel: 4.5 cubic ft

One 1.5 cubic foot bag fills about twelve 12-inch pots.

Buying Bagged or Bulk Soil Conditioner

When buying soil conditioner, you can purchase bagged product or bulk material. Bulk buying is more cost effective for large projects. Bagged soil is more convenient for small gardens and doesn’t require special delivery or equipment to move onsite.

You can use the soil calculator guidance above to determine how many cubic feet or bags you need based on your project size. Most bagged soil conditioners come in 1, 1.5, 2 or 3 cubic foot bags.

You might want to order bulk loads of soil and amendments for very big projects like new gardens, lawns that have been fixed up, or commercial sites. Bulk quantities are usually sold by the cubic yard. One cubic yard is equal to 27 cubic feet. A landscape supply company can bring and spread large amounts of materials for you.

Key Takeaways

• Add soil conditioners like compost to improve soil texture, moisture retention, aeration and nutrition for optimal plant growth
• The amount needed depends on your specific project
• Use 1/8 inch layer for topdressing lawns, 1 inch layer for mulching, and 3 inches to suppress weeds
• For new raised beds, fill entirely with high-quality potting mix
• When amending existing soil, use a ratio of 20% amendment to 80% current soil
• Use portion guidelines to estimate amounts for containers
• Purchase bagged conditioner for small projects, bulk for large gardens or jobsites

With this handy soil conditioner calculator guidance, you can determine the right amount of amendment material needed for your next gardening or landscaping project. Improving your soil will lead to healthier, thriving plants!