Soil Amendment Calculator & Garden Soil Guide

How to Use This Soil Amendment Calculator

Enter your garden bed dimensions and the depth you plan to till amendments into (typically 3-6 inches). Then adjust the amendment ratio sliders to match your target mix. A common ratio for vegetable gardens is 60% compost, 20% peat moss, and 20% sand. The calculator shows cubic yards for each component.

How to Amend Garden Soil for Better Growth

Most garden soils benefit from amendments that improve texture, drainage, and organic matter content. The goal is to create loamy soil, a balanced mix of sand, silt, and clay with plenty of organic matter. Clay soils need sand and compost to improve drainage; sandy soils need compost and peat moss to retain moisture.

Common Amendment Ratios by Soil Type

• Clay soil: 30% compost, 20% peat moss, 50% coarse sand (sand opens up dense clay)
• Sandy soil: 50% compost, 30% peat moss, 20% existing sandy soil (organic matter holds water)
• New raised bed: 60% compost, 20% peat moss, 20% coarse sand (balanced mix for vegetables)
• Flower beds: 70% compost, 20% peat moss, 10% sand (more organic matter for blooms)

Amendment Tips

Add amendments in fall for spring planting, this gives organic matter time to break down. Till amendments into the top 6-8 inches where most plant roots grow. After amending, test your soil pH (most vegetables prefer 6.0-7.0). One application typically lasts 2-3 growing seasons before organic matter needs replenishing.

Understanding Soil pH and Nutrient Availability

Soil pH controls how well plants can absorb nutrients, even nutrient-rich soil starves plants if the pH is wrong. Most vegetables and flowers thrive in slightly acidic to neutral soil (pH 6.0-7.0). Below 5.5, aluminum and manganese become toxic to roots, and phosphorus becomes locked up. Above 7.5, iron, zinc, and manganese become unavailable, causing yellowing leaves (chlorosis). To raise pH (more alkaline): add garden lime at 5-10 lbs per 100 sq ft. To lower pH (more acidic): add elemental sulfur at 1-2 lbs per 100 sq ft, or incorporate peat moss which is naturally acidic (pH 3.5-4.5). Always test soil pH before amending, a $15 soil test kit saves you from applying the wrong treatment.

Compost vs. Peat Moss vs. Manure: What Each Amendment Does

• Compost: The all-rounder. Adds organic matter, beneficial microbes, and slow-release nutrients. Improves both clay and sandy soils. Well-made compost has a neutral pH and can be applied at any rate, you literally can't use too much. Make your own or buy by the cubic yard from landscape suppliers ($25-45/yd³).
• Peat Moss: Excellent at holding water (up to 20x its weight) and acidifying soil. Light and fluffy, it dramatically improves clay soil texture. However, peat moss is essentially sterile, it adds no nutrients. It's also a non-renewable resource harvested from peat bogs. Coconut coir is a sustainable alternative with similar water-holding properties.
• Aged Manure: High in nitrogen and organic matter. Must be aged/composted for at least 6 months, fresh manure is too "hot" (high in ammonia) and will burn plant roots. Cow manure is the mildest; chicken manure is the strongest (use at half the rate of cow). Never use cat, dog, or pig manure on food gardens, they can carry pathogens harmful to humans.
• Coarse Sand (builder's sand): The only amendment that permanently improves clay soil drainage. Unlike organic matter which decomposes over time, sand stays put. But you need a LOT, adding less than 3-4 inches of sand to clay soil actually creates a concrete-like mixture. Use only sharp, washed sand; never play sand (too fine) or beach sand (salt kills plants).

No-Till vs. Tilling: The Great Debate

Traditional advice says to till amendments into the top 6-8 inches. But the no-till (no-dig) movement argues that tilling destroys soil structure, kills earthworms, and brings weed seeds to the surface. For new garden beds on compacted or poor soil, tilling once to incorporate amendments is justified. For established beds, a no-till approach, spreading amendments on the surface and letting worms and water incorporate them, preserves the soil food web and reduces weed germination. If you do till, do it only once to establish the bed, then maintain with surface applications thereafter.