Large outdoor planters offer the space to grow impressive displays of flowers, vegetables, shrubs, and even small trees. But to ensure your plants thrive, understanding how much soil they need and selecting the right soil mix ratio is crucial. Too little soil can restrict root growth and lead to nutrient deficiencies, while the wrong mix can result in poor drainage or excessive moisture retention, both detrimental to plant health. Let’s dig into the details.

Determining the Right Amount of Soil for Large Outdoor Planters:

The amount of soil your large outdoor planter needs directly correlates with the size of the planter and the type of plants you intend to grow. Here’s a general guideline:

• Fill it Up (Almost): Unlike in-ground planting, where roots can spread extensively, container plants rely on the soil within the planter. Generally, you should fill your large outdoor planter almost to the top with soil, leaving about an inch or two of space below the rim. This “headspace” makes watering easier and prevents soil from spilling over.
• Consider the Plant’s Root Ball: When transplanting a plant into a larger planter, ensure the new container can comfortably accommodate the existing root ball with room for future growth. The depth of the planter should be sufficient for the deepest roots of your chosen plants.
• Volume is Key: For very large planters, you might be wondering about specific volumes. Here are some rough estimates:
  • 24-inch diameter x 24-inch deep planter: This size typically holds around 15-20 gallons or approximately 2-2.7 cubic feet of soil.
  • 36-inch diameter x 24-inch deep planter: This can hold approximately 30-40 gallons or roughly 4-5.4 cubic feet of soil.
  • Larger trough planters: The volume will vary greatly depending on dimensions, but you can calculate it by multiplying the length, width, and depth (in feet) to get cubic feet, then converting to gallons (approximately 7.48 gallons per cubic foot).

Choosing the Appropriate Soil Mix Ratio:

The ideal soil mix for large outdoor planters is different from garden soil. Container soil needs to provide good drainage, aeration, and water retention. Here’s how to choose the right ratio:

• Start with a High-Quality Potting Mix: The foundation of your container soil should be a good quality commercial potting mix. These mixes are specifically formulated for container gardening and are typically lighter and drain better than garden soil.
• Amend for Drainage: Especially in large, deep planters, drainage is critical to prevent root rot. Consider amending your potting mix with drainage materials:
  • Perlite: This lightweight, volcanic glass improves drainage and aeration. Aim for a ratio of about 1 part perlite to 2 or 3 parts potting mix.
  • Coarse Sand or Horticultural Grit: These can also enhance drainage, but use sparingly as too much sand can make the mix heavy. A ratio of 1 part sand to 3 or 4 parts potting mix is generally sufficient. Avoid fine beach sand, which can compact.
• Enhance Water Retention (If Needed): For plants that prefer consistently moist soil or in hot, dry climates, you might want to improve water retention:
  • Peat Moss or Coco Coir: These materials hold moisture well. Use them sparingly, as too much can lead to waterlogged soil. A ratio of 1 part peat moss or coco coir to 3 or 4 parts potting mix is usually adequate. Be aware that peat moss is not a sustainable resource, so coco coir is often a preferred alternative.
• Add Nutrients and Improve Soil Structure: Incorporating organic matter can benefit your plants:
  • Compost: This adds nutrients, improves soil structure, and helps with both drainage and water retention. Aim for a ratio of about 1 part compost to 3 or 4 parts potting mix. Well-rotted manure can also be used similarly.

Typical Soil Mix Ratios (Adjust to Your Specific Needs):

Here are a few examples of soil mix ratios you can adapt:

• General Purpose Mix: 2 parts potting mix, 1 part perlite, 1 part compost.
• Mix for Plants Needing Excellent Drainage (e.g., succulents, herbs): 2 parts potting mix, 1 part perlite, 1 part coarse sand or grit.
• Mix for Plants Preferring Moisture (e.g., some vegetables, ferns): 3 parts potting mix, 1 part peat moss or coco coir, ½ part perlite.

Key Considerations When Choosing Your Soil Mix:

• Type of Plants: Different plants have different soil requirements. Research the specific needs of the plants you intend to grow.
• Climate: In hot, dry climates, you might need a mix that retains more moisture. In rainy climates, excellent drainage is paramount.
• Watering Habits: If you tend to overwater, a mix with more drainage materials is safer. If you sometimes forget to water, a mix with better water retention might be beneficial.
• Planter Material: Some planter materials, like terracotta, are porous and can dry out soil more quickly, requiring a mix with better water retention.

In summary: Fill your large outdoor planters almost to the top with a well-draining potting mix amended with materials like perlite and compost. Adjust the ratio of these components based on the specific needs of your plants and your local climate. Avoid using garden soil on its own, as it typically doesn’t provide the necessary drainage and aeration for container gardening.