Soil amendments are materials added to soil to improve its physical properties, such as water retention, permeability, water infiltration, drainage, aeration and structure. Using soil amendments is essential for creating and maintaining an optimal environment for your plants to thrive.
In this comprehensive guide, we will discuss what soil amendments are, the different types of soil amendments, reasons for using them, and how to apply various soil amendment products to improve your garden or landscape soil quality.
What Are Soil Amendments?
Soil amendments are materials added to soils to alter their chemical or physical properties and improve plant growth. They make soils easier to work with and improve moisture retention, drainage, aeration, and nutrient availability.
In other words, soil amendments help create the ideal environment for plant roots to thrive by altering the soil structure and chemistry. They make heavy clay soils lighter and more workable. They help sandy soils retain water and nutrients. Overall, amending soil enables better plant growth than can be achieved in unimproved native soils.
Some common soil amendment categories include:
- Organic amendments – peat moss, compost, aged manure, cover crops, and diverse organic materials added to soil to improve moisture retention, drainage, and nutrition. They increase soil organic matter content.
- Inorganic amendments – vermiculite, perlite, tire crumb rubber, calcined clay. These are added to improve moisture retention and drainage in soils.
- pH adjusters – lime, sulfur, and other materials added to raise or lower soil pH to optimal levels for plant growth.
- Fertilizers – organic and inorganic amendments added to provide nutrients and enhance plant growth.
Soil amendments provide a growing foundation that landscape plants need not only to grow, but to express their maximum genetic potential for health and beauty. Amending helps plants better withstand stresses like drought, high rainfall, foot traffic, disease and pests.
Different Types of Soil Amendments
Many types of materials, both organic and inorganic, can be used as amendments to improve soil quality:
Organic Soil Amendments
Compost – Decomposed organic matter that provides nutrients for plants and beneficial microbes. Compost also improves soil structure.
Aged manure – Provides organic matter and nutrients once fully composted. Should be aged 6+ months to avoid burning plants.
Sphagnum peat moss – Absorbs and retains moisture well. Also provides organic matter. Acidifies soil.
Coir – Coconut husk fiber is helpful for moisture retention and provides organic matter. Has a neutral pH.
Grass clippings – Fresh clippings can be mixed into garden soil; avoid clumping.
Leaves – Shredded leaves make good mulch and amendments when mixed into soil.
Cover crops – Grasses and legumes grown to cover soil then tilled as green manures.
Biosolids – Nutrient-rich organic matter recycled from wastewater treatment. Must be processed correctly for safety.
Wood chips – Best used as surface mulch; avoid mixing directly into soil to prevent nitrogen depletion during decomposition.
** Straw** – Provides organic matter when worked into soil. May contain weed seeds so best to compost first.
Inorganic Soil Amendments
Perlite – Lightweight volcanic glass that improves drainage and aeration. Does not affect fertility.
Vermiculite – Lightweight mica product used to improve moisture retention and aeration.
Builder’s sand – Mixed with heavy clay soils to improve drainage. Rinse well before using.
Biochar – Charcoal produced from pyrolysis of organic matter. Stores carbon long-term and increases nutrient retention when mixed into soil.
Crushed gravel or pea gravel – Best for improving drainage in very wet soils. Use gravel without fine particles.
Calcined clay granules – Porous granules that hold moisture and air. Stable at high temperatures.
Tire crumb rubber – Recycled tires made into pellets to improve soil structure and moisture retention. Avoid inhaling rubber dust.
Expanded shale – Heating shale produces porous, lightweight aggregates to enhance drainage and aeration.
Sand or soil conditioners – Combination products to mix into native soils to create looser texture.
Reasons for Using Soil Amendments
There are several key reasons gardeners and landscapers utilize soil amendments:
Improve soil structure – Amendments like compost and manure create soil aggregates, improving porosity and enabling movement of water, air, and roots through the soil. This is especially helpful for tight, dense, clay soils.
Increase moisture retention – Sandy soils drain too quickly and dry out fast. Amendments like peat moss and coir help retain more water for plant availability.
Enhance drainage – Amendments like perlite, vermiculite and biochar improve drainage and aeration for planting in heavy clay or compacted soils.
Adjust pH – Lime raises pH of acidic soils, while sulfur lowers pH of alkaline soils. Getting to the optimal 6.0-7.0pH range maximizes nutrient availability.
Boost soil fertility – Organic amendments like manure and compost add essential macro and micronutrients that plants need for growth and productivity.
Improve plant growth – Healthier root zones and improved fertility enable plants to better express their genetic potential for vigor, beauty, flower production, and overall growth.
Increase organic matter – Added organic materials feed beneficial soil microbes. They create humus that holds nutrients in plant-available form and retains moisture.
Promote soil biology – Diverse organic amendments foster a thriving soil food web of micro and macro organisms that release nutrients for uptake by plant roots.
Recycle organic resources – Amendments utilize leftover products high in carbon like wood chips, as well as high nitrogen materials like grass clippings that would otherwise go to waste.
How to Apply Soil Amendment Products
With so many soil amendment products on the market, it can get confusing to figure out which to use and how to apply them correctly. Here are some tips:
Incorporate Evenly When Possible
Ideally, amendments should be mixed into soil evenly so their benefits extend to the entire plant root zone. This is easy for small gardens and beds – simply rototill amendments into the top 6 inches of soil before planting. For larger areas like lawns, use a core aerator to expose soil before topdressing with compost.
Focus Amendment Areas Near Transplants
For trees, shrubs, and perennials planted into native soil, dig wide enough holes to loosen soil on sides. Mix amendments into soil removed from holes before returning amended soil around transplants.
Topdress Lawns and Gardens
Where tilling is not possible, apply 1/4 to 1/2 inch of compost, manure, or other amendments as topdressing. Over time, this will work down into soil. Topdressing is also used to improve established lawns and gardens annually.
Create Amended Beds
For raised beds, rotate and fill with a custom blend of amendments like compost, topsoil, peat moss and fertilizer. Mix thoroughly before filling beds. Amending entire bed areas provides ideal conditions for vegetable and flower roots.
Follow Package Instructions
Bagged products like vermiculite, peat moss and fertilizer have recommended application rates. Follow instructions carefully to avoid problems like creating soggy soil or burning plants with excess fertilizer.
Soak Clay Soils Before Tilling
Dry clay soil can become concrete-like when tilled. Soak areas overnight before working in amendments to allow clays to hydrate and achieve a crumbly texture that incorporates amendments better.
Adjust Soil pH Before Amending
Have soil tested to determine pH. If highly acidic or alkaline, amend with sulfur or lime as needed to reach optimal pH. This ensures any added nutrients remain plant-available after amending.
Use Moderation
Avoid over-applying organic amendments like manure that can burn plants if added in excess. Follow rates on bags or do soil tests to determine how much amendment to apply.
Using Specific Soil Amendment Products
Here is more detail on using some common soil amendment products:
Compost
Compost is one of the most beneficial all-purpose amendments for improving soil structure, moisture retention, and fertility. Use compost derived from plants, not animal manures, if concerned about pathogens. Apply 1-3 inches of compost and till into soil before planting beds and gardens. Replenish annually with 1/2 to 1 inch of fresh compost as topdressing.
Manure
Cow, horse, chicken, and other manures provide nitrogen and other nutrients once composted 6 months or longer. Avoid fresh manure that could burn plants or leach nitrogen. Mix well-aged manure into garden beds at ratio of 1 part manure to 2 parts soil. For lawns, use 1/4 inch topdressing annually.
Peat Moss
Mix peat moss into soil at ratio of 1 part peat to 3 parts soil to improve moisture retention. For containers, use up to 1/3 peat moss in the potting mix. Soak peat moss before using – dry peat is hard to wet and repels water.
Biochar
Mix in 1-5% biochar by volume into garden beds and 5-10% into potting mixes. More is not necessarily better – observe plant response. May initially lower soil nitrogen as it decomposes – topdress with compost or manure. Lasts decades in soil.
Perlite
Mix into garden beds and containers at 25-50% ratio with other amendments like peat moss. Improves drainage of heavy soils. Blend thoroughly when mixing into soil. Perlite floats, so take care when watering to avoid it washing to the surface.
Wood Chips
Spread 2-4 inch layer of wood chip mulch around plants to retain moisture and suppress weeds. Avoid thick layers (>4 inches) as they can deplete nitrogen during decomposition. Best not to directly mix into soil – compost chips first.
Lawn Topdressing
For existing lawns, spread 1/4 to 1/2 inch layer of compost, manure, biosolids or mix of amendments to add nutrients and organic matter without thatch buildup. Topdress in early fall or spring. Follow with core aeration to work down into soil.
Frequently Asked Questions
What is the fastest way to amend soil before planting?
For small areas like gardens, rototilling or digging in amendments provides immediate improvement before planting. For larger areas, core aeration and topdressing amendments speeds the process so soil can be amended and planted within the same season.
How often should soil amendments be applied?
Annual light topdressing maintains the benefits of amendments in existing gardens and lawns. Heavy clay soils and new gardens may need more – amending beds 2-3 years initially establishes the desired loose soil texture. Container plants need fresh amended mix each year.
Is fall or spring better for amending soil?
Fall is an excellent time to amend garden beds you plan to plant the following spring. This gives amendments time to blend with native soil over winter. Spring amendment is ideal directly before planting. For lawns, amend in early fall or early spring.
What are biosolids and are they safe amendments?
Biosolids are treated sewage sludge from wastewater processing plants. Safe for gardens if properly processed to remove pathogens and pollutants. Check local regulations – some municipalities prohibit use. Introduce gradually to avoid salt damage.
Can too much compost or manure burn plants?
Yes, fresh manure and even heavy compost applications can burn plants due to high salt, ammonia and organic acid content. Age manure 6+ months before use and limit to 1/4 inch topdressing. Mix compost into beds, don’t leave thick layers on top.
How do I know if store-bought garden soil is appropriate as an amendment?
Read contents on bag – quality topsoil is fine but avoid garden soils with chemical additives. Look for soil mixes blended with compost and organic fertilizer. Test unknown mixes on a small area first to observe plant response before using extensively.
What soil amendments should be used to help plants tolerate drought?
Organic materials like compost, manure, peat moss, and biochar increase moisture retention during droughts. Adding natural minerals like humates and paramagnetic rock dust also help plants resist drought through complex effects on soil that boost plant vigor and health.
Conclusion
Amending soils with compost, manure, peat moss and other organic and inorganic materials provides a broad range of benefits. The ideal soil environment created through careful amending enables plants to flourish and express their full genetic potential.
Follow these research-based guidelines on how and when to apply the leading soil amendment products to significantly improve garden and landscape soil quality. Avoid over-application of any amendments. Test unfamiliar products on small areas first to gauge effectiveness.
With careful selection and balanced use of soil amendments, you can produce thriving gardens and landscapes. Plants with healthy, robust root systems better tolerate stresses like drought and disease – and reward you with an eruption of flowers, fruits, foliage, and beauty.