What is a soil amendment?
Whether you’re looking to grow a beautiful garden or an exotic indoor jungle, healthy soil composition is a top priority in growing strong, healthy plants. Soil amendments can be essential when planting or repotting various plants. They modify your soil to better suit the different needs of various plants. A soil amendment is a material, often organic, added to soil to improve its texture, structure, nutrient content, drainage, water retention, or pH level to encourage the growth of strong, healthy plants. Curious about what soil amendments are and how they can benefit your plants? Read on!
Let’s go over some common amendments like manure, peat moss, dolomitic lime, and more to learn about their properties and uses.
Manure
Manure is one of the most common - and likely oldest - amendments in use. It is known for its ability to improve soil texture and structure, as well as for adding organic matter, which increases the microbial count and slowly releases nutrients into the soil. The organic matter in manure binds soil particles, increasing drainage and aeration in clay soils, while improving water retention in sandy soils. If you are using manure in your soil, do keep in mind that this organic matter is quite rich in nutrients. As such, it is best to apply manure moderately to avoid negative effects on plant health due to possible nutrient imbalances. Applying in the fall or early spring will give adequate time for organic matter breakdown and nutrient release. A word of caution, well-composted manure is preferrable, as fresh manure can damage plants with its high nitrogen content.
Peat Moss
Another popular soil amendment, peat moss, is often used to improve soil structure, aeration and water retention. Peat moss’s unique ability to retain several times its weight in water makes it ideal for sandy soils, keeping garden beds evenly moist and plants hydrated without waterlogging. This is because peat moss has a light and airy structure which creates air pockets in the soil, providing more oxygen to the roots and decreasing the risk of root rot. One important note about peat moss: its naturally lower pH level can benefit acid-loving plants (like blueberries, azaleas, and rhododendrons) but may harm plants that prefer a more neutral pH if not balanced with other amendments. Lastly, a pro tip for applying peat moss: moisten it slightly before adding to soil, as it can be quite hydrophobic when dry.
Dolomitic Lime
This amendment made from crushed limestone provides calcium and magnesium and raises the pH of acidic soils to a more neutral level, which benefits most vegetable gardens, many flowering plants, and lawns. Sandy soils are more acidic and often benefit from application of dolomitic lime. The calcium carbonate and magnesium carbonate found in dolomitic lime encourages the creation of air pockets in the soil, reducing its density, decreasing compaction, and increasing drainage. This is especially useful for soils that are rich in clay. It is best to apply dolomitic lime in the fall to give it sufficient time to breakdown and impact the soil’s pH before the upcoming growing season. A moderate application is key to avoid the soil becoming too alkaline, which can impact uptake of nutrients.
Aluminum sulfate
Aluminum sulfate is used as an amendment to acidify the soil for acid loving plants like blueberries, azaleas, rhododendrons, and hydrangeas. Due to its chemical composition, aluminum sulfate reacts quickly with water, immediately lowering soil pH. The change in pH caused by this amendment is responsible for the production of blue flowers on Mountain hydrangeas (Hydrangea macrophylla). It is recommended to apply aluminum sulfate with caution in slow and gradual applications to avoid causing aluminum toxicity or shocking plants.
Vermiculite & Perlite
Vermiculite, a lightweight mineral-based amendment, is commonly used in seed-starting mixes, with potted houseplants, and in vegetable gardens to encourage healthy root growth. It does so by improving aeration and moisture retention in the soil. Its unique shape and porous structure allow vermiculite to hold onto water and nutrients such as calcium, magnesium and potassium. It slowly releases moisture and nutrients for the plant to absorbe. Vermiculite is most efficient when used in combination with other amendments like compost, peat moss, or perlite. It is typically mixed in at a ratio of 20-25% to avoid the retention of too much water, and overly wet soil.
Perlite is a lightweight amendment composed of volcanic mineral that is commonly used in potting mixes, especially for houseplants, to improve aeration and drainage in the soil, and lessen compaction. Its gritty, porous structure allows excess water to drain more easily from the soil which makes it ideal for plants that prefer a drier soil, like cacti or succulents. Perlite, when used in combination with peat moss, compost or vermiculite, is a useful amendment to improve aeration and lessen compaction in clay soils in order to make it better suited to growing vegetables and flowering plants.
So what’s the difference between vermiculite and perlite? It comes down to their ability to better retain or drain water. Moisture loving plants will benefit more from vermiculite than perlite given its stronger moisture retention properties, whereas plants needing drier, well-draining soil will benefit more from perlite and the drainage it offers.
Orchid Bark
Orchid bark, a chunky, and woody soil amendment made of smaller pieces of various tree barks, is typically used to improve aeration and drainage for healthier root growth in the mix of orchid and other epiphytic houseplants. Epiphytes, which include plants from the Hoya, Monstera, Philodendron, and Epipremnum genera, are plants adapted to thrive without soil. They climb trees and absorb moisture and nutrients from the air through unique structures like aerial roots. The chunky structure of orchid bark gives roots something to grab onto, as they would in their natural environment. It also helps increase drainage, while creating air pockets in the soil to provide more oxygen to roots and decrease the chance of developing root rot. Orchid bark is often used in combination with other amendments like perlite and moss to create a custom soil blend for tropical houseplants.
Earthworm castings
Earthworm castings, also known as vermicompost, are the byproduct of earthworms digesting organic matter. As a nutrient-rich soil amendment, it is commonly used to improve soil fertility and structure. The various nutrients found in earthworm castings help promote the development of strong, healthy root systems, contributing to stronger plants with better resistance to pests and diseases. They increase the ability of soil to evenly retain moisture, without being waterlogged, making earthworm castings ideal in both sandy soils that drain too quickly, and clay soils that can become too wet. Earthworm castings can be mixed directly into the soil, or used as a top dressing for established plants in both the spring and the fall.
Fertilizers
Fertilizers are a soil amendment used to increase nutrient content in the soil, and come in various formats including organic fertilizers, synthetic formulations, and slow release formulations, as well granular and liquid or water soluble formats. While most organic, slow release and granular fertilizers can be applied directly to the soil following the application instruction on the product label, liquid and water soluble fertilizers will need to be diluted and added into the soil by watering. Organic fertilizers provide a slower release of nutrients, and may need to be applied prior to the growing season for optimal results, while nutrients in synthetic fertilizers are released and absorbed immediately for instant results.
Final Thoughts
Soil amendments are valuable tools for modifying soil structure and properties to suit your growing needs. However, it's important to consider each plant's unique needs to select the most appropriate amendments for an optimal growing environment.
Still have questions about which amendments to use? Contact us here.