And Then There are Amendments

by Nanette Londeree, Master Rosarian

Late spring and early summer may get you itching to plant new roses, spiff up your flower beds, or mix up some great soil to transplant a container rose on your patio. Unless your garden is blessed with oodles of vibrant soil just waiting to be used, you may want to give it a boost before you do any planting. You’ve got plenty of plain old dirt, so what do you add? Soil conditioner? A fertilizer? Gravel or sand? Will you mix it in or lay it on top? Depending on what you want the added material to do, you’ve got plenty of choices.

First some terminology – an amendment is any material mixed into the soil that indirectly aids plant growth by improving the condition of the soil, like its structure or texture, water retention or microbial activity. To do its work, an amendment must be thoroughly mixed into the soil. The terms soil conditioner and amendment are often used interchangeably, both serving to improve the chemical, physical or biological properties of soil. Mulches are organic or inorganic materials placed on the soil surface to help prevent weed growth, conserve moisture and add organic matter to the soil as they break down. Some materials used as soil amendments can also act as a fertilizer by providing nutrients to the soil, or be applied to the soil surface as mulch.

In the process of determining what amendment to use, your first decision is what you want from the amendment – are you looking to loosen up heavy clay soil? Improve drainage? Lower soil pH? It’s a good idea to do a soil test before making any significant changes. Simple test kits for soil pH and the major nutrients of nitrogen, phosphorus and potassium are available at local nurseries. Next consider how long the material will last in the soil, whether it retains water and / or improves permeability (the rate at which water moves through the soil), or if it may present any problems from excess salts, weed seeds, plant, animal or human pathogens. Finally, think about the practical aspects of the material like availability, cost and ease of handling.

Many amendments are used to change soil texture – the way a soil feels. Our ubiquitous clay soil has tiny particles that feel sticky. When wet, it will retain moisture and drain slowly, though once dried it’s hard to penetrate the surface. Most often with these heavy soils, the aim is to increase permeability, improve aeration and drainage. The fastest and easiest way to do this is by adding lots of organic matter – compost, humus or tree bark (with nitrogen added) are good sources. You can also add inorganic materials like perlite or vermiculite to lighten the soil and facilitate drainage. WARNING: Don’t use sand as an amendment for clay soils unless you want to create your own clay bricks! Any mixture less than 70% sand in 30% clay actually packs more densely than straight clay and turns the soil into concrete! If you choose to use sand, make sure it’s coarse builder’s sand.

Roses enjoy a soil with a pH that is slightly acid to neutral (6.5 – 7.0); pH affects availability of nutrients to plants (especially iron) and the activity of soil microorganisms. If you want to raise your soil pH, incorporate materials with calcium and magnesium carbonates – dolomitic or calcitic lime, into the soil. If you’re after lowering pH, consider peat moss or coffee grounds.

Watch out for unwanted salts, weed seeds and pathogens when selecting your amendments. Wood ashes and fresh manures may contain an over-abundance of soluble salts that can change soil structure and damage roots. Fresh manures, or composted manures that have not reached an adequate temperature, may be riddled with weed seeds or carry potential pathogens.

Many amendments are readily available at little to no cost – you can make your own compost and earthworm castings and harvest wood ashes from your fireplace or wood-burning stove. Collect your coffee grounds after your morning cup of joe; or hit up the myriad coffee houses that give spent grounds away for free. Other materials may cost you, especially if you purchase in small quantities (all that packaging is expensive!). Some local landscape supply places allow you to bring your own container and shovel to fill up large bags of compost making it much more economical; and if you’ve got a lot of soil to amend, you can purchase it by the truckload. Nurseries, garden centers and home improvement stores carry easy to handle packaged materials – from quart-sized to large compression packed bags (they can be heavy!) They’re also most likely to have the less common amendments like lime, peat moss, perlite and vermiculite. But buyer beware! There are no regulations about the quality of the product, salt content or other beneficial or harmful qualities of bagged products.

How much of the amendment you add will vary depending on the condition of the soil you’re working with, what you’re adding and what you want it to do to the soil. Over-amending can be a problem if you’re trying to fix a soil problem by adding large quantities of amendment in a single season, and result in soil that is high salts or ammonia that burns roots and leaves, low nitrogen levels from the tie-up of nitrogen due to a carbon to nitrogen ratio imbalance or retaining too much water.

Now that you’re filled with possibilities of what you can add to create that vibrant soil, the table below with common amendments and their key attributes may help you select a material that will meet your soil needs.

Scroll to Top