The Ultimate Guide to Soil Amendments
If you’ve followed along, you know our stance on soil. Good soil is the literal foundation of any successful garden. Quality soil teams with biodiverse life. It is nutrient-rich, well-draining, and creates a nice, crumbly tilth (the term used by experts to speak to the condition of soil, specifically its ability to support plant roots.)
Unfortunately few gardeners are lucky enough to have ideal soil in their garden plots. And, since we cannot always rely on mother nature to supply all the essentials, they usually have to be added by the gardener.
This is where soil amendments come in. Good soil amendments used properly can produce results that feel like magic. Using the wrong amendment at the wrong place and time turns soil to cement, damages delicate microbiomes, and may even pollute your soil. The following article will help you to understand, when, why, and how to use soil amendments to achieve a bountiful garden.
What Are Soil Amendments?
Often confused with fertilizers, soil amendments are products mixed into the topsoil to promote healthy plant growth. They function in several different ways. For example, they may be used to alter soil’s texture or structure, increase nutrient content, improve drainage or water retention, and adjust the pH level.
Fertilizers, on the other hand, are primarily used for their ability to supply nutrients. In short, they each play a different but important role. Understanding what soil amendments offer, is the first step in using them to make the most of your garden.
How do Soil Amendments Work?
Unlike fertilizers, which add nutrients to the soil, amendments are used to modify and improve the condition of the soil itself. Used properly, amendments alter the soil to create an easier growing environment for plants and gardeners alike.
For example, with soil that is correctly amended, roots can penetrate more easily and water infiltration is improved. Soil amendments also have the ability to change soil in ways that affect the availability and usability of both naturally occurring and added plant nutrients.
Soil amendments are meant to be added to the topsoil of a garden before planting begins. From there they are watered into the soil and filtered in naturally to the ground below. Soil amendments are not meant to be used as fertilizer substitutes. Instead, they function to alter the soil to help fertilizers do the job they're intended for.
How Do I Know if I Need Soil Amendments?
If your garden isn’t growing well or fails to thrive, it may be time to consider amending your soil. If the soil is difficult to work with, lacks biodiversity, stays too wet, or dries out too rapidly, it’s probably time to consider adding some soil amendments. In slightly more scientific terms, the two main reasons to add soil to your garden are to improve the soil's texture or to correct the soil's pH.
Why Soil PH Matters
Plants rely on 17 “essential” nutrients for sustained and healthy growth. Soil pH plays a vital role in determining whether your plants get the nutrients they need. Generally speaking, most plants do best when soil pH is in the 6.0 to 7.0 pH range, neutral to slightly acidic.
In high-alkaline soils as well as overly acidic soils, essential nutrients can become “tied up" so that plants can't absorb them. Soil amendments are intended to improve soil quality, but unnecessary amendments can have an adverse effect. For instance, raising soil pH too high can set off a domino effect of nutrient imbalances. Conversely, lowering soil pH too much can create toxic conditions. Even organic matter can cause imbalances if too much of them are applied.
Before you start adding amendments ad hock, always test the pH of your soil first. This can be done easily with a home testing kit. (I believe we bought our tester at a local hardware store for around $15.) Or there are a couple of DIY solutions in the link below. If your soil is otherwise well-draining and has good crumble, this may be the only test you need to determine where to start.
Why Soil Texture Matters
When considering soil “texture”, what we are really looking at is its composition. Soil is composed of three primary mineral elements clay, silt, and sand. Each component is necessary for good soil composition or tilth, but too much of any one ingredient can spell disaster in the garden.
Overly silty soils can be prone to compaction and erosion. Overly sandy soils lose moisture quickly and retain nutrients poorly. Clay-heavy soils are easily waterlogged when moisture is present and concrete like when dry.
You can use the DIY methods in the article above to test the composition of your soil or you can check with your local extension office. They should be able to provide you with a complete soil analysis as well as amendment recommendations for a minimal cost.
If you find that you have less than ideal soil, you will aim to reach a composition of 40% sand, 40% silt, and 20% clay. Great soil is cultivated purposely over time, depending on how far off balance your soil is, this may take a few years to achieve.
Why Biodiversity Matters in Soil
It may come as a surprise to you, but garden soil is not inert. A healthy soil teams with life beneath the surface in the form of micro and macro-organisms including bacteria, fungi, tiny insects, and the almighty earthworm. Scientists estimate that about ¼ of all species on the planet exist within the soil. And, a single teaspoon of garden soil may contain thousands of species, millions of individuals, and hundreds of feet of mycelium (fungal networks).
All of this life is essential in the garden. Each piece plays its part to aerate the soil, decompose organic matter into nutrient-rich waste, and even provide nutrients themselves.
What are the 17 Essential Nutrients That Plants Need?
As we said above, there are 17 “essential” nutrients that every plant needs to survive. They are listed below along with what their primary function in plant nutrition is.
Boron - helps in calcium uptake & aids in the absorption of nitrogen
Calcium - promotes early root development & encourages seed production
Carbon - a primary energy source and building block for plant tissues
Chlorine - helps to regulate water uptake, & improves disease resistance and tolerance
Copper - regulates respiratory activities in plants
Iron - essential for the formation of chlorophyll and synthesis of proteins
Hydrogen - necessary for building sugars & other molecules to produce glucose for plant energy
Magnesium - helps in the uptake of phosphorus & regulates the uptake of other nutrients.
Manganese - helps in chlorophyll formation
Molybdenum - essential for nitrogen-fixing organisms both symbiotic & non-symbiotic
Nickel - a component of plant urease.
Nitrogen - makes plants dark green & succulent, promotes vegetative growth
Oxygen - plays a crucial role in photosynthesis
Phosphorus - Stimulates root development & increases disease resistance
Potassium - increases vigor and aids in disease resistance
Sulfur - stimulates root growth as well as seed and nodule formation
Zinc - helps in the formation of growth hormones and chlorophyll
Organic vs. Inorganic Amendments
Soil amendments fall into two primary categories: organic and inorganic. As the name suggests, organic amendments come from something that is or was alive. Inorganic amendments, on the other hand, are either mined or man-made.
Organic amendments increase the organic content of the soil. Used properly over time they can improve soil aeration, water infiltration, and both water- and nutrient-holding capacity. Many organic amendments contain plant nutrients and can act as organic fertilizers as well.
Organic amendments include:
Alfalfa Meal A natural source of readily available plant nitrogen. It can be spread on the soil, or made into a tea and watered in.
Biochar Biochar improves soil moisture holding capacity, has a liming effect on soil, improves nutrient retention, softens the soil, and boosts harvests. This is one of my favorite amendments as it lasts in the soil for hundreds of years. The upfront cost is high, but it’s one-and-done unless you create new beds.
Blood Meal is a byproduct of the meat industry that is extremely high in readily available nitrogen. A little goes a long way. If overused it can and will “burn” your plants. Also note that if you live in a bear-populated area, blood meal in the garden will act as an attractant.
Fish Meal A byproduct of America’s catfish industry, this amendment provides an excellent source of nitrogen.
Coconut coir A byproduct of the commercial coconut industry, coconut coir can be used to improve the drainage and structure of the soil.
Compost Compost is naturally microbe rich. Once added, over time It will slowly acidify the soil, feed plants, and make soil softer as it increases the organic matter.
Feather Meal This is a byproduct of the poultry industry. It provides a solid source of slow-releasing nitrogen.
Manure Manure is naturally microbe-rich, add nutrient to soil, and will slowly acidify soil over time. Remember that manure, like compost, must be aged before use as fresh manure will “burn” plants.
Mushroom compost Mushroom compost is a byproduct of the mushroom industry. It improves nutrients, soil structure, and beneficial microorganisms.
Rice Hulls Used as a mulch, or added to the soil, rice hulls can help to aerate your soil.
Straw This is an amendment that takes a while to break down, so it is best added in the fall, to aerate the soil and add organic matter.
Sphagnum peat Peat moss can add texture, aid in moisture retention, and raise the ph of almost any soil.
Wood chips Wood is naturally low in nutrients when fresh and must be partially decomposed before adding it to the soil. The use of wood chips will change soil chemistry, gradually acidifying it over time.
Wood ash In addition to having a high nutrient content, wood ash can help in neutralizing soil acidity. Use this sparingly, or not at all as wood does have a high salt content.
Worm Castings Castings add minerals, inoculate the soil with living micro-organisms and improve soil structure by boosting organic matter.
Inorganic Soil Amendments
Inorganic amendments are used to improve soil texture and drainage as well as increase the water and air permeability of the soil. These amendments with not decompose within your soil in your lifetime.
Inorganic amendments include:
Dolomite Lime Lime acts to raise pH in acidic soils and is also a good source of calcium and magnesium.
Gypsum Gypsum gets deep in the soil layers very quickly when used as an amendment to improve structure and drainage. It also provides calcium and sulfur.
Perlite Made from expunged volcanic glass, this amendment improves soil structure by providing drainage and aeration.
Vermiculite Vermiculite is a naturally occurring mineral that helps to aerate the soil while simultaneously retaining water and nutrients.
Using Soil Amendments
As you can see from everything you’ve read up until now, there is a lot more to amending soil than just picking a product and throwing it on. Below we’ll touch on some guidelines for use.
Improving Soil Texture
Often we find that soil is low in the levels of organic matter necessary for structure, water retention, and biodiversity. Using organic amendments can quickly change the texture of your soil as well as encourage beneficial organisms to set up home in your garden plot.
Adjusting Soil pH
The pH of your garden soil needs to be within a range that will allow plants to access the nutrients in the soil. The first step in correcting soil pH is knowing where you start. Once you know your soil pH, you can select the appropriate amendments to restore balance.
It’s important to note that garden soil is in a constant state of flux. Each season plants deplete it of nutrients, rain washes away delicate topsoil, and even the most heavily amended soil will eventually revert to its natural state. Once you get to know your garden soil, as well as the nutrient requirements of your specific crops you will be able to determine what amendments your garden will need over time.
Let’s Get Gardening!
Now that you hopefully have a good grasp of soil amendments, what they do, and which ones to use, it’s time to get our hands dirty and get out in that garden! Early Spring and late Fall are the best times to add in soil amendments to give them the time they need to work their way into the soil and start making the desired changes.
As garden season fast approaches, and you consider your soil, what amendments do you think will be most beneficial to your crops this year? Tell us all about your gardening plans, in the comments below, and if you are just getting ready to start a garden for the first time be sure to hop over to our Gardening for Beginners page for more helpful tips and advice! And, as always, until next time,