The Why & How of Garden Soil Amendment

“If a healthy soil is full of death, it is also full of life: worms, fungi, microorganisms of all kinds …  Given only the health of the soil, nothing that dies is dead for very long.”
–  Wendell Berry,  The Unsettling of America, 1977

In my opinion, there is nothing on Earth more underrated than dirt. We walk on it every day without any thought, spend money and time trying to eradicate it from our homes, and dump our trash and waste into it. But the truth is, dirt, or more correctly, soil, is one of the most amazing and important things on our planet.

Why is soil so important?

Soil is the richest source of life. Hidden in soil are fungi, bacteria, protozoans, plant systems, insects, worms, mammals, and reptiles. All of these life forms affect its structure and aid in its development. Interestingly enough, this life in the soil depends on dead stuff in the soil. Bacteria and fungi are present whenever the soil contains organic matter in the form of decaying plants and animals.

Without soil we wouldn’t have food. Soil is the source of a vast majority of our food (note, this may change as hydroponic technology advances). Soils provide the perfect environment for plants and crops to survive, without which we (and the livestock we consume) would starve.

Soil helps prevent flooding and filters water. Another important function of soil is to act as a sponge for water and rain that would otherwise flood the land. The captured water is not only another integral part of plant growth but it also filters out nasty compounds that would otherwise make water essentially poison.

Soil protects us from harmful bacteria. Soil is filled with minor bacteria that helps toughen our immune systems when we are young (and even when we are old), making us able to withstand much tougher infections down the line. Another good reason to go play in the dirt. 🙂

Wow, soil is some pretty amazing stuff! Not only does it play many essential roles here on Earth, it also plays the most essential role in a garden. Because as every gardener knows, it all begins with the soil. The healthier the soil, the better your plants will grow and the fewer problems you will have. So just how does one go about checking and maintaining the health of their soil? For the answers to that, we will start with the basics.

Amending Soil – The Basics

I’ve had good crops and I’ve had bad crops. The one thing they have in common is the condition of my soil. I learned long ago that if I didn’t take the time to properly maintain and check the health of my soil, I couldn’t expect a good harvest in return. Skipping proper soil amendment and opting for a quick fix of fertilizer (a lazy approach that I have used many times) will only get you so far. In the words of Marion Cran:

“If you want to have a happy garden, you must ally yourself with the soil; study and help it to the utmost, untiringly. Always, the soil must come first.” 

You might be thinking – is it really necessary to amend my soil?  Short answer is – yes! Vegetables, herbs, and fruits all take nutrients from the soil as they grow. Each year you need to replace those nutrients to insure a healthy harvest. There are many ways to improve your soil, including adjusting the pH and adding soil amendments. By doing a little work now, you’ll save time and energy come springtime.

Fall is a great time of year to tune up your soil. The year’s harvest is winding down, the cool weather makes gardening enjoyable, and many of the soil amendments you add now will have all winter to break down before your next planting. Presently, I am working on amending the soil in my own garden.

Soil Composition

To understand how to amend your soil, first you must understand its basic composition.  Soil is composed of weathered rock, organic matter, water, and air. But the hidden “magic”, as previously discussed, in a healthy soil are the organisms that flourish when the other soil elements are in balance.

Minerals

Roughly half of the soil in your garden consists of small bits of weathered rock that has gradually been broken down over time. Soil type is generally classified by the size of these inorganic soil particles: sand (large particles), silt (medium-sized particles) or clay (very small particles). The proportion of sand, silt and clay particles determines the texture of your soil and affects drainage and nutrient availability, which in turn influence how well your plants will grow. (Source.)

Organic Matter

Organic matter is the partially decomposed remains of soil organisms, plants, and animals. Although it only makes up a small fraction of the soil, organic matter is absolutely essential. It binds together soil particles into porous crumbs or granules which allow air and water to move through the soil. Organic matter also retains moisture, and is able to absorb and store nutrients. Most importantly, organic matter is food for microorganisms and other forms of soil life. (Source.)

Soil life

Soil organisms are essential for plant growth and help convert organic matter and soil minerals into the vitamins, hormones, disease-suppressing compounds and nutrients that plants need to grow. Their excretions also help to bind soil particles into the small aggregates that make a soil loose and crumbly. As a gardener, your job is to create the ideal conditions for these soil organisms to do their work. This means providing them with an abundant source of food (the carbohydrates in organic matter), oxygen (present in a well-aerated soil), and water (an adequate but not excessive amount). (Source.)

Air

A healthy soil is about 25 percent air. Insects microbes, earthworms and soil life require this much air to live. The air in soil is also an important source of the atmospheric nitrogen that is utilized by plants. Well-aerated soil has plenty of pore space between the soil particles or crumbs. Fine soil particles (clay or silt) have tiny spaces between them – in some cases too small for air to penetrate. Soil composed of large particles, like sand, has large pore spaces and contains plenty of air. But, too much air can cause organic matter to decompose too quickly. To ensure that there is a balanced supply of air in your soil, add plenty of organic matter, avoid stepping in the growing beds or compacting the soil with heavy equipment and never work the soil when it is very wet. (Source.)

Water

Healthy soil will also contain about 25 percent water. Water, like air, is held in the pore spaces between soil particles. Large pore spaces allow rain and irrigation water to move down to the root zone and into the subsoil. In sandy soils, the spaces between the soil particles are so large that gravity causes water to drain down and out very quickly. That’s why sandy soils dry out so fast. Small pore spaces permit water to migrate back upwards through the process of capillary action. In waterlogged soils, water has completely filled the pore spaces, forcing out all the air. This suffocates soil organisms as well as plant roots. Ideally, your soil should have a combination of large and small pore spaces. Again, organic matter is the key, because it encourages the formation of aggregate, or crumbs, or soil. Organic matter also absorbs water and retains it until it is needed by plant roots. (Source.)

Every soil has a different combination of these five basic components. By balancing them you can dramatically improve your soil’s health and your garden’s productivity. But first, you need to know what kind of soil you have.

Soil Texture and Type

Identifying your soil texture: Soil texture can range from very fine particles to coarse and gravelly. You don’t have to be a scientist to determine the texture of the soil in your garden. To get a rough idea, simply place some soil in the palm of your hand and wet it slightly, then run the mixture between your fingers. If it feels gritty, your soil is sandy; if it feels smooth, like moist talcum powder, your soil is silty; if it feels harsh when dry, sticky or slippery when wet, or rubbery when moist, it is high in clay.

My soil feel gritty – a good indication I have sandy soil.

Identifying your soil type: Soils are generally described according to the predominant type of soil particle present: sand, silt or clay. By conducting a simple soil test, you can easily see what kind of soil you’re dealing with. You may want to repeat this test with several different soil samples from your lawn and garden.

1. Fill a quart jar about one-third full with topsoil and add water until the jar is almost full.

2. Screw on the lid and shake the mixture vigorously, until all the clumps of soil have dissolved.

3. Now set the jar on a windowsill and watch as the larger particles begin to sink to the bottom.

4. In a minute or two the sand portion of the soil will have settled to the bottom of the jar. Mark the level of sand on the side of the jar.

5. Leave the jar undisturbed for several hours. The finer silt particles will gradually settle onto the sand. You will find the layers are slightly different colors, indicating various types of particles.

6. Leave the jar overnight. The next layer above the silt will be clay. Mark the thickness of that layer. On top of the clay will be a thin layer of organic matter. Some of this organic matter may still be floating in the water. In fact, the jar should be murky and full of floating organic sediments. If not, you probably need to add organic matter to improve the soil’s fertility and structure.

My garden soil is a sandy loam (predominantly sand, with small amounts of silt and clay). I took this sample after my Summer harvest, in preparation of planting my Winter/Spring crops. This sample indicates that my soil could use a bit more organic matter.

Now that you have identified your soil type and texture and understand the basic components of healthy soil, it’s on to the fun part – playing with dirt!

Soil Amendment – A basic recipe

I found a great website, Gardner’s Supply Company, that outlines a basic “soil recipe” for amending different soil types. I like to use their recipe as a starting point for amendment.

To improve sandy soil:

• Work in 3 to 4 inches of organic matter such as well-rotted manure or finished compost.
• Mulch around your plants with leaves, wood chips, bark, hay or straw. Mulch retains moisture and cools the soil.
• Add at least 2 inches of organic matter each year.
• Grow cover crops or green manures.

To improve clay soil:

• Work 2 to 3 inches of organic matter into the surface of the soil. Then add at least 1 inch more each year after that.
• Add the organic matter in the fall, if possible.
• Use permanent raised beds to improve drainage and keep foot traffic out of the growing area.
• Minimize tilling and spading.

To improve silty soil:

• Add at least 1 inch of organic matter each year.
• Concentrate on the top few inches of soil to avoid surface crusting.
• Avoid soil compaction by avoiding unnecessary tilling and walking on garden beds.
• Consider constructing raised beds.

In addition to the amendment recipe above, here are a few other “ingredients” you might want to consider based on the needs of your soil. (Source.)

• Alfalfa Meal – Primarily a plant source of nitrogen, Alfalfa Pellets (5-1-2) also contains trace minerals and triacontanol, a plant growth promoter.
• Blood Meal – A strong, slow release source of nitrogen, Blood Meal (13-1-0) is also chock-full of trace minerals.
• Bone Meal – Granulated for easy application and quick uptake by plants, Bone Meal contains 20% phosphate and up to 23% calcium.
• Chicken Manure – Composted Chicken Manure (3-2-2) provides a well balanced supply of nutrients and is excellent for mulching and moisture retention.
• Coconut Coir – Made from compressed coconut fiber, coir is an eco-friendly peat alternative! Works anywhere you would normally use peat moss, rockwool, vermiculite, perlite or pumice.
• Gypsum – Used to loosen heavy clay soils, Agricultural Gypsum contains about 23% available calcium and 18% sulfur.
• Kelp Meal – A great source of micronutrients and beneficial plant growth promoters. Kelp Meal also encourages tolerance to stresses such as pests, disease, frost and drought.
• Dolomite Lime – Sweetens soil (raises pH in acidic soils) and is a quality source of calcium (Ca) and magnesium (Mg). Promotes healthy plant growth.
• Shellfish Meal – A source of calcium (23%), nitrogen, phosphorus and trace minerals. Contains chitin, which stimulates the growth of soil microbes that inhibit root-knot nematodes.
• Sulphur – Lowers pH in alkaline soils. Elemental Sulfur contains 90% sulfur with 10% bentonite as a binder. Great around acid loving plants such as blueberry, azalea and rhododendron.
• Worm Castings – Gardeners know Worm Castings to be the most nutrient dense organic compost available. In soil, they retain water and release nutrients in a form that is easily used by plants.

Soil pH

The pH level of your soil indicates its relative acidity or alkalinity. To improve the fertility of your soil, you need to get the pH of your soil within the 6.5 to 6.8 range. You can’t change the pH of your soil overnight. Believe me…I’ve tried. 😦 Instead, gradually alter it over one or two growing seasons and then maintain it every year thereafter. Liberal applications of organic matter is a good idea too, because it helps to moderate pH imbalances. (Source.)

Acidic Soil. If the pH of your soil is less than 6.5, it may be too acidic for most garden plants (although some, such as blueberries and azaleas require acidic soil). Soils in the eastern half of the U.S. are usually on the acidic side.

The most common way to raise the pH of your soil (make it less acidic) is to add powdered limestone. Dolomitic limestone will also add manganese to the soil. Apply it in the fall because it takes several months to alter the pH.

Wood ash will also raise the pH, and it works more quickly than limestone and contains potassium and trace elements. But if you add too much wood ash, you can drastically alter the pH and cause nutrient imbalances. For best results, apply wood ash in the winter, and apply no more than 2 pounds per 100 square feet, every two to three years. (Source.)

To raise the pH of your soil by about one point:

• In sandy soil: add 3 to 4 pounds of ground limestone per 100 square feet.
• In loam (good garden soil): add 7 to 8 pounds per 100 square feet.
• In heavy clay: add 8 to 10 pounds per 100 square feet.

Alkaline Soil. If your soil is higher than 6.8, you will need to acidify your soil. Soils in the western U.S., especially in arid regions, are typically alkaline. Soil is usually acidified by adding ground sulfur. You can also incorporate naturally acidic organic materials such as conifer needles, sawdust, peat moss and oak leaves. (Source.)

To lower soil pH by about one point:

• In sandy soil: add 1 pound ground sulphur per 100 square feet.
• In loam (good garden soil): add 1.5 to 2 pounds per 100 square feet.
• In heavy clay: add 2 pounds per 100 square feet.

Soil Testing

A professional soil test will provide you with a wealth of information about your soil, including the pH and amount of different nutrients. Your local Cooperative Extension Service office may offer a professional soil testing service. The advantage is low cost and results that are specifically geared to your location. If this service is not available, you can also have your soil tested by an independent soil lab. Soil test results usually rate the levels of soil pH, phosphorus, potassium, magnesium, calcium, and sometimes nitrogen.

To get the most accurate test results, take a soil sample from each garden area: lawn, flower garden, and vegetable garden. Spring and fall are the best times to perform a soil test. The soil is more stable, and these are good times to incorporate any recommended fertilizers. Many labs will give recommendations for specific organic amendments upon request. If not, you will have to compare labels to find organic substitutes for the chemical fertilizers that may be suggested.

 To Till or Not To Till

Traditionally gardeners till their gardens in fall to expose overwintering insects, bury plant parts, and mix in soil amendments. It may be healthier for the soil, however, to be left untilled. (Source.) Tilling injects air into the soil, which speeds the breakdown of beneficial organic matter. It also disrupts the air and water channels in the soil, leaving it susceptible to erosion. Tilling also can kill earthworms. So, what to do? If you have a small or raised-bed garden, consider digging by hand. Remove weeds, old plants, and debris from the beds. Add compost and soil amendments, and lightly turning the soil with an iron fork to mix the amendments. If you have a large garden, like me, tilling may be the best option. Just be sure to add organic matter before tilling, and consider mulching the whole garden to prevent soil erosion afterwards.

This year I tilled my garden soil by hand.

Amendment Example – My Garden

To end this post, I will share how I recently amended my sandy loam garden soil. In an attempt to kill weed seeds and disease, I spent 6 weeks after my summer harvest solarizing my soil. After this process, my soil was left dry, hard, and nutrient deficient – definitely in need of some serious amendments to get it back in shape. To figure out the general state of health my soil was in, I did the following.

First, I began by testing my soil’s pH. To do so I broke up my 12′ x 12′ garden plot into four 6′ x 6′ sections. I tested each section using an at home pH test kit. My soil’s acidity tested pretty good, roughly between 6.0-7.0 on the pH scale. Therefore, I didn’t need to make any modifications regarding acidity.

Second, I tested my soil using the mason jar method described above in Identify Your Soil Type. I noted that there was very little organic matter floating in the jar, which meant I needed to add a lot more to boost my nutrients.

Third, I picked up the soil and felt it in my hands. Simple, yet effective. My soil was hard and clumpy, indicating a need for more organic matter to improve structure.

After this basic diagnosis, I decided to amend my soil using the following:

Worm Castings. I added worm castings (worm poop) because they add nutrients and help suppress soil diseases.

Composted Cow Manure. I added this component to increase the organic matter and to improve the soil structure.

Compost. Homemade compost is the best ingredient you can add to any soil. A little goes a long way to improve soil structure, moisture retention, and nutrition.

After a thorough tilling to mix these ingredients, my soil is back in business and ready for my Winter/Spring garden!

Hope you find this tutorial not only useful but that it inspires you to look at soil in a whole new way. As always, happy gardening!