Healthy soil is the basis of a productive garden. Increasing the amount and improving the quality of the air, water, and organic material in the soil leads to increased water infiltration and retention, nutrient cycling, and soil biological diversity in the soil, resulting in healthier, more nutritious crops and higher yields.
With a sound, long-term approach, healthy soil can be achieved regardless of initial soil quality. It is important to understand soil basics:
Soil is a living substance that must be nourished, managed, and protected. Soil organisms help break down crop residues and plant matter into soil organic matter (SOM).
SOM improves the structure of the soil, increases microbial activity, increases water infiltration, enables water retention, and increases nutrient availability for plants.
Soil fertility management is achieved through conserving soil, water, and SOM; increasing SOM content; and supplementing nutrients through soil amendments.
Deep soil quality is necessary for plant roots to maximize nutrient and water uptake and to maximize production. Double digging can improve deep soil quality when combined with compost and other amendments (see page 18).
The ‘ideal’ soil is 25% air, 25% water, 45% mineral particles, and 5% organic matter. Analyzing the existing soil helps unlock its potential to grow more crops. The critical elements of healthy soil are its physical properties, nutrient availability, biological activity, and deep soil quality.
Soil physical properties
Soil physical properties are broken down into soil texture and structure.
Soil texture
Soil texture refers to the size of the soil particles. The soil textural classes, from the largest to smallest soil particle size, are sand, silt, and clay. Knowing the textural class of the soil gives clues to the farmer about its current nutrient and water-holding capacity, and opportunities for improving the soil.
Sandy soils (soils with a higher percentage of sand than clay and silt) drain well but do not hold nutrients for long periods of time. As water moves through sandy soils, plant nutrients are washed away. Sandy soils are very susceptible to wind erosion but are less vulnerable to water erosion.
Silt soils (soils with a higher percentage of silt than sand and clay) are relatively fertile compared to clay and sandy soils, but are most at risk to water erosion.
Clay soils (soils with a higher percentage of clay than sand and silt) hold onto plant nutrients, but are easily waterlogged and compacted.
Loam soils are made up of a percentage of the three types of soil textures. The different percentages of sand, silt, and clay in the loam soil varies. They drain well, but also hold water better than sandy soils. They have high soil fertility and are best suited for agriculture.
Organic matter, incorporated into the soil in the form of compost, crop residue, manure, and biochar, can be used to improve soil quality if it is too sandy or clayey.
Soil structure
Water- and air-holding ability
How gardeners manage the soil, however, can create (or destroy) soil structure and the resulting soil quality and soil health. Soil structure refers to the way soil particles are put together. Some of this is natural arrangements, or groupings of particles into clumps based on the way the parent rock has degraded into soil, and the rest is influenced by soil and crop management.
There are many benefits to good soil structure, such as good water infiltration and root growth, reduced erosion, enhanced nutrient cycling, and increased biological activity in the soil, all leading to better crop production.
Soil structure can be protected or improved through good soil and crop management practices, such as:
Using permanent planting beds and pathways to avoid walking on and compacting the soil.
Practicing close plant spacing and mulching to create a protective plant canopy to decrease raindrop impact on the soil.
Tilling the soil when it is dry or slightly wet.
Having plants on the beds, which increases the soil biology and the resulting actions that form individual aggregates that improve soil structure.
Adding organic matter (especially from compost), crushed charcoal and/or lime to the soil.
Nutrient availability
Soil acidity
Too much acid in the soil limits plant roots’ ability to absorb beneficial minerals due to the presence of too many hydronium ions. These ions are removed
from the root zone once the acidity is removed, which can be done through the addition of liming agents, such as calcium carbonate. Most minerals are
available to plants in soils that are just slightly acidic (pH 6.0–6.8).7 Soil acidity on either side of this range prevents nutrient retention in soil and limits uptake in the roots. Soil acidity can be changed by adding numerous amendments. For example, wood ash, which contains calcium carbonate, decreases acidity, while coffee grounds increase acidity. Farmers need to connect with local agencies
to test the acidity of the soil in their gardens, but, in the meantime, they can increase a plant’s ability to grow in either acidic or basic soils by adding large amounts of compost, manure, wood ash, and charcoal dust.8
Nutrients needed for healthy soil
While soil can appear inactive, there is actually a whole world of activity that supports agriculture going on every day under the surface. Plant roots search for water and critical nutrients. Soil microorganisms break down dead material
and compete with each other for resources. All this activity means that there is a constant exchange of energy and nutrients between the soil, the air, and living and dead things. These exchanges are called cycles, and by following them step by step, we can see that soil is constantly being formed and reformed by the surrounding environment. By understanding these cycles, practitioners can better understand how to build long-term soil health.
Carbon cycle
Carbon is a basic element of living things, but it does not exist in just one form. It can be found in many things, from sugar to soil particles to gases. The movement of carbon from one form to another is called the carbon cycle, and all stages of life, from birth to death, are involved in this cycle. Without the carbon cycle, life would not be possible.
The carbon cycle plays an important role in agriculture. When a young plant grows, it takes in carbon from the air to make its food. When a dead plant decays, most of its carbon is released back into the air, but some carbon remains in the soil and becomes SOM. Over time, as more and more plants decompose in the soil, the level of SOM increases, along with the levels of soil microorganisms.
- pH (which stands for ‘power of hydrogen’) is a measure of the amount of these acidifying ions found in the soil. Although pH can sometimes extend past 14, the typical range for soil pH is from 5.5 to 7.5.
Acidic soils have a pH less than 7. The lower the number, the more acidic the soil. Basic soils have a pH greater than 7. The higher the number, the more basic the soil.
- More information can be found in Test of your soil with plants 2nd edition by John Beeby.
As organic matter increases, the soil is able to support more plant life, which then returns more carbon to the soil. This cycle is the foundation for fertile soils.
Gardeners can use the carbon cycle to their benefit. Composting, crop residue management, and the addition of other organic matter all encourage the creation and utilization of SOM. These techniques are key to ensuring long-term soil health.
Nitrogen cycle
Nitrogen is another basic element of life. Without nitrogen, plants are not able to convert sunlight into food. The nitrogen cycle follows the movement of nitrogen from one form to another, just like the carbon cycle.
The air is mostly made of nitrogen gas, but plants cannot use nitrogen in this form. Nitrogen gas must be converted to other forms before plants can use it. This important conversion is carried out by certain soil microbes. These microbes live inside the roots of legumes (groundnuts, peas, beans). These microbes live inside the roots of legumes (groundnuts, peas, beans), where they have access to food created by the plants from sunlight.
By including legumes in crop rotations, gardeners can increase the level of nitrogen in the soil. Crops planted where legumes have been previously planted can then use the nitrogen stored in the soil for their own growth.
Many local resources exist around the home or the community that can help grow healthy plants and sustain productive activities. Animal manures, ash, charcoal dust, burned crop residue, and organic plant material are all valuable assets available locally that can be used to improve the long-term quality of the soil. These local soil amendments contain the same benefits as inorganic
fertilizer: nitrogen (to improve leaf growth and help leaf formation), phosphorous (to improve root development and flowering), and potassium (to improve seed formation). Many of these resources can be added directly to the soil when tilling or preparing the permagarden (see page 26). They can also be used to help amend the soil when planting berms or other areas around the homestead with useful trees or shrubs. Other waste materials may need further decomposition, burning, or crushing before adding to the garden.
Applying woodash increases the soil’s pH level.
Photograph: Thomas Cole
Wood ash is a primary source of calcium carbonate, which can increase the soil pH level (i.e., reduce the soil’s acidity). A soil with a low pH (i.e., high soil acidity) blocks the uptake of beneficial minerals by the plant. Wood ash from cooking fires, found almost everywhere, counteracts this pH imbalance. Wood
Making biochar (above).
Gathering soil amendments (right).
Photographs: Thomas Cole
ash is also an important source of potassium and supplies trace amounts of phosphorous and magnesium. Wood ash can be applied directly to the soil and mixed in before planting. The application rate should be no more than 1kg per 10 m2. Wood ash works best when it is in contact with as much soil as possible. Broadcasting ash on the surface without mixing it into the soil does not raise the pH effectively.
Charcoal is an important soil amendment, especially the small chips and dust no longer useful as fuel. The addition of charcoal dust helps increase water retention, creates habitat for microorganisms, and permanently improves the soil’s ability to hold nutrients by increasing its Cation Exchange Capacity (CEC). This becomes even more important in sandy soils, which have a difficult time holding nutrients, and in gardens in subtropical or humid areas where compost or SOM breaks down rapidly in the soil. Charcoal dust helps keep these nutrients from leaching out and makes them more available for crops in a permagarden.
Charcoal dust can be added annually to the garden, 1–2 cm spread over the top of the bed and then mixed into the top 10 cm of the soil. In most communities, charcoal dust is readily available where vendors sell bags of charcoal.
Biochar is charcoal made specifically for soil improvement using whatever abundant and underused crop residue, such as maize or sorghum stalks, rice husks, and sugarcane bagasse, is available. If not made on purpose, it can often be found in large burn piles within crop fields. In areas with high levels of usable crop residue, making biochar is a good way to provide this amendment for both the permagarden and the field crop garden. Instructions to make biochar can be found in Appendix 3.
Coffee grounds are an excellent and, depending on location, abundant source of organic nitrogen. It is already in a stabilized form and is slowly released into the water in the soil for uptake. Coffee grounds can be added to the compost pile, mixed directly into the soil during the double digging process, or added as a soil amendment when planting trees or other plants around the homestead.
Egg and oyster shells are an excellent source of calcium, important for healthy flowers. Both must be dried and, in the case of oyster shells, slowly burned, before being crushed into powder. For the calcium to become available for plant uptake, however, it must first be digested by soil microbes, one more reason to make sure the garden soil has ample organic matter and compost.
Kitchen scraps are a tremendous source of macro- and micronutrients, as well as organic material for the compost pile. These include Irish and sweet potato peelings; the tops of beets and carrots; the stems from kale, chard, and other greens; and the rinds or peels of fruit, such as melons or bananas. In addition to composting, kitchen scraps can be converted into nutrient- and microbe-dense ‘tea’ for irrigation and leaf application to garden crops following the method described in the plant health section. Collecting kitchen scraps in rubbish pits around the compound is an effective way to store these materials and help them decompose before putting into a compost pile.
Manures are good sources of organic material for the garden, and they provide small amounts of nutrients in the form of nitrogen, phosphorus, and potassium. Manure can be sourced from cows, pigs, chickens, goats, rabbits, horses, sheep, ducks and other fowl. It is important to use well-aged (dry) manures in the garden, after they have decomposed or composted for several months, as fresh manure can contain harmful pathogens and may harm plants if applied directly to crops. In addition, manure from dogs should not be used in the garden, because it is more likely to contain pathogens. Fresh manure, as well as a slurry of fresh manure and cow urine, can be used to make a liquid manure tea. This tea can then be applied through leaf feeding or through drenching the roots of the crops.
Applying well-aged (dry) manure to the garden:
Locate a source of well-aged manure (poultry or cow manure may be the most available in the community).
Gather enough to apply 2–5 cm over the area where crops will be planted. Mix into the top 20 cm of the soil.
Repeat before every planting cycle.
Biological activity
Microbes
Although invisible to the naked eye, the fungi and bacteria (collectively known as microbes) in soil play a critical role in the carbon and nitrogen cycles. Without them, dead plants would not decay; they would simply remain on the soil surface for years. With them, dead plant material is incorporated back into the soil as organic matter and nitrogen is cycled into forms that plant roots can absorb.
By conserving soil moisture, rotating crops, and increasing SOM, bio-intensive agriculture provides the conditions that allow soil microbes to thrive. In doing so, a gardener can reap the benefits through healthier crops and higher yields.
Soil organic matter
SOM is a critical component of healthy soil. The most productive agricultural lands in the world are areas with high SOM. SOM consists of plant, animal, and microbial residues at various stages of decomposition. SOM can range in age from a few months to thousands or even millions of years. It can be a maize plant that has partially decayed over the last 6 months or tiny particles that have been formed over centuries. The final product of decomposition, humus, is a dark and crumbly material that has stabilized over time.
Without SOM, soils would be much less hospitable to plant growth. SOM increases a soil’s ability to retain nutrients, improves soil structure, increases air flow, and provides a rich environment for soil microbes active in nutrient cycling and disease suppression. All these factors work together to provide ideal conditions for healthy root growth.
In a general sense, SOM can be seen as a buffer against extreme conditions. It protects the soil against sudden changes in acidity. It improves water drainage of clay soils and water retention of sandy soils. Soils with low SOM do not have these benefits, and crops grown in soils with low SOM more quickly succumb to environmental stresses. Managing SOM in this respect is a building block of a resilient household. Decreasing the amount of tillage, adding organic matter to the soil, and keeping plant cover on the garden are all ways to maintain or increase SOM.
Compost
Compost is a key way to add SOM to the garden. Organic matter, best applied as compost or rotted manure, is the most important ingredient that can be added to improve garden soil. In addition, it combines with other processes to ensure superior levels of air, water, microorganisms, and minerals essential for vigorous root health and corresponding growth and crop yield. Just a single tablespoon of
finished compost contains over 7 billion beneficial microbes that work to ensure long-term soil and plant health. As discussed below, compost plays many roles in the soil. It is also easy to make if enough materials are available. Over the course of just a few months, locally gathered waste materials can be converted into a soft, nutrient-dense asset (Appendix 2).
Decomposition is a natural process of nutrient cycling. When dead plants or animal manures are left in the field, their nutrients eventually return to the soil as they decompose. Composting is just a managed process of decomposition that maximizes the benefits for the gardener. With the right materials and proper maintenance, a compost pile produces nutrient-rich material that can help improve soil over the long term.
Effective compost requires a good mixture of plant materials.
Photograph: Peter Jensen
Composting relies on soil microbes to break down material. Therefore, it is important to provide the right materials in the right amounts for the microbes to work efficiently. The basic ingredients for good compost are brown, carbon-rich materials; green, nitrogen-rich materials; air and water. Composting works best when the pile has one-third ‘green material’ and two-thirds ‘brown material’.
Up to 10% of the composting material can be soil as well. As a general rule of thumb, green material is moist, flexible, and high in nitrogen, while brown material is dry, brittle, and high in carbon. A variety of greens and browns can be used. The green material can include vegetable scraps, fresh crop residues,
manure, and weeds that have not gone to seed. The brown material can include corn cobs, straw, and dry leaves. Too much of either green or brown material slows the decomposition process and lengthens the time until the compost is ready. Supplying sufficient water for the microbes to live is important. Too much
water and they will drown with little access to air. Too little water and they cannot function as well. A simple method to gauge the right amount is to hold the early compost materials in your hand and squeeze it hard. If water runs out, it is too wet. If your hand does not glisten when you open it up, it is too dry.
A well-made compost pile heats to 120–140° Fahrenheit (49–60° Celsius) after just 2 days. One 1m3 pile, after 2 or 3 months (turned over once every 1–2 weeks, adding water as needed), provides ten 20 L buckets of finished compost for use in the garden. This is enough for three 5 m garden beds. During garden bed renovation, following the removal of the previous crop, one 20 L bucket of finished compost should be added per two square meters.
When blended and managed over the course of 2–3 months, a compost pile creates a material that improves the health of the soil and the plants that grow in it.
Compost provides many benefits to the garden:
Improved soil structure Compost is an important soil conditioner. It breaks up heavy clay soils and binds together sandy soils. This improved structure allows a sandy soil to hold water and a clay soil to drain water, promoting proper root growth and health.
Soil moisture retention The organic matter in compost allows it to hold six times its weight in water. A soil with good organic matter content soaks up rain like a sponge and regulates the supply of water to the plants. A soil stripped of its organic matter resists water penetration, leading to crusting, erosion, and flooding.
Aeration A well-aerated soil assists in the diffusion of air and moisture into the soil and in the exchange of nutrients. Carbon dioxide released by organic matter decomposition diffuses within and above the soil.
Fertilization Compost contains many elements that are essential to plant growth, including nitrogen, phosphorous, potassium, magnesium, and sulfur, and is especially important as a source of trace elements, such as molybdenum, zinc, and iodide. In addition, compost increases a soil’s CEC, which increases its ability to hold nutrients.
A well-made compost heap provides nutrients to enrich garden soil
Nutrient release Related to fertilization, organic acids released by decomposing organic matter dissolve soil minerals, making them available to plants as food. As organic matter breaks down, it slowly releases key nutrients for plant uptake and a healthy soil microbe population. Nitrogen, one of the most important plant nutrients, is also the most easily lost to leaching and gasification.
Soil acidity and toxin buffer Plants have specific tolerances in terms of toxins and soil acidity. Organic matter, in particular the carbon molecule, draws these toxins and acidity out of the soil water, allowing plants to have a broader range of tolerances to these elements, which are common in the world’s poorest soils.
Germination and early seedling growth Steady moisture levels are required for a seed’s coat to crack and germinate. Compost in the soil acts as a sponge, absorbing water and keeping the seed moist. This increases the speed of germination and the likelihood of the fully developed young plants — called seedlings — to survive periods of dry weather that would otherwise destroy the tender stems, roots, and leaves.
How to make compost
Materials needed:
Brown/dry leaves or grasses – about six large sacks.
Green grass, leaves, or weeds from garden area (no seeds!) – about two large sacks.
One 20 L bucket of manure or good topsoil (source of bacteria). Three or four 20 L buckets of water.
A 1 m stick to use as an aerator and thermometer.
A machete or hoe to chop the material into small pieces.
Materials that should NOT be added to a compost pile:
Plants known to be diseased or under severe insect attack.
Plants that are toxic to other plants and microbial life, such as hemlock, acacia, juniper, bamboo, gmelina, onion, citrus, castor bean, and eucalyptus. Plants or plant materials that may be too acidic, such as pine needles.
Perennial, invasive weeds and their root systems, such as wild morning glory, and Kuch, Bermuda, Striga, or Kikuyu grass.
Soap, oil, meat, or the manure from meat-eating animals, like cats and dogs, which may contain pathogens.
Human excrement.
Compost instructions
- Select a place in the shade. Too much sun dries out the compost pile and slows down the decomposition process.
- Gather brown and green materials. Large leaves should be chopped into small pieces to speed the decomposition process and release moisture and minerals. A properly made compost pile contains one-third green materials and two-thirds brown materials.
This is only one way to make compost. Another method is to have a high C:N ratio. This method can be learned from the additional resources listed at the end of the manual.
- Put down an initial 5-15 cm layer of coarse sticks. This helps aerate the pile from below, enabling air movement through the pile during decomposition.
- Begin to layer and mix the brown and green materials. Start with a 20 cm layer of brown.
- Add a 10 cm layer of green.
- Add 2 cm of topsoil, manure, or finished compost (approximately 4 large handfuls).
- Blend all layers together while water is added to moisten well.
- Repeat Steps 4 through 7 until the pile is 1 m wide by 1 m deep by 1 m high.
- Cover with 2.5 cm of topsoil and a sheet of plastic to help hold the moisture in the pile. If plastic is not available, then cover with dry grass.
- After 2 days, the pile becomes very hot. This means that the bacteria are working to break down the materials. DO NOT MIX. Measure the temperature only if desired.
- WAIT 1 WEEK. Gently mix while adding more water to keep moist. Cover well.
- WAIT 1 WEEK before mixing and applying water again. Cover well.
- Allow pile to rest for at least 2 weeks before mixing again. Cover well.
- Continue to mix every second week, watering and covering until the inside of the pile is brown, crumbly, and cool to the touch. At this point, the compost is ready to be used in the garden.
Deep soil quality
Deep soil quality (healthy soil in both the topsoil and subsoil) is important for good plant root growth. The average garden is tilled to the depth of the
equipment normally used to work the land. At best, this means soils are aerated to about 20 cm, the length of the average hoe blade. Over time, with successive
tillage to the same level, a nearly impermeable subsoil—a ‘hard pan’—is created that blocks the movement of air and water through the soil profile, which in turn stunts the growth of the roots of plants. If plant roots are not able go deep into the soil (which they can do only if there is a good air-water dynamic in place), then they must be planted farther apart so as not to compete with neighboring plants for air, water, and nutrients. When planted farther apart, sunlight easily reaches the soil surface, causing weed germination, moisture loss, and overall weaker, underproducing plants. All these losses are avoided by having deep soil quality.
When the starting soil is unhealthy, there are some techniques the farmer can use to speed up the process of having deep soil quality, such as double digging. By preparing the soil deeply, i.e., breaking through and amending the compacted subsoil layer, ample air, water, and carbon allow healthy plant roots to go much deeper into the soil.
When starting your permagarden for the first time, or working with unhealthy soils, the permagarden method recommends practicing double digging.10
Double digging requires a lot of effort, but ultimately it creates a garden bed that is easier to manage.11 Since double digging requires so much work, it is important for a gardener to work during the cool part of the day or to dig after a rain when the ground is soft. A gardener can also make the process easier by teaming up with neighbors and family members.
Often when a garden bed is prepared using double digging, the bed will increase in height due to increased aeration in the soil. However, in arid areas, the finished beds should not be raised; rather, they should be kept at soil level, or even slightly sunken, to help conserve moisture. The beds still need to be double- dug, but the finished height needs to be lower to avoid the soil drying out when hot winds blow.
Double digging allows for closer plant spacing, as the roots grow down rather than to the sides. Double-dug beds are permanent, which allows them to absorb and retain water more effectively. They can be amended with important nutrients and SOM that is sourced locally. Crops can then be rotated between beds or from one place in a bed to another place in the same bed from season to season to maximize pest control and achieve higher yields.
Double digging.
- It is important to note, however, that once the farmer has obtained deep soil quality, benefits from double digging decrease, and increased oxidation and disruption can cause losses in SOM.
- A double-dug garden bed does not need to be dug again once deep soil quality is obtained.
How to double-dig a permagarden bed (Appendix 4)
Begin by marking the edge of the permagarden beds using sticks and string or a hoe in the dirt. Make sure that all the beds, pathways, swales, berms, and holes are measured and marked before beginning to double dig.
- Once a bed is marked, measure 40 cm segments along the bed lengthwise. Put small stakes at the 40 cm marks or simply mark with a hoe.
- Remove 20–30 cm of topsoil from first 40 cm section (A1 in the diagram below), digging down until the hard pan is reached. The removed topsoil should be kept at the end of the bed.
- When possible, have a partner dig the next 20–30 cm of subsoil from the same section, loosening and digging but not removing the soil. Keep loosening the whole section until most of the larger pieces have been broken up.
- Add compost, manure, wood ash, charcoal dust, or any other soil amendments to the loosened subsoil. One shovel or several handfuls of each amendment is enough.
- Using a shovel or a hoe, mix these amendments into the subsoil.
- When possible, have a partner dig 20–30 cm of topsoil in the next 40 cm section. As it is dug up and loosened, place this topsoil on top of the subsoil section that was just amended. Make sure that all the topsoil gets removed.
- When possible, have a new person loosen the subsoil as in Step 3. Repeat Step 3.
- Amend the soil, following Steps 4–5.
- Repeat Steps 3–6 until bed is complete. The saved topsoil from the start of the bed should be used to build up the bed in the last 40 cm section.
- Once the double digging process has been completed, add more manure, compost, and soil amendments to the finished bed. Add one shovel full or several handfuls every 50 cm.
- Smooth out the top with a rake or hoe or by hand, creating a flat planting space. The garden is now ready for planting or seeding.
