Soil Amendment 101

MARCH 2020 - Soil Amendment 101

When was the last time your soil had a check-up? Do you have plants that are underperforming despite planting them as advised? Are you overwhelmed by all of the choices and advice? It’s likely that your soil (and plants!) will benefit from some form of amendment and here are some soil basics, to get you pointed in the right direction!

Soil provides physical support, nutrients, water and air space (for gas exchange) necessary for plant growth. The quality of your soil greatly affects the growth and health of your plants. Contrary to popular belief, roots do not “sense” nutrients or seek them out per se, but they will increase growth and branching when in contact with nutrients.

Soil nutrients are constantly changing. Different types of organic matter and nutrients are available in many forms, and some take longer than others to be converted or released for plant uptake. I liken it to making dinner happen; you and the plants are hungry. Some meals are fast and ready within minutes. Others require fresh ingredients that still need to be purchased or harvested, cleaned, prepped and prepared. Ultimately, your goal is a balanced diet.

  • Synthetic fertilizers provide a simple and direct way of adding any of the essential mineral nutrients.
  • Additions of organic matter improve soil structure as well as providing nutrients, but it must be broken down by soil organisms first.

Synthetic (manufactured) fertilizers have a guaranteed nutritional analysis of nutrients, and are usually sold in forms that are easily taken up by  most plants (like fast food, it has a small place in your life). Generally speaking, well decomposed organic matter is preferable. It helps bind soil minerals into aggregates and improve your soils structure. The nutritional analysis will vary, depending on the source material.

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Common Amendment Materials

Compost is the quintessential soil amendment, probably because nature is the most efficient and often elegant recycler, returning nutrients directly back to the earth and its inhabitants. If the compost is not adequately broken down, it won’t have as many nutrients readily available for plants to use. Materials that are more thoroughly decomposed will contribute more to the cation exchange than fresh organic matter or coarse material (more on CEC in the next section). Another bonus is that quite often, organic soil amendments are recycled materials that have been diverted from landfills.

Soil microorganisms are responsible for a great deal of the “work'' done in soil. They decompose organic matter, and use the carbon, nitrogen and other nutrients in the organic matter for their growth. They get what they need and release any extra for the plants. Soil microorganisms are like that great roommate who always orders way too much food and then shares with their friends. Beneficial organisms such as earthworms and nematodes can be found at your local garden centre or online.

Sheep manure is high in nitrogen (N) and Potassium (K)

Chicken manure is high in nitrogen (N) and phosphorus (P)
*disclaimer: be careful when using animal manures and compost. If too much nitrogen is released at once, plants will suffer (too much lush growth too quickly, welcomes a bug buffet on a stressed out plant). Mix with soil to dilute the potency before applying. The rate at which nutrients are released can vary depending on soil temperature, moisture and soil type.

Bone meal is high in phosphorus (P)

Cover crops tilled into the soil during their flowering stage are known as green manure. They serve as protection or mulch during their growing stage.  Annual legumes (peas, lentils, or fava beans) and perennial legumes (clovers and alfalfa) barley, rye and buckwheat are common. Once tilled, the fresh, green plant material quickly decomposes, improving soil organic matter. When growing legumes, you should always inoculate the seeds with the appropriate Rhizobia bacteria prior to planting to assist with nitrogen fixation.

Mulches should not be mixed into the soil but placed on top as a protective barrier. Coarse mulch materials like wood chips, bark or straw contribute to gradually improving your soils structure in addition to keeping roots cool, preventing evaporation and erosion and impeding weed growth. These materials often take many seasons to break down. Sawdust and coniferous needles for example, take longer to break down than animal manure and may temporarily immobilize nitrogen in your soil.

Sand is often added to clay soil to improve water infiltration, in potting mixes and garden beds. Be careful to use a light hand in applying, and mix in well with native soil as too much sand will create drought conditions with poor nutrition.

Lime is used to slowly increase soil pH are accomplished through liming (a single application of lime will remain active for several years.

Acidic materials like peat moss act to lower soil pH and are practical and effective in garden beds and potting mixes.

How do I choose which one?

That depends on your project; a porch planter will have different requirements than a heavily treed backyard. Knowing a little bit about the soil type and texture that you’re working with is ideal.

Soils can be divided into two major types: mineral soils, and organic soils.

The mineral portion of a soil is composed of sand, silt, and clay particles formed over long periods of time from broken and eroded rocks.

  • A typical loamy soil has approx. 20% clay, 35% silt and 45% sand.
  • Only sand particles are large enough to be visible to the human eye. For example:  if a clay particle were the size of the head of a pin, a typical sand particle would be the size of a basketball.
  • Clay particles have the greatest influence on soil properties.

Organic soil matter consists of decomposed remains of plants, animals, and microorganisms. It is constantly changing and cycling.

  • Organic matter functions as a pantry of nutrients for plants to use over the course of time.
  • Organic matter also improves soil structure, by acting as a glue to hold soil mineral particles together into aggregates. A soil with good aggregate structure improves root growth, seedling emergence, water infiltration, and aeration.
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What type of soil do I have?

The relative proportion of sand, silt, and clay in a soil determines the soil texture. Soils are grouped into textural classes according to the particle type which most influences the soil properties. In general, the feel of

  • moist sand is gritty
  • moist silt is slippery
  • moist clay is sticky

A simple test can help you determine your soils texture!

Does it really matter?

It really does. Soil texture influences your soils quality, which directly affects plant growth and health.

Clay (aka fine or heavy) textured soil has a higher water holding capacity, so is less susceptible to drought but susceptible to pooling and subsequent plant drowning. Wet soils require more energy to warm and are slower to warm up in spring. They are more prone to compaction as well as damage from heavy equipment and foot traffic. Clay soils can also be physically difficult for roots to grow through. Clay soil can sound like a bit of a drag, until you find out that they’re teeming with nutritious possibility.

With sandy (aka coarse or light) textured soils, water drains more readily but almost excessively so - they are often nutrient deficient and require more frequent fertilizer. They do not bind together and are more prone to erosion.

Most clay particles and organic matter are colloids (soil particles with a negatively charged surface) that play a vital role in plant growth. Tiny in size with a correspondingly large surface area, they are like tiny superheroes, doing what other soil particles can’t do; attract positively charged nutrients (cations) and repel negatively charged nutrients (anions). If you want to dive deep into CEC (Cation Exchange Capacity), get your geek on here (Youtube video) 

If you’d rather take our word for it, you can expect increased C.E.C. in your soil under the following conditions:

  • high clay content
  • high soil organic matter content
  • additions of well decomposed organic matter (humus)
  • high soil pH

In practical terms, acidic soils have a pH of less than 5.5, pH between 6.5 and 7.5 as neutral and strongly alkaline soils have a pH of over 8.5. Soil pH also affects the form and availability of plant nutrients and nitrogen fixation begins to decrease. The ability of a soil to resist changes in pH is referred to as its buffering capacity. You can find an inexpensive PH testing kit at many garden centres and retailers or online.

Now that we have explored the tip of the iceberg in terms of soil health, you will have a good foundation of understanding to assist in your gardening endeavours. At the end of the day, it's often easiest to work with nature rather than against it. Selecting site appropriate species is the first step in ensuring success. Regularly assessing and amending your soil will help ensure robust growth and performance in your plantings for years to come.

Happy Gardening!

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