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what we must know about garden soil

Knowing about soil

Soil is important to plants–it supplies them with water and nutrients, and it anchors their roots. Here’s a primer to help you understand one of your garden’s most basic ingredients.


Sandy soils are made of relatively large rock particles that fit loosely together. These soils tend to warm faster in the spring and drain quickly during wet periods. Unfortunately, they don’t hold water well during drought and lose nutrients more quickly than other soil types. Sandy soils feel gritty to the touch.


Silty soils are made from medium-sized particles. They shed excess water more quickly than clay, but not as quickly as sand. Silty soils tend to feel slick to the touch when they’re wet.


Soils with a high clay content are made of small particles that fit tightly together. Clay soils hold water and nutrients during times of drought, but stay wet longer during wet periods. They’re more susceptible to winter heaving (moving around) during periods of freezing and thawing, which exposes and harms roots of perennial plants.

Knowing about soil
Knowing about soil

Organic matter

Organic matter helps eliminate the disadvantages of both sandy and clay soils. Organic materials such as compost, decomposed manure, and shredded leaves hold moisture when the soil is dry, but still let soils shed excess water. They reduce soil compaction, allowing plant roots to spread more easily.


Loamy soils are those rich with organic matter. In addition to regulating water better than both sandy and clay soils, loamy soils encourage beneficial microorganisms such as mycorrhizal fungi, which reportedly help plants absorb nutrients and resist disease.


This is a layer of soil so compacted that plant roots can’t grow through it. In extreme cases, water won’t permeate through the layer. Hardpan can occur when sand is mixed with clay-unless there’s a very high percentage of sand, the small clay particles will cling to the larger sand particles and eliminate the spaces between the particles where water moves through. It can also occur by compaction, especially with heavy equipment.

Acidic soil

An acidic soil has a low pH (lower than 7.0 on a scale of 0 to 14). The pH of a soil determines, in part, what nutrients plants can take from the soil. For instance, plants absorb iron readily in acidic soils, but have trouble absorbing molybdenum. Some plants, such as blueberries and rhododendrons, have adapted to acidic conditions and require acidic soil to grow well. Sulfur and aluminium sulfate tend to increase a soil’s acidity.

Alkaline soil

An alkaline soil has a high pH (higher than 7.0 on a scale of 0 to 14). The alkalinity of a soil also determines what nutrients plants can absorb. For example, plants absorb potassium more readily in alkaline soils, but have trouble absorbing manganese. Some plants, such as some dianthus, thrive in alkaline soils, but most garden plants prefer a neutral or slightly acidic soil. Lime and wood ashes are two materials that increase a soil’s alkalinity. Mulch

While not actually a soil component, mulch (such as compost, shredded bark, and shredded leaves) relates closely with the soil. Mulch helps prevent soil erosion, holds moisture, and reduces drastic temperature changes in summer and winter. An organic mulch will break down, adding organic matter to the soil.

Blue oat grass works well in Autumn

Blue oat grass

Blue and gold aren’t just colours on a Notre Dame football jersey. They describe the fall garment of blue oat grass, a sedate beauty with thin, narrow, blue-grey leaves that grow in a rounded tuft. Imagine a mound of blue fescue, but twice as big, with tall, nodding spikelets that bloom in early summer. Blue oat grass is a well-behaved clump-forming grass, so it won’t take over your flowerbed, and it tolerates heat and drought once established. Gold comes into play in autumn, when the flower spikes mature to the colour of wheat.

Common name: Blue oat grass

Botanical name: Helictotrichon sempervirens

Plant type: Perennial grass

Zones: 4 to 9

Height: 3 to 4 feet (in flower)

Family: Poaceae

Growing conditions

Sun: Full sun 

• Soil: Average, well-drained

• Moisture: Average to dry


• Mulch: None needed.

• Pruning: Cut back last year’s foliage in early spring.

• Fertiliser: None needed.


• By seed or division

Pests and diseases

• Vulnerable to rust

Garden notes

• Blue oat grass is a clump-forming grass that generally stays put. Under ideal conditions, it may spread by seed, but it doesn’t spread by rhizome and it’s not considered aggressive. 

• Use blue oat grass in rock gardens or borders, as a ground cover or a specimen plant.

• The bluish foliage can be used as a background for spring bulbs, or as a companion for plants with blue, silver, or purple. It’s also good in the fall garden, where the mature flower plumes add a touch of gold.

All in the family

• Blue fescue (Festuca glauca) is a smaller lookalike of blue oat grass. This member of the grass family does well under similar conditions—dryish soil in full sun—but stays less than half the size of blue oat grass.

• Poaceae, the grass family, also contains bamboo, rice, corn, wheat, and millet—some of the world’s most important food crops.     

Purple prairie clover can really improve a garden

Did drought make crispy chips of your perennials last year? This summer, fight fire with fire. Purple prairie clover (Dalea purpurea), a North American perennial that shoots out fiery purple-pink bloom spikes from early June to September, is adapted to dry sites open to the blazing sun. Its deep taproot—often 5 or 6 feet long—allows it to thrive in conditions that would fry your average garden flower. Butterflies and bees like the flowers, and neighbouring plants benefit from the nitrogen that this legume fixes in the soil. It’s found in the wild from Canada to Texas.

Common name: Purple prairie clover

Botanical name: Dalea purpurea

Plant type: Perennial

Zones: 3 to 8

Height: 1 to 3 feet

Family: Fabaceae

Growing conditions

• Sun: Full sun

• Soil: Prefers average, well-drained soil, but tolerates both sand and clay.

• Moisture: Average to dry


Mulch: Mulch to preserve moisture in the soil.

• Pruning: None needed.

• Fertiliser: None needed.


• By seed

Purple prairie clover can really improve a garden
Purple prairie clover can really improve a garden

Pests and diseases

• Vulnerable to rust.

Garden notes

• Under the right conditions, purple prairie clover will reseed, but it’s not a bully.

• Purple prairie clover is small, so plant it where it won’t be overwhelmed by tall perennials. Good companions include butterfly weed, dianthus, mountain mint, and little bluestem.    

• Use it in rock gardens, prairie gardens, or perennial beds.

• Leave seed heads on through the winter.

All in the family

• The Fabaceae, or legume family, is one of the largest families of flowering plants. There are more than 19,000 species in the family. Some legumes that might be in your garden already are beans, peas, lupines, and sweet peas. Some that might be in your kitchen are lentils, edamame (soybeans), and peanuts. Some that you probably hope aren’t in your garden are clover and kudzu.