materials

Amendments vs. mulch

Both amendments and mulches can be either organic, such as ground bark, or inorganic, such as gravel. Although amendments and mulches are often similar materials, amendments have a finer texture and are worked into the soil to improve tilth in the plant root zone, while mulches are placed on top of the soil to conserve moisture, moderate soil temperature, reduce weed seed germination, protect soil structure, and reduce erosion. After organic mulch decomposes, it can be worked into the soil as an amendment.

Choosing amendments

Adapted from CMG GardenNotes 241 Soil Amendments and CO CES Fact Sheet Choosing a Soil Amendment.

To improve aeration, infiltration, and structure in clay soils, consider both permeability and water retention. Ideally, use a mix of materials

• something with high permeability like pine bark to improve infiltration and drainage
• compost or manure to aid structure and aeration and to encourage beneficial soil life.

Frequency of application

Will the product be used repeatedly to improve the soil over time, as in a vegetable garden, or as a single application before planting lawns, perennials or shrubs? If the latter, consider adding an inorganic amendment as well.

Longevity

Products that decompose rapidly, like grass clippings and manure, give quick results, while products that decompose slowly, like fine pine bark, give longer-lasting results. Use a combination of materials to get both rapid and persistent benefits.

Salts

Products containing manure and/or biosolids are often high in salts, while plant-based products are low in salts. Application rates and frequency will depend on the salt content of the material and the depth to which it will be tilled.

Need for supplemental fertilization

Soil amendments can tie up nitrogen until they decompose. Supplemental nitrogen should be added to reflect the decomposition rate.

Amendment decomposition rate

Recommended amendments

Adapted from Managing Soil Tilth: Texture, Structure, and Pore Space

Organic

Clay soil particles pack too tightly to form the large pore channels where oxygen and roots are found. When organic matter is added, the particles aggregate into larger clumps that pack loosely, improving drainage and air and root penetration. Organic matter also fosters beneficial soil life (microorganisms and earthworms) that further improve structure. Note that structural improvement cannot be achieved in a single session or season because there are limits to how much organic matter can be added (50% by volume) without creating a drainage barrier.

Compost

Well-rotted compost is the best single organic amendment for clay, but mixtures of organic amendments are also a good choice. Although compost also enriches the soil, it releases nutrients slowly and often does not contain enough nutrients to supply all the needs of growing plants. The structural benefits of compost are numerous:

• creates a better environment for root growth
• increases the ability of the soil to hold and release essential nutrients
• promotes the activity of earthworms and soil microorganisms beneficial to plant growth
• improves seed emergence and water infiltration by reducing soil crusting

Pine bark

Finely-ground pine bark, sold as “pine bark soil conditioner”, is another excellent amendment and is a native and renewable resource. A pea-size grind (¼–½″) is best. Pine bark soil conditioner is available in bags or by the truckload. Straight pine bark is a better value than pine bark mixed with other ingredients. Other soil conditioner products are often too fine or coarse to provide the needed pore space.

Manure

Composted manure has no odor. It is excellent for improving soil structure and supporting beneficial organisms like earthworms and also supplies some minor nutrients. Manure is only a modest fertilizer, with an NPK of about 1-1-1. Make sure that the manure has been tested to be low in salt.

Recently, herbicide carryover in compost and composted manure has become a problem. An Extension factsheet discusses herbicides of concern, sensitive plants, and sources to avoid. The US Composting Council has a certification program that tests compost for many features including pH, soluble salts, nutrient value, fecal pathogens, and heavy metals.

Inorganic

Inorganic amendments like gravel complement the functions of organic amendments. They are permanent additions used to improve drainage, make soil more resistant to compaction and, to varying extents, moderate temperature and help conserve moisture. Inorganic amendments should be incorporated into the soil just like organic amendments, ideally raising the level of the bed to above grade. Putting them in the bottom of a planting hole does not improve drainage — the hole still forms a “bathtub without a drain”.

Gravel

The best size is a pea gravel called “78” that is about ⅜″ diameter. Gravel can also deter tunneling by moles and voles.

Expanded shale

Sold under brand names such as “Perma-Till”, it functions like gravel but is lighter in weight.

Sources for amendments

Local sources for amendments
Certified compost sources
Certified compost program

Ineffective/harmful amendments

Peat moss

Peat moss retains too much water when initially added, then decays rapidly and leaves the soil in its original state.

Sand

Any mixture less than 70% sand/30% clay creates a soil that compacts more easily than pure clay or subsoil.

Gypsum

Gypsum is useful in the alkaline clay soils of the western US but is not effective in our acidic clay.

Fresh manure

Fresh manure is high in salts, which draw water from plants. The result is root damage and burned leaf edges.

Fresh wood chips · fresh sawdust

These have a high carbon content and therefore use a lot of nitrogen while decomposing. If you incorporate either without adding enough compensatory nitrogen, none will be available for your plants. Similarly, barn bedding is sometimes made of wood chips. Although the manure provides additional nitrogen for decomposition, salt is still an issue. Bedding should be composted before use as an amendment. Of course, undecomposed materials can be used on top of the soil as mulch. Hardwood chips break down faster than pine chips.

Purchased topsoil

There are no laws regulating the content of topsoil, so buyer beware. Purchased topsoil is frequently no better than your existing soil, and unsterilized topsoil can contain weed seeds. Generally, it is better to amend the existing soil with organic materials. When using amended soil, avoid a sharp interface with your native soil by thoroughly mixing the two. Otherwise, plant roots will remain in the amended layer and dry out in hot weather.

Undecomposed organic matter

Undecomposed organic matter such as wood chips or shredded leaves is fine as mulch but should not be worked into the soil during the growing season.

• Decomposition uses nitrogen that your plants need.
• Decomposition collapses the volume, leaving your plants in a depression that can drown roots.
• Note that partly-decomposed organic material can be tilled into the soil in the fall in preparation for spring planting. Time and freeze-thaw cycles will break it down.

Improperly composted organic matter

Anaerobic decomposition is often a problem with materials high in cellulose such as leaves or hardwood chips. Anaerobic decomposition is rare for pine bark because lignin is the main component rather than cellulose. Pine bark is unique because it can be used without composting as either mulch or soil conditioner. Only the particle size differs.

• Properly composted organic material has a pH of 6.0–7.2. In contrast, organic material stored in a large pile often undergoes anaerobic (low oxygen, high moisture) decomposition and becomes very acidic, with a pH of ~ 3.0. It will smell like vinegar, ammonia or sulfur.
• The methane, hydrogen sulfide, ammonia, and/or alcohol found in improperly composted mulch or compost are rapidly toxic to plants, with damage usually evident within 24 hours of application. Symptoms include marginal leaf chlorosis or scorch, defoliation and/or plant death.

How to amend clay soil

Clay soil is easiest to till when it is slightly moist. If the soil is difficult to break up, try irrigating the area, waiting a day or two for the soil to dry, then spading again. The size & depth of the area to be amended should be large enough to readily accommodate the roots of the mature plant. It is better to amend an entire bed rather than individual holes.

• First, test to see if your soil is workable. Digging, walking on or working with wet soil (even good soil, but especially clay soil) compresses it and destroys the fragile pore network. Dig down 6″, grab a handful of soil, and squeeze it. If the soil can be crumbled after pressure is released, it can be worked. Otherwise, wait for several days without rainfall to begin digging (test again first).
  
  Undisturbed soil

  Trampled dry

  Trampled wet

  U Idaho recommending readings Hydrology and Erosion
• Remove existing sod by sliding a spade under the roots. If the area is infested with weeds or weedy grasses in active growth, consider applying glyphosate (a short-lived nonselective herbicide), following the label instructions. Wait until the growth has died, then either remove the top growth as you would for sod or till it under. If the area is very weedy, you may want to try 3 cycles where you encourage weeds to sprout (by watering) and then kill them with glyphosate.
• Because the top 8–10″ of soil is where most plant roots live, spade or till the soil to at least this depth (the full length of a shovel blade), but preferably 12″.
• Add organic matter, 25-50% by volume (for example, if you dug to 10″, spread a layer of organic amendment at least 2.5″ deep over the area). Less than 25% will not provide enough aeration, and more than 50% creates too great a texture difference. Work the organic matter thoroughly into the soil to almost the depth of the bed. A very shallow layer of broken-up, unamended soil at the bottom will aid water movement. If compost is not available, use peat moss to amend sandy soil and finely ground pine bark to amend clay soil.
• Spread and mix a second layer as before, adding any necessary fertilizer or lime as indicated by the soil test report. If you dig the bed just prior to planting, water it several times to settle the soil first.

If not amending

There are two situations where you will not amend soil: when amendment is not practical and when planting trees.

Where amending is not practical

In poor sites where amending is not practical, plant growth will be compromised and plants will require increased maintenance. For best success in poor sites:

• Select plants tolerant of the soil conditions, including low soil oxygen and reduced root spread (compaction), poor drainage (wet soils), drought (dry soils), and low fertility. These are characteristics of some rock garden or alpine garden plants.
• Space plants further apart to reduce competition for limited soil resources.
• Try small transplants, which often adapt to poor soils better than either larger transplants or plants grown from seed.
• Consider raised-bed / container gardening.
• Minimize additional soil compaction with the use of organic mulches and management of foot traffic flow.
• Use organic mulch, which helps improve soil tilth over a period of time as the mulch decomposes and is worked into the soil by soil organisms (do not put weed fabric under the mulch), and add material periodically.

Planting trees

The current recommendation is to not amend soil with organic matter when planting trees. The roots must ultimately grow in native soil and amending the planting hole can retard this process. Because of this, the most important part of planting a tree is choosing a tree that is adapted to your site and soil conditions.

Potted

B & B

Bachman’s Landscaping

Soil prep

Loosen the soil by digging or tilling an area 3–5 times the diameter but slightly shallower than the root ball height — 4″ shallower for potted trees, 8″ shallower for balled-and-burlapped (B&B) trees. This compensates for any subsequent settling. To avoid water penetration problems due to the texture interface between the root ball and the native soil, the root ball must be visible at the surface, not covered with any backfill soil.

Root ball prep

If potted, gently press on the container to remove the tree or use a sharp knife to cut away the pot.

If B&B, once the tree is placed and straight, cut ropes from around the trunk and from the top of the basket. Remove the top of the wire basket if it will be exposed above ground. Remove burlap from the top of the ball so that none will show above the native soil level. Otherwise, the burlap will form a wick that draws water out of the soil and away from the tree roots.

Planting

• Place the plant, confirming that the root ball is at the correct height and the plant is straight.
• Backfill with native soil, tamping gently with your hand or watering to settle the fill periodically.
• Create a shallow berm around the amended soil so that applied water will be retained in the prepared soil. This berm can be removed once the plant is established.
• Mulch the ball and surrounding soil, leaving a few inches bare around the tree trunk to discourage voles and to prevent bark rot. A 3–4″ layer of mulch benefits the soil in many ways:
  • Keeps the soil surface from eroding
  • Prevents disease organisms from splashing onto leaves & flowers
  • Keeps roots cool
  • Conserves soil moisture
  • Keeps the soil surface from crusting & preventing water penetration
  • Reduces weed germination
  • Supplies organic material & nutrients as it decays
  • Provides an attractive finish

links

Soil Management and Fertility