The Structure of Soil

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Structure describes how soil hangs together. pore clump into crumbs or clods. Loose crumbs and c(f ^^ecan space, no matter what your soil texture. As a result, compensate for less-than-ideal texture. Sod with good ^

more rainfall more quickly, and excess water drams ^

and soil organisms push through more easily, and gardeners dig With ease. Good structure makes good gardens.

Organic matter - in partnership with soil organisms - is the mam agent behind good structure. It greatly increases the pore space m soil. It not only helps crumbs (aggregates) form, it also increases their stability. Earthworms and microorganisms break down organic matter into gelatinous substances that gently hold soil particles together. Roots of plants and fungi also contribute, in two ways. They push soil particles together as they push through. They also manufacture gummy substances that help hold these particles together.

Managing Structure

As with texture, the easiest way to improve soil structure is by-adding organic matter. Once you've achieved good structure, it's impossible to maintain it without maintaining humus levels. Organic matter (plus a few months' time) may be all you need to improve powdery soil or large, hard clods. Compacted structure, soil that's crusted on the surface or forms a dense layer (hardpan) below the surface, is harder on plants and takes more effort to fix. Rainwater and roots can't enter easily, and it can be hard to dig. Adding organic matter isn't enough; vou'U also need p°ige'l43)lble d,gginS (See Page U1) °r !00Sening vvith a broadfork (see When to Work Your Soil hours (ideally, overnight) before digging w! it SeVeral squeeze, or do you see a solid stirkv f rU" out when you

-t Wait a day or so^S^ZIZ too a mostly solid lump that easilv bre£ aSrS t ?«** Do ** see Your soil is slightly moist. ,>s"t nght ^ ^ * °ig

How to Promote Ideal Soil Structure

  • Keep levels of organic matter high: Add compost, manure, or plant residues regularly, or grow green manure crops.
  • Encourage abundant, healthy soil organisms (by adding organic matter).
  • Cultivate soil only when moist: Avoid walking on. or driving machines over, wet soil.
  • Make sure soil contains a good balance of calcium and magnesium (but avoid overtiming).
  • Cover bare soil: Mulch areas that aren't covered by plant leaves to minimize pounding from heavy rains.
  • Keep soil in use as much as possible: Plant a .green manure or cover crop if an area will be bare for a couple of months.
  • Minimize rototillingand other forms of cultivation, as these break up desirable soil crumbs and channels.


Examining Soil Structure

The best way to learn about the structure of your soil is to look at it. Start by digging a hole and comparing the soil in place on the sides with the loose soil removed from the hole. To test the stability of your soil's structure, see if its crumbs hold up when you add water.

Dig a hole about a foot (30 cm) deep. Can you see small (16- to V2- inch, or .5 to 1 cm) crumbs in the top few inches? Ideally, you'll see some crumbs even 12 inches (30 cm) down. A crust on the surface, or a dense layer (hardpan) partway down, is a sign of poor structure.

Does the soil removed from the hole contain lots of '/2-inch (1 cm) crumbs? Do larger clods break opart easily? These are signs of good structure.

Place a handful of soil crumbs in each of two glasses. Gently pour water into one of the glasses to cover the soil. If your soil has good (stable) structure, crumbs will hold together even when wet, so the wet and dry samples should look similar.

Pour water onto bare soil in the garden. Does it flatten all the crumbs into a fine, even iayer? This effect, called puddling, is a sign that your structure needs improving.


Master Gardening

Promoting Drainage

Dealing with Topography

The shape of the land surface, or topography, has a big effect on drainage. Where ground slopes forms a small hill, drainage of heavy-textured soils will improve. Slightly sandy soils, though, may behave more like very sandy so-h-Iri low spots or where land fornix bowl, water will tend to collect regardless of how sandy the soil k You can modify the topography to correct all but the most extreme drainage problems. In low spots, in heavy soils, or where the water I table is high, building raised beds I will improve drainage. In sandy I (light) or gravelly soils and dry climates, creating sunken bed$$H I help gardens capture all available rain or irrigation wafer. (See", page 145 for instructions.) When? topography causes severe problems. get advice from a landscaping professional about regrading ¡your yard. j

Son Drainage Classes


Excessively drained Well drained Hourly drained

Very poorly drained


Sandy or gravelly

Loamy: lop few in Soil is wet tor sevt

Soil stays wet aim on steeply sloping land); top few inches dry out. quickly s stay moist after rains months of the year V^SBMP

constantly; water forms long-lasting puddles on surface

Plants Tnat Indicate Poor Drainage

Getting to know wild plants in your yard can tell you a lot about what's going on below the surface. The most reliable indicator plants are those for poorly drained soils. While areas that form puddles may be easy to spot after heavy rains, indicator plants ran show you these areas at any time. IWt rely on a single plant, or on unhealthy speomens. You need to see several thriving plants of one type, or at least three different types, to know your drainage is poor

WHdflowers and Weeds

Bulrushes (Sarpus. spp.) Buttercups {Ranunculus spp.) Cardinal flower (Lobelia cardinalis) Cattails (Typha spp.) Coltsfoot (Tussilago farfara) Docks [Rumex spp.) False hellebores (Veratrum spp.) Horsetails (Eijuisetum spp.) Ironweed ( \ remonia noveboraccnsis) Joe-Pye weeds (Eupatorium spp.) Mosses (many spp.) Plantains (Plantago spp.) Poison hemlock (Conium maculatum) Ragged-robin (Lychnis flos-cuculi) Rushes (Juncus spp.) Sedges {Carex and Cyperus spp.) Skunk cabbages (Symplocarpus spp.) Smartweeds (Polygonum spp.) Water hemlocks (Cicuta spp.)

poison hemlock horsetail

plantoin horsetail

Shrubs and Trees

Buttonbush (Ccphalanthus occidentalis) Chokeberries (Aronia spp.) Meadowsweets (Spiraea spp.) Summersweet (Cleihra abufolia) Willows (Salix spp.)

Winterberry (ilex vertidUala and I. laevigata)

poison hemlock plantoin

Dig two holes, each about a foot (30 cm) deep and a foot across, spaced several feet apart.

2 Fill one hole to the top with water Veos/e depth of the water and write it down, noting« Repeat with the second hole.

Continue to check each hole periods write down the time when all water has d<o» away. If ¡t takes more than eight houtfs, y^ijf poorly drained.

Dig two holes, each about a foot (30 cm) deep and a foot across, spaced several feet apart.

2 Fill one hole to the top with water Veos/e depth of the water and write it down, noting« Repeat with the second hole.

After an hour, write down depth and time for both holes. Repeat after two and three hours. Calculate the average inches (cm) bst per hour: Add the six numbers for inches (cm) lost per hour (three from each hole) and divide by six. Water in well-drained soil should drain about an inch (2.5 cm) per hour. If your water disappeared much faster, your soil is excessively drained

Testing Soil Drainage though vour texture *st -hows a gnr . * w „ „,, »«,r ^ ^» «SSKT- UK W - - *

areas ,„ s.V if to y..* 1 • » „,, wbm I he »» «> >' hooae an av„,K„ m3

Fbr the most accurate results, «7 " kv so d lump Also.'ry

Continue to check each hole periods write down the time when all water has d<o» away. If ¡t takes more than eight houtfs, y^ijf poorly drained.

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