soil horizons garden soil science

Planning a vegetable garden: a quick peek at garden soil science

Garden design is the fun stuff: the layout, the pretty, the la-la-la.  It’s also the nitty gritty science stuff that makes the garden function (your end goal is, after all, success). The two bits of site assessment science that will increase your organic gardening success are climate and soil.  This post focuses on soil.

Garden soil science looks at 4 different but strongly interrelated qualities:

  • Soil Structure (what size and shape bits is it made of?),
  • Soil Moisture (how much will it hold AND how much will it share?),
  • Soil Nutrition (what food can plants get from it?),
  • and Soil acidity or Soil pH.

Soil Structure

sand particle size what is soil made of
Sand is the largest soil grit. Picture one of those 7" 45 rpm records.

All the tiny gritty bits of soil are grouped into 3 particle sizes: Sand, silt, and clay.

While these particles are tiny bits of any number of minerals, most minerals have a “preferred” size that they breakdown to. It’s the mix of bits that define a soil’s character.

A soil scientist can hear someone just talking about very sandy soils (for example) and know something about

silt soil particles garden soil make up
Compared to sand, silt is about the size of a nickel.

the sorts of bedrock that soil came from, which will tell them about the mineral composition, the geologic history, and all sorts of other interesting stuff.

For a gardener or a landscape designer, what really matters is that the size of the garden soil’s particles has everything to do with how well the soil can feed and water your plants. Plants pick up nutrition from soil through a cation [cat-eye-on] exchange.

clay particle size what is garden soil made of
Beside that nickel-sized silt, clay is most like rice.

While I neither expect you to know that word nor plan to tell you exactly what it is, I do know what you need to know: the important plant-feeding nutrition activity (that exchange) is happening around the edges of the particles of soil.  Because sand has a large grit, there are fewer edges-of-particles in a cup of sand than there are in a cup of clay.  This means sand is not very nutritious, but clay can be very nutritious.  Clay, of course, has some other issues.

Soil Moisture

Water can only move between the bits of soil where the air is, and it can only get caught and held when the gaps are just right for holding water.  The biggest difference between silt and clay may not actually be the particle size.  Although clay is smaller, it’s possible the particle shape is the really crucial bit.

Silt tends to be pretty uniform in size and shape (giving it a very silky feeling, like rubbing flour between your fingers), but clay is more flat, more flakey.  This ‘platelet’ quality is what allows clay to be so viscous, so slippery and sticky at the same time.  Of course, in order to slip, it has to be holding moisture.  Clay’s moisture holding capacity is so good it’s more accurately a form of moisture hording.  It won’t share.  It can’t share.  That water molecule is deeply, deeply attached to that clay particle and by gum ain’t no plant gonna sip it up.

Of course, few soils are pure anything, most are a mix of clay, silt, sand, and even gravel sized pieces.  When any type of soil has so much water that the water is filling up even the bigger air pockets, the soil is passing ‘Field Capacity’. It’s holding as much water as this soil field can reasonably hold, probably more.  In a sandy soil, that is very, very wet indeed.  In a heavy clay soil, there were fewer airspaces to begin with, so field capacity is reached that much sooner.

Once the rains stop, the water percolates through the soil.  While a sandy soil lets the water slip through and dries out rapidly, a clayey soil develops the strong chemical bonds with water mentioned earlier.  Plants start wilting (hence the name, ‘Wilt Point’) not for lack of water, but for lack of ability to break that chemical bond.

The soft, loamy soils prized in gardens are built of a wide range of soil particle sizes.  This allows large gaps for air, and lots of surface area for sharing nutrition.  Whether a gardener is beginning with a sandy site or a boggy, boot-sucking clay pit, the answer is the same: add organic matter.

soil moisture graph garden soil quality assessment

Soil Nutrition

Most gardeners acquire their organic matter from compost, which is basically the junk drawer of the garden: old coffee grounds, fruit peels and cores, lawn clippings, weeds and fall leaves (best mowed over a few times to speed up the mechanical portion of the breakdown), wilted salads, squash rinds, rinsed out eggshells (rodents will come for the un-rinsed shell), and more.  Another option is to plant cover crops, which most organic farmers rely on strongly.

I’ll spend more time on the specifics of soil nutrition later (nitrogen, phosphorus, potassium, etc), but there’s one key concept to get across right now: starved from above begets starved from below.

Soil organizes itself into various layers, called soil horizons.  The exact makeup and rhythm of a soil’s horizons is so distinct that soils around the world have been given their own specific names, but a generalize-able pattern exists.  There are more specifics and sub-layers (there is nothing so fine a good scientist can’t parse it further) but the basic layers are these:

soil horizons garden soil scienceThe very top layer, the roughest bit that was just leaves just a few seasons ago, is known as O, (think “Organic”).  Below that, the rich topsoil: A (“A+”, the good stuff). The B layer (“Below”) is a zone of transition (“Between”), I’ll come back to it.  C is regolith, which is the fancy name for the “Crumbling” bedrock, and “D” is the bedrock itself (“Dino” lived in Bedrock? I’m stretching.)

organic matter breakdown soil nutrients nutrition
Organic material breakdown releases carbonic acid, which rain flushes lower in the soil.

As water moves through the soil, it pulls nutrients from the organic layer down into the soil, which darkens the A and, to a lesser extent, B horizons.  Too much rain will wash the nutrients deep down, as happens in the rainforest. Along with the nutrients, the rain washes carbonic acid, the natural outcome of the breakdown of organic matter, down into the deeper layers as well.

It is this carbonic acid that is responsible for the chemical weathering of the underlying bedrock, causing it to break up and release to the plants the minerals so necessary for their health.

soil horizons soil layers
Carbonic acid weathers the rocks, releasing minerals which earthworms and deep-rooted plants bring toward the surface.

Numerous farmers find themselves in a real jam as they try to move to organic methods.  Not only have the synthetic nitrogens hidden (and likely caused) the die off of the soil organisms that breakdown organic matter, but the years of paltry organic breakdown have reduced the supply of minerals being leached from the rocks below.

I have heard of some farmers (and some gardeners) applying sea salts to the land in an effort to supplement this deficiency.  This is a stop gap measure, to be used for a year or three at most while the underlying problem is addressed.  Add organic matter with the fervor of a nurse or nun, and the soil will recover its ability to heal itself.

One gardener I know includes her compost pile in her vegetable bed rotation pattern- it’s simply one of beds for the year, then left to simmer down for the winter while a new bed begins receiving her kitchen’s largess.

There is one final soil quality measurement: soil pH.  Because soil acidity impacts so much, especially soil nutrition, I’ll leave it for another day.

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