Soil is partly earth but not all earth. Soil is a living thing. It grows, gets sick, and becomes well again. Like both garden and gardener, the soil changes dramatically from winter through spring, summer, and autumn—back to winter.
As the cycles march along, year after year, our garden soil may grow stronger or weaker.
Complex Body Of Soil
First, let us look at the soil, at what it is.
A soil has depth quite beyond the part stirred in tillage. Its body has a complex skeleton.
This body holds living things—the feeding roots of plants, microorganisms, and little animals—as well as water, air, and nutrients to nourish them.
A soil has shape. It is flat, sloping, or hilly. The body of each natural kind of soil is unique. The world has many thousands of them.
Each is the product of a specific combination of climate and living forms, acting upon a body of weathered parent rock, with a specific shape, over some time.
If any of these five factors change very much—the rock, surface shape, climate, living forms, or time—we have another kind of soil with a different body.
Because of its different body, this soil’s behavior is unlike the first one. In addition to the multitude of natural soils, we produce those from them.
All our garden soils are made from natural soils through man’s modification of the natural processes.
Some differences are small, while others are very great.
The gardener may make his soil better or worse for gardening, or perhaps just different.
Cycles Of Change
Many natural processes move in great cycles. So it is with soils.
First are the great geological cycles of mountain building, erosion, and deposition, modified by great pulsations of climatic change.
Only some 25,000 years ago, great ice sheets covered much of our country north of the Ohio River, for example.
Plant Growth Cycle
Next come the cycles of plant growth, all the way from a year to a hundred years or more.
Trees, for example, feed on the whole soil, bringing up mineral nutrients that are brought into their stems, leaves, and fruits.
These fall to the surface and furnish food for micro-organisms. As they decay, the mineral nutrients are again available.
In this one way alone, plants greatly modify the soil they live on, generally (but not always) making it more productive for themselves.
Our garden soils go through a rhythm of change in temperature and growth from morning until evening and back to morning again.
The Seasonal Cycle
The seasonal cycle of change in the soil, say from New Year’s Day around to New Year’s Day again, is the most interesting one of all.
We see it over and over again. As gardeners, we are a part of it.
Let us look at the familiar one in much of the United States and Western Europe.
To follow the seasonal cycle, we need to choose a kind of soil and a kind of season or region.
Although all soils with this kind of season have some characteristics in common, still there are hundreds of kinds, including:
- flat ones and hilly ones
- clayey ones and sandy ones
- wet ones and moist ones
- deep ones and shallow ones
Most are acid, while a few are sweet.
Some are relatively rich in plant nutrients, while others are poor in fertility.
The Normal Soil
Within the region having our season, let us look at a natural soil somewhere in the “general middle” at what the scientist calls “normal” soil.
It is gently sloping. It has a mixture of minerals in the parent rock and both sand and clay with a good supply of silt particles, partway in size between sand and clay.
We can easily find such soil in a natural forest between St. Louis and Boston and between Ottawa and Knoxville.
Except when the surface is frozen hard, we can dig a hole and look at it anytime.
Look At The Soil Layers
This is the first rule for making a proper garden.
First, look at the soil—at the whole depth that influences the roots of plants.
First of all, we see from our pit that the soil is made up of layers or horizons that vary from one another in color, thickness organic matter content, sand, silt, and clay, and in the arrangement of the particles into plates, blocks, or granules.
Our soil in the woods has a 2” to 3” inch surface layer of leaves that fell last autumn.
Just beneath and above the mineral soil is a 1” to 3” inch layer of leaf mold—partly decomposed leaves and twigs.
The mineral layers of the soil are commonly grouped into 3 principal horizons called A, B, and C, roughly corresponding to the terms, which are:
- Surface soil
- Subsoil
- Substratum
This deepest one, the substratum, is the partly weathered parent material of the soil.
It may be rotten rock formed in place or loose material brought to its present place by the wind, water, or glacial ice.
The so-called true soil or solum consists of the A and B horizons.
A garden soil made from such soil—and it is a good soil—is commonly quite different from its natural parent.
The surface layers are mixed by plowing or spading into one 6” or 7” inch layer.
With the destruction of the surface organic covering, the mineral soil is exposed at the surface, but it can be made even more productive than it was naturally.
Let us now follow such a garden soil through the seasons with the gardener.
The Soil In Winter
Life is dormant in the soil. As the surface froze, water moved up from the lower layers.
Tender plants are protected from frost by coverings of partly the following:
- rotted leaves
- sawdust
- mixed soil
- compost
Then winter shakes itself with a few warm days.
The freshly thawed surface soil is wet. Then frost again.
Ice crystals reform, prying under stones and surface roots and bulbs.
Some plants are thrust out an inch or more with freeze-and-thaw and more freeze-and-thaw. Gaps appear amongst stones in the rock garden.
The gardener carefully pushes these plants back into the soil on warm days before drying winds can kill them.
The frost has made it more granular where the soil has not been walked on or tilled.
If a warm, hard rain comes while the lower soil is still frozen, let us hope that the gardener has little terraces and good mulch to slow down the water on the slope.
Otherwise, he will lose soil and shallow-rooted plants because of the water which cannot soak in runs rapidly off the surface.
Once the frost has gone, the excess water percolates through the soil.
Except where the gardener has plants growing, such as grass, hardy evergreens, or hardy cover crops in the open beds, this water carries soluble plant nutrients, especially nitrogen and potassium, which are essential to plant growth.
As spring approaches, the rains help warm the lower soil, raising it above 42° degrees Fahrenheit, so that the microorganisms and roots can begin to grow again.
New Roots Need Nutrients To Push Out When Spring Comes
As soon as the sun and warm air of spring start, the new roots push out need water, air, and nutrients.
In the natural soil, part of the nutrients come from the minerals in the soil but, more importantly, from the decomposing organic matter.
For the plant roots to have nitrogen, the microorganisms must have first split off the available forms from the organic matter.
In the natural soil, the new nitrogen from the air, fixed by microorganisms and that comes with the rain, balances any leached from the soil.
However, in the garden soil, we must also make up for any removed by harvest of fruits, stems, and flowers, either as organic matter brought to the garden from the outside or as fertilizer.
A large part of the phosphorous and other nutrients needed by plants becomes available as the organic matter decomposes.
Thus cold soils or wet soils in which the air spaces are filled with water are “slow” soils.
Plants cannot grow much until the microorganisms have started to grow.
Functions Of Organic Material
The organic matter in both the natural and garden soil has two competing sets of functions.
On the one hand, it serves as food for microorganisms and releases nutrients for plants.
To perform this function, it must decompose.
The other general purpose of organic matter is to serve as a physical conditioner:
- to protect the surface as a mulch
- to control temperature
- to keep the soil open and granular
As it decomposes completely, it fails to carry on this physical function.
So new supplies are added to both the forest and the garden.
Plant Nutrients As Fertilizer
As soon as growth begins, the gardener adds plant nutrients as fertilizer.
This hastens the supply to the roots, even if the microorganisms arc slow, besides raising the natural level.
Soil Acidity
Along with organic matter and fertilizer, the gardener also thinks about the soil’s acidity.
Most vegetables, flowers, and lawn grasses prefer soil that is only slightly acidic.
In such soils, the microorganisms that fix nitrogen and release the nutrients from organic matter grow best, and the mineral nutrients are most available to plants.
Yet most natural soils in our climatic region are more acid than that.
To make the change, the gardener adds lime, especially ground dolomitic limestone, which contains magnesium.
This doesn’t need to be done in spring, but it is done best when the soil is spaded and prepared for planting.
But some plants do not like lime.
Here again, the skillful gardener fits his garden soil to his plants, either by changing the soil or selecting the plants.
Although most common garden plants do best in slightly acid soil, azaleas, rhododendrons, mountain laurel, blueberries, and many others want a strongly acid one.
Many common wildflowers of the Eastern states and other plants prefer something in between.
Preparing Your Soil
When the plants (or bulbs) are set out, the careful gardener prepares granular soil to the depth that the plants root (not counting big trees and large shrubs).
Compost or leafmold and mineral fertilizer are mixed with the lower soil before planting, and enough lime to reduce the acidity or enough sulfur to increase it, as needed by the plant.
Seeded rows are given a band of fertilizer beneath and to one or both sides of the seeds.
Once plants are well up and seedlings established, the surface can be mulched in forested soils.
Thus, as the spring fades into summer, part of our garden soil has been spaded and worked into a granular but firm condition.
All of it has been fertilized according to the needs of several kinds of plants.
The plants are properly placed. Terraces have been made on sloping places to prevent too rapid water run-off.
The surface and the plants growing in it are mulched as much as possible to conserve water, keep roots only moderately warm, and suppress weeds.
The Summer Season
This is the real testing period for both garden soil and gardener.
Many spring gardens that look nice fade away with hot weather because of hard soil, shallow roots, and lack of water and nutrients.
Now the big problem is water. Our region has a “continental” climate.
Showers are irregular and often sharp.
Unless the garden soil is open and crust-free, as in the forest, the water cannot soak into the soil quickly, and much is lost.
At the same time, our plants are using a lot.
If the gardener is careless, growing weeds rob his plants of nutrients, light, and water.
They must come out.
Proper Watering To Prevent Summer Droughts
Summer droughts can be harmful to fine plants even with terraces to slow down the run-off water, a well-prepared deep surface soil, mulch, and weed control.
Without these conditions, the garden soil is certain to be dry in most years.
The roots are now deep in the soil.
Our watering should prevent the lower soil from drying out and keep the plants from wilting at all. As a plant wilts, it is injured even though it recovers growth after watering.
Worst of all, for lawns or beds, is a light surface sprinkling of dry soil. Surface roots are stimulated only to die when the surface dries again. When watering is done, it should be done well to moist the whole rooting zone.
Careful Inspection During Summer
During summer, the skillful gardener watches his plants carefully.
It has been said that “the best fertilizer is the shadow of the master.”
He can control diseases and insects when they first appear.
He checks his plants’ vigor, form, and color for indications of nutrient deficiencies.
For example, dark green, weak, spindly plants with few flowers suggest too much nitrogen to the other nutrients.
If the growing tips of shrubs turn yellow, and the yellowing gradually spreads to the older leaves, the plant is probably deficient in iron.
He can add chelated iron to the soil or ferrous sulfate to spray the leaves.
If yellowing starts first on the old leaves and spreads to the young ones, and if water and nitrogen are satisfactory, probably the plant is too low in magnesium.
As the plants grow, the gardener can note how well soil and plants get on together.
He can change the soil and select the most nearly adapted plants.
This adjustment is the basic key to gardening, and each successful gardener finally developer their own “handbook.”
This handbook is for their kind of garden soil, not some other kind.
Autumn Planting
This is the second planting season and the “hardening” season for perennials and shrubs.
For these, nutrients in the soil, especially nitrogen, should be low, otherwise, late tender growth will freeze in winter.
Autumn is also the season when we must be sure that no weeds scatter their seeds on the soil.
Early autumn is a good time to fertilize the lawn in areas where it is hot and moist in late spring or early summer.
With abundant nutrients in the soil, the grass goes into the winter with heavy food reserves in its roots, ready to push new spring growth.
Now grass responds to spring fertilizers with nitrogen in them, but such luscious grass is very susceptible to fungus diseases, damping-off, and brown patches, especially on clayey soils.
Thus in shade and where summers are cool, high nitrogen in the soil during spring is fine.
But where winters are fairly mild and the early summer is hot and wet, the Iawn soil should have fairly low nitrogen in spring, and high nitrogen in autumn.
For a long period in autumn, the soil can be reworked tor spring-blooming bulbs.
Each one needs a bit of mixed fertilizer an inch or so below the bulb.
Transplanting Shrubs
Autumn is a good season for transplanting many shrubs and perennials after the danger of hot dry weather is passed.
The soil gets settled tightly around the roots during the autumn rains and the freezing-and-thawing of winter and before the heavy water use of late spring and summer.
During this season, we can usually find a time when the soil has the ideal moisture content for preparing new beds, reworking old ones, building rock gardens, and laying out terraces and walks.
In early spring the soil is likely to be too wet and late spring is, well, too late.
If our garden soil freezes deeply and stays frozen all winter, autumn-planted evergreens may suffer from a lack of water because the soil has not settled tightly around the roots.
Many gardeners near Washington, D. C., for example, like to move evergreens in autumn a bit more than in early spring, while most gardeners further north, in New England or Canada, prefer to move them in the spring after the soil is thawed and dried a bit.
Autumn is the big compost time.
All disease-free plant remains saved during summer and autumn are now mixed with the leaves of the autumn fall.
The compost that was started in a pile last autumn, a year ago, and turned last May or June, is now ready for use in preparing the soil for new plantings and for mulching the perennials, but enough needs to be saved for spring.
Mulch Your Plants
For shrubs and trees, a sawdust mulch can be added in autumn.
This is especially good for acid-soil lovers.
In the intimate garden, a light covering of compost over it improves the appearance.
Where shrubs are under trees, this final mulching comes late—after the freshly fallen leaves have been put away into the new compost pile.
Generally, as autumn closes, the garden should go into winter with a good cover of living plants, mulch, or both.
Big beds for vegetables and annual flowers can be sown in early autumn to rye, oats, wheat, or ryegrass for cover.
Such a cover protects the surface soil from beating rain and wash.
When spaded under in spring, the fresh organic matter soon decomposes and releases the nutrients.
As the autumn changes to winter, plants become dormant, except the evergreens, the micro-organisms go into rest, and insects and small animals seek the lower levels below the frost.
As the surface cools, while the soil beneath is still warm, water moves up to it.
Normally, freezing is followed by snow.
The garden is asleep. The gardener reviews his successes and failures and plans for a better garden in the next cycle.