Soil consists of rock particles (sand, silt, or clay, in descending order of size) plus organic matter, as well as the air and water that move in the pores between these solids, and perhaps most important, the animals, microbes, and fungi that live in the mixture. Most of us do not think of the dirt beneath our feet as living, yet it supports an extraordinary array of living creatures. It is these living creatures within the earth, from earthworms to microorganisms, that help to decompose organic matter and thus make soil capable of supporting the perfect lawn.
A full description of the topsoil in a particular location would include soil texture (what size are the rock particles?) its structure (how do those particles clump or aggregate?), its density (how compacted is it?), its pH (how alkaline or acidic is it?), its mineral content (what nutrients does it contain? what toxic minerals, if any?), its organic content (how much organic matter does it contain?) and its depth. And that is just the topsoil.
Texture, structure, and density together determine how porous a soil is and thus how easily water and air move through it. pH and mineral content influence fertility, or how many nutrients are present in the soil and how easily plant roots can absorb them.
Texture in the soil refers to the size of the mineral particles in soil — the relative amounts of sand (coarsely ground rock), silt (medium-fine rock particles) and clay (finely ground rock). The pores in very sandy soil can be so large that water quickly drains through it, falling beneath the reach of plant roots. On the other hand, fine clay particles tend to form a dense, non-porous soil that is poor in oxygen, absorbs water only reluctantly, and then holds onto it tenaciously. The following table indicates what percentage of each is present in different types of soil. Loam, at 20% clay, 40% silt, and 40% sand, is generally considered ideal for growing.
Soil structure deals with how the particles in soil clump, or aggregate. Structure can be described as platy, prismatic, columnar, blocky, granular, or, get this, structureless, but despite the undeniable appeal of these terms, they are not central to our purpose here, as we are concerned not with the exact form that soil structure takes but with what it is and what influences it.
The various solid components that make up soil — the rock particles and organic matter — tend to aggregate, or come together, in irregular bundles, called peds. Many factors, including soil texture (the average size of the rock particles in the soil), type and amount of organic matter, pH, and salinity, influence how these components aggregate. In turn, the resulting structure helps determine many other factors, including drainage, density, and aeration, which in their turn influence how deeply roots grow and how easily they take up nutrients. Soils with good structure are described as being “in good tilth,” a term related to “till” and “tillage,” and used to indicate soils that are ready for crops or other plantings.
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The purpose of soil amendments (as opposed to fertilizers) is to improve soil structure. Fertilizers add nutrients to soil; to balance the ph and improve the circulation of air and water. These things determine how easily plants can make use of whatever nutrients are present.
Ideal soils have the following characteristics, all of them dependent on structure:
- They are highly porous, so that they contain large amounts of air (up to 25%) and are easily permeated by water; yet they are also absorbent, so they retain water when wet. (Sandy soils are porous but not absorbent; clay soils are not porous, but they are highly absorbent, in this specialized sense.)
- They are friable — easily crumbled by hand into smaller particles. Sand is not friable because it has no structure to crumble; neither are clays, which resist crumbling, being hard when dry and sticky when wet.
Neither clay nor sand tends to aggregate easily, which is one reason why neither is satisfactory as a garden or lawn soil. Pure clay and pure sand are the structureless soils. In both cases, the addition of organic matter (compost!) helps enormously.
Compost improves soil structure not only physically, by adding absorbent material to sand and porous material to clay, but chemically as well. In clays, organic matter contains long molecules that help bind clay particles together into peds. In sandy soils, negatively charged particles of humus provide binding sites for positively charged ions of magnesium and calcium, thus preventing those nutrients from leaching away; the ions, in turn, bind humus particles together, aiding in aggregation. Compost also contributes microbes and other organisms, which have their own role to play.
Living creatures help to maintain and improve soil structure in ways too numerous to count, both chemical and physical. Here are just a few of those ways: Microbes decompose organic matter, producing compost; worms feed on that compost and produce, with their castings, the richest compost of all; worms and others move in the soil, mixing different levels and aerating as they go; all, in the end, contribute their bodies to the organic matter in soil. Healthy soil is impossible without these beings, most of them too small to see without a microscope.
Yet another key factor in soil structure is the chemistry of salts. Some of this is described in the discussion of gypsum. (See The Special Case of Gypsum). Loosely (very loosely) put, a number of key nutrients (primarily ions of magnesium and calcium) can bind to more than one particle of clay or humus, so they can actually pull particles together, helping aggregates form and thus improving soil structure. Other ions, and the key culprit here is sodium, can only bind to one particle, so they inhibit aggregation. When sodium levels rise too high, sodium ions displace magnesium and calcium, damaging soil structure and creating what are known as sodicsoils, which have very poor structure.
Each of these factors — compost, living organisms, salts — plays a more involved role in soil structure than that described here. This overview, however, gives some idea of how complex soil structure is, and how central it is to plant health.
Healthy earth is porous; compacted earth is not. Neither air nor water move easily through compacted earth, and even earthworms can find it tough going. Roots themselves have a harder time moving through tightly packed dirt, and the lack of oxygen and water make nutrients harder for roots to access.
Compacted earth is therefore a serious problem for lawns and one that needs to be corrected if the grass is to be truly healthy. The single most important treatment is aerating, but best results are achieved through a combination of approaches. Aerating, amending, and topdressing all help.
pH is a measure of how acidic or alkaline soil is, conditions which help determine plant health and happiness. It’s measured on a scale of 0 to 14, though if your results are anywhere near either of those numbers, you probably can’t grow anything at all — but you might be able to sell tickets to soil scientists, curious to see this phenomenon.
Chemically, pH is a measure of the presence of free hydrogen ions in a solution. ‘Free’ here means “free to bond with other ions,” while the solution, in the case of soil, is always a water solution. The hydrogen atoms in water are in constant motion, leaving water molecules and then recombining with them hundreds of times a second. When a hydrogen atom breaks its bond with a water molecule, it leaves its electron behind, becoming a positively charged ion. The water molecule it left, now known as a hydroxide ion, has an extra electron, so it is negatively charged. In a pH-neutral solution, the concentration of hydrogen ions (H+) is equal to that of hydroxide ions (OH-). A greater concentration of hydrogen ions yields an acidic solution; a greater concentration of hydroxide ions yields a basic solution. (Basic and alkaline are not synonymous, but the distinction isn’t important here.)
On the 0-to-14 pH scale, the middle number, 7, is neutral. Lower numbers indicate acids and higher ones basics. It’s important to know that this is not a linear scale, but a logarithmic one, which means that each number on the scale indicates a ten-fold increase in the concentration of hydrogen ions over the number above it. A solution with a pH of 6 has ten times as many free ions as one with a pH of 7; a solution with pH 5 has a hundred times as many.
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Soil pH frequently affects how minerals behave or what form they’re in, and thus how easily plants can absorb them. There are no simple summaries of these effects, as change in pH often initiates not one chemical process but several, even for a single nutrient. Just as an example, five of the seven micronutrients — manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), and boron (B) — become increasingly unavailable as pH rises. This results not from a single process, but from several, which include the formation of less soluble compounds, conversion to ionic forms not usable by plants, and an increase in bonds to negative soil particles (clay and humus). Zinc and copper are both affected by all three.
As a result of such chemical processes, nutrients can be present but unavailable to plants; they are said to be tied up. Other minerals (including a number of nutrients) become increasingly tied up as pH falls. Neutral or slightly acidic pH generally allows an optimal range of nutrients to be available to plants.
All plants need varying amounts of the three primary nutrients in most plant foods, nitrogen (N), phosphorus (P), and potassium (K), and smaller amounts of the secondary nutrients calcium (Ca), magnesium, (Mg), and sulfur, (S). Beyond that, they need trace amounts of seven micronutrients boron, chlorine, manganese, iron, zinc, copper, and molybdenum.
Both excesses and deficiencies of nutrients can cause problems. It’s axiomatic that deficiencies of necessary nutrients would cause problems, but not obvious that excesses would. If two chemicals behave the same way chemically — binding to the same sites on the same molecules under similar conditions — an excess of one can mean that it beats out the other in the chemical bonding game. The excess of one nutrient, then, can cause a deficiency of another. In other cases, excesses themselves can be toxic, for several of these minerals, such as chlorine and copper, to name two obvious examples, are poisonous to both plants and humans in sufficient quantities, though they’re essential in small quantities.
Different soils supply different amounts of these nutrients, so if problems appear it helps to get your soil tested.
Nothing, but nothing, can substitute for organic matter in soil. That matter can be present as living, dead, or decomposing organisms, or as humus, the final, most stable stage of decomposition. According to William Bryant Logan, author of Dirt: The Ecstatic Skin of the Earth, no one quite knows what humus is, a curious statement on the face of it. Apparently humus molecules, while sharing certain properties, differ slightly each to each. Logan quotes two soil scientists, one of whom sighs, “Humus is imperfectly understood,” while the other states that “It is very possible that no two humus molecules are or have ever been alike.”
In his book Life in the Soil, James B. Nardi takes up Logan’s implicit challenge, defining humus as such “hard-to-digest plant materials” as “oils, resins, lignins, and waxes” that have already passed largely unchanged through at least one decomposer, an animal or organism that feeds on dead organic matter. These organic materials can take hundreds of years to break down into their constituent elements, and in the meantime, they play essential roles in building soil aggregates and in the complex chemistry that makes elements available to plants.
Compost contains decaying matter at a number of stages, and even when it is mature, it consists of a huge spectrum of organic constituents, humus being only one. The chemical diversity of organic compost makes it profoundly different from any other soil amendment. So does the rich array of living organisms within it. These two things in part explain why compost is so adaptable, so widely useful. No matter what soil problem you’re facing — pH imbalance, too much sand, too much clay, compaction, nutrient poverty, thin topsoil, poor structure — compost will help. It may not solve the problem, but it will always help. Learn all you need to know about making compost here.
Compost contains millions of micro-organisms, the microscopic beasties that do the work of breaking down dead organic matter into humus. When you add compost to your soil, then, you are also adding the micro-organisms that produced it. These micro-organisms can then go to work in your lawn, turning grass clippings and thatch to soil, making nutrients available to your grass, and in the end adding their bodies to the compost in which they lived. Compost also provides the food for earthworms, whose castings are the best compost of all, whose tunnels aerate soils, and whose presence alone provides a good index of the health of a soil.
Depth brings us back to earth: it’s all very well to go on about lovely humus-rich dirt, but if there’s not enough of it, you’ll have problems. It’s rare, though, that you’ll have a thin layer of rich earth. So if your top-soil is thin, take hope; as you improve it, it will get thicker. This is one case in which it’s possible to kill two birds with one stone.
If you’re starting a new lawn, depth is something you can test and correct before planting seed or laying sod. If you’re improving an established lawn, you can add an inch or so of soil as an amendment each spring and fall, until your soil reaches the desired depth. Eight inches is generally considered adequate; twelve is good; twenty-four is a rare treasure.
The Way of Water
How It’s Lost
Starting with the obvious, let’s state that you put water on grass to water the grass, not to raise the water table, rinse your driveway, or augment the neighborhood stream. Lawn sprinkling water which does not end up in the grass has been wasted (see Water Saving Tips For Lawns).
In the grandest scheme of things, of course, nothing is wasted; the principle of the conservation of energy sees to that. On a slightly more local, planet-wide level, the water that evaporates in Texas does fall as rain somewhere else on earth. But most of us live on a more human, less metaphysical scale; as the Ogallala aquifer under Texas (and New Mexico, and Oklahoma, and Nebraska, Kansas, Colorado, Wyoming, and South Dakota) falls, the fact that its water reaches earth again as rain in the Atlantic ocean, or as snow in Iceland, offers little comfort.
Water can be lost through four routes:
- percolation, when it sinks through the earth below a level to which plant roots can reach;
- transpiration, the “breathing” of a plant’s leaves;
- evaporation from the surface of the soil itself, and finally
- run-off, where it moves across the surface of the earth, often to a body of water, instead of sinking into the soil.
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