Soil

Soil Components

There are many components to soil. Perhaps the most obvious are the solid particles that support house foundations, plant roots, etc. A closer look would reveal that the solid particles forming soil exist in two types. Organic particles consist of chemical molecules that contain atoms of carbon; these organic particles are either produced by living organisms or are degradation products of once-living organisms. Examples of organic particles would be worm castings, twigs, leaf litter, compost, and humus. Inorganic particles consist of chemical molecules that generally do NOT contain carbon atoms; these inorganic particles are produced by the weathering of earth's bedrocks. Examples of inorganic particles would include rocks, pebbles, sand, silt, and clay.

The solid organic and inorganic particles are not compressed enough to exclude spaces between the particles. So another soil component is air in these spaces. This air will include all of the atmospheric gases, and for most of the organisms that live in the soil the critical gas will be oxygen (O2). Thus most productive soils are an aerobic environment conducive to the growth and welfare of a wide range of soil organisms.

The surfaces of the particles interface with the air though a thin film of water. So water is also a critical component of soils. Water is obviously critical for all of the organisms that live in the soil environment. This will especially be true for plants. So the spaces between the solid particles must include a film of water as well as pockets of air exchanging with the atmosphere if this soil is to support life.

Our soil is not complete without its complement of living organisms. It has been estimated that if we were to scrape away the soil down one-foot in depth over a one-acre area of soil, and sift out the living organisms, there would be over three tons (!) of living material in that acre-foot of soil. So soil is a very lively place! Indeed in walking over soil, you compress spaces between the particles and maim and kill a large number of soil organisms!

A closer look at inorganic soil particles

We realize quickly that some sizes of soil particles are irrelevant to plants in terms of support and growth. So we exclude from our thinking those particles that are too large for plants to interact directly with. So glacial erratic boulders, rocks, gravel, pebbles are all excluded here. I would only add here that in New England, the soil is loaded with these large particles and because they interfere with farming, they were pulled out and put around the edges of the fields, creating the low walls you find running through the Connecticut woods today. These fields fed the European colonists in the early days of the invasion. Once the pathway to the excellent soils in the midwest was opened, Connecticut farmers could no longer compete. The fields were let to go back to forest. The inorganic soil particles of a size to interact with plant roots are sand, silt, and clay.

Sand is the largest particle plants interact with, ranging from 0.1 to 1 mm in diameter. This particle packs loosely with others of its kind, providing large are spaces in the soil. This means that sandy soils have good gas exchange and excellent drainage of water. I'm sure you have observed this at the shore when you observed a wave wash up the sandy beach and disappear into the sand. But sand particles have limited surface area compared to their volume and therefore do not hold water or minerals very well. So sandy soils dry out and the nutrients leach out very quickly. So a sandy soil requires more frequent watering with less volume of water. Fertilizer should be dilute and applied more frequently.

Silt is a medium sized particle, ranging from 0.001 to 0.1 mm in diameter. This one is adequate in terms of drainage, air capacity, water holding, and nutrient capacity. But it lacks the superior drainage and air capacity of sand. It also is inferior to clay in terms of holding water and minerals. So a middle size is NOT ideal!

Clay is the smallest inorganic particle in the soil, and is less than 0.001 mm in diameter. These microscopic particles do pack tightly so that a clay soil does not percolate water (does not drain!), holds really small amounts of air and can be easily waterlogged. Clay soils become anaerobic very easily killing the organisms living in the soil. However because there are myriad tiny spaces and lots and lots of surface area to attach mineral nutrients, this soil has high water-holding and nutrient-holding capacity. So a clay soil has to be worked carefully to grow plants. One must water sparingly, to avoid compressing the air spaces out of it by walking (running equipment) over it when it is wet.

The following chart summarizes the three sizes of soil particles relevant to plant growth. As is quite obvious from this chart, a soil that is all sand, or all silt, or all clay would be mediocre at best and likely to have really poor characteristics! So no single soil particle is going to serve well to grow plants!

Inorganic particles

SizeDrainageWater
Retention
AirNutrient
Capacity
Sand1-0.1 mm+++0+++0
Silt0.1-0.001+++++++
Clay< 0.0010+++0+++

So if a soil made of only sand, or only silt, or only clay is not really very good (for different reasons) for growing plants, then what kind of soil IS good for growing plants. Obviously we need a mixture of these three particles for optimum soil texture. Such a mixture of the three particles that is suitable for growing plants is called loam. This word rhymes with "foam" in most of the USA...except in parts of New England. Here in Connecticut, for example, most farmers will talk about loam and you will hear the word "loom." Curiously New England was the location of vast textile industries. Every town near a river had a mill with looms for making fabric. Yet the farmers of New England call their soil "loom" too. This is very odd...but then there are lots of odd things about New England.

Soil Classification

If loam is an ideal kind of soil mixture, how does one determine whether the local soil texture or soil type is some kind of loam? There are several ways to do this and some are more accurate or quantitative than others. Some people would rub the soil between their fingers and assess the soil subjectively this way. With some experience a person can at least get a crude classification of a soil sample this way. One could use a series of sifters to divide the sand from the silt from the clay...but if the particles clump together (as soil tends to do!) some "sand" would actually be "clay" that is conglomerated. So sifting is inaccurate.

The method we will use in laboratory is one of the better methods for assessing soil texture. We will sift the pebbles, sticks, large organisms, etc. out of a soil sample so that we mostly have particles of sand, silt, and clay in whatever small clusters as may exist. We will put these into a glass cylinder and mix them with a solution of Calgon&tm;. The active ingredient of Calgon is sodium hexametaphosphate. This chemical compound surrounds soil particles and coats them over their entire surface interacting with water molecules. Sodium hexametaphosphate is a wonderful surface-active agent (surfactant). The Calgon thus separates all of the particle clusters into their individual particles. Adequate mixing of the slurry of Calgon and soil particles is critical to separating all of the clusters.

At the end of the mixing of soil and Calgon, the cylinder will be stood on a level counter overnight. The Calgon-coated particles will settle to the bottom of the cylinder based upon their weight and density. Obviously sand particles fall first and collect in a layer at the bottom of the glass cylinder. This happens in just a few minutes. Over an hour or two the silt particles will then precipitate to the bottom forming a layer over the sand. Overnight, the clay particles will then precipitate and form a layer over the silt.

The depth of the three layers can be measured to quantify the amount of sand, silt, and clay in the sample. In our graduated cylinders, we can use the volume markings on the side of the glass to determine the volume of each layer. We will then calculate the percentage of the total soil column that is sand, silt, and clay, respectively. These three percentage values should total 100% if our calculations are accurate. We can then use these three percentages to determine the soil type from a standard soil triangle.

SOIL TRIANGLE

Let's take an example. Imagine that your cylinder had no volume marks (maybe you used a straight sided jar or glass tumbler at home). The sand layer would be at the bottom and because these particles are large, you can see them easily and can also see the voids between the particles. You measure the sand layer to be 5 cm deep. The silt layer starts where the voids disappear. These smaller particles pack tightly enough that you can no longer make out the voids between them. You measure the silt layer is 2 cm deep. The clay layer begins where even the particles are no longer evident. The clay layer looks uniform in color and texture and extends to the bottom of the the Calgon water layer above it. Your measure the clay later to be 3 cm.

The 5 cm of sand out of a total column height of 10 cm represents 50% of the soil. The silt is therefore 20% and the clay is 30%. Notice how the total percentages sums to 100%. These percentages can then be used on the triangle above to determine the soil texture. Draw a line straight across to the right from 30% clay. Draw a line down and to the left from 20% silt. Draw a line up and to the left from 50% sand. Be sure to keep your angling lines parallel between the existing lines on the triangle. Do NOT cut cross-lots! In this case, the soil type works out to Sandy Clay Loam. This soil classification is one that has enough sand for good drainage and gas exchange, but enough clay to hold water and minerals too. So it is indeed a loam and is suitable for growing plants. It may not be a "perfect" loam, but it will grow plants nicely!

Organic Particles

Organic particles...those that were produced by living organisms or are breakdown products of living organisms...are harder to classify and analyze. In your soil column much of this material floats on top of the Calgon solution, or precipitates as a flocculent layer just at or above the clay layer.

The smallest organic particles are called humus but are the size of clay particles and have some similar properties. Humus provides excellent water and nutrient holding for sandy soils. But they also improve drainage for clay soils. The humus particle then is desirable for almost all soil types explaining the popularity of "organic gardening" which emphasizes conditioning the soil with humus.

Humus comes from the breakdown of leaves, grass clippings, kitchen scraps, rototilled plants (green manure), or spreading manure and working these materials into the soil. Horse manure is considered the premier soil texture improver in the arsenal of the organic farmer. It has a high percentage of undigested plant fibers etc. Organic farmers who lack horses put their scraps of materials above into compost heaps or compost bins to degrade into a black material called compost. This is spread over the soil and worked into the top few inches to condition the soil.

Locally all of our autumn leaves are vacuumed from the streets and composted over the winter. In spring, homeowners can pick up for free the finished compost from the recycling center to work into their gardens prior to spring planting. Also, spent compost is available for sale from our local (Franklin) mushroom farm. Again, many avid gardeners make their own compost in the backyard.

Nutrients Needed: Macronutrients

From a plant perspective, in addition to water and air, the soil also provides mineral nutrients dissolved in the water. As plants take in the water they also take in the minerals dissolved from the solid soil particles. These minerals provide the plants with essential atoms. The essential atoms can be divided into two classes. The macronutrients are those that are reqired in abundance for the survival of a plant.

The macronutrients are not only important but they are needed in abundance. Each nutrient has a name and a chemical symbol that is found in the periodic chart of the elements. The macronutrients needed abundantly by plants include: C H O P K N S Ca Fe Mg.

C is for carbon. This atom is taken in by plants mostly in the form of CO2 from the atmosphere. Because you exhale this gas, plants grow faster when people are breathing around them. It isn't that plants "like" you, or "hear" you reading to them, or talking to them. But in fact they are growing faster because of what you exhale (pollute!) into the atmosphere. A lot of kids try to do experiments where plants are supposedly growing in response to music, and surprisingly enough, the plants grow better in the environment with the music of choice of the kids! Of course all this really says is that the kids spend more time with the plants experiencing the music the kids enjoy and therefore grow faster! Their projects have other problems, but this is a major one.

H is for hydrogen...the most abundant atom in the universe...the fuel for the stars including our sun. O is for oxygen and these two atoms are taken in by plants mostly in the form of H2O, water! As long as the soil is moist enough and not too salty, plants get what they need from soil water. So C, H, and O are not macronutrients we worry about getting to our plants.

P is for phosphorus. This atom is taken from the soil solution as a dissolved phosphate (PO4-). But most soils are poor in phosphorus content and so this is one that really needs to be supplemented by a gardener in most places. This atom is critical as a component of DNA and RNA (hereditary molecules), of cell membranes (phospholipids), and ATP (adenosine triphosphate) the energy currency. A lot of steps in plant biochemistry including photosynthesis need phosphates to function. So plants with insufficient phosphorus are often dwarfed. So phosphorus is a critical part of most fertilizers.

K is for potassium. Yes I know the letters are confusing, but chemists chose K for Kalium, the German name for this atom because P was already used for phosphorus. This atom is absorbed as a potassium ion (K+) dissolved in the water film from the soil particles. Again many soils are impoverished in potassium, so we include it in our fertilizers for soil improvement. Potassium is used for ion balance and help control turgor pressure to support young plants, and help the mechanism for opening and closing the stomata on the leaf surface to control water loss and gas exchange.

N is for nitrogen. The atmosphere is close to 80% N2 gas, but nevertheless most of life on earth is limited in its growth capacity by a lack of this atom. In humans if the diet is short of nitrogen the disease kwashiokor is observed. It is a sign of starvation. Nitrogen cannot be taken from the atmosphere because N2 gas is virtually inert. Instead plants must receive nitrate, nitrite, or ammonium ions from the soil. Herbivores in turn must eat plants to get nitrogen. Carnivores eat herbivores for their nitrogen. So getting nitrogen into plants is critical. Because so many soils lack this nutrient, nitrogen is another of the elements found in fertilizer. The whole food web depends upon adequate nitrogen in the soil. The nitrogen is a critical part of many biomolecules including DNA, RNA, and proteins.

Fertilizers that are sold in the USA are required by law to give a guaranteed analysis which tells the customer what is being purchased in terms of nitrogen, phosphorus, and potassium. The packaging on the fertilizer shows the analysis as three numbers separated by hyphens. The numbers represent the percentage of each element in the bag in the sequence N-P-K. So fertilizer with an analysis of 5-15-10 tells you that 5% of what is in the bag is nitrogen, 15% is phosphorus, and 10% is potassium. So this fertilizer has a nice balance of the three nutrients and might be suitable for a vegetable garden. A fertilizer that has two 0s in its analysis is called a single fertilizer. For example 23-0-0 is a fertilizer consisting of 23% nitrogen (the rest is inert stuff) and this high-nitrogen fertilizer is probably all you need for a typical lawn of grass. Flowers and fruits need balanced fertilizers.

Now before you go out and buy some balanced fertilizer for your garden, I'd just like to comment that single fertilizers are often very cheap at farm stores. You can purchase these and make your own blend to spread on the garden for far less money than the bags of branded fertilizer at the same farm store.

S is for sulfur. Plants absorb sulfur as a sulfate ion (SO42-) from the soil water. Because soil particles (sand, silt, and clay) are composed of iron sulfides, most soils have plenty of sulfur and weather out plenty of sulfate for plants to grow just fine.

Ca is for calcium. Plants absorb calcium as an ion (Ca2+) from the soil water. Some kinds of rock have lots of calcium (limestone is an example), others have virtually none (granite for example). Many soils have plenty of calcium because the underlying bedrock weathers and releases the ions into the soil for plant use. But in areas with a granite bedrock (Eastern Connecticut for example), calcium can be growth-limiting. So people will often spread crushed limestone on their land to help supply this critical element to their plants. In fact while plants do not have bones that need calcium, nor muscles that operate with calcium, they do need a fair bit of calcium for many other processes and as a cofactor for lots of enzymes. So plants in Connecticut can be short of this mineral and so "liming" of lawns and gardens with powered limestone is a very common spring and fall practice.

Fe is for iron. This metal ion (Fe2+ or Fe3+) is absorbed from the soil water. Again, these ions weather out of soil particles that are mostly iron sulfides, so that most soils have plenty of iron for plant growth. We do not need to worry about iron availability in most soils. There are exceptions...especially those with extremes in pH.

Mg is for magnesium. This metal ion (Mg2+) is absorbed from the soil water and is weathered out of many mineral particles. In most soils we don't have to worry about getting enough, but here in Connecticut we have granite as the base rock and acid rain thanks to pollution in Ohio, Pennsylvania, and New York states. So here we often do not have enough magnesium to supply plant needs. Magnesium is a macronutrient for plants because it is the central atom for chlorophyll to carry out photosynthesis. So plants need quite a bit of this atom compared to a human (for us Mg is a micronutrient). Here in CT, we often choose dolomitic limestone when we lime our lawns and gardens. The dolomite provides the extra boost of Magnesium that our plants need.

Here is a handy table of the macronutrients. You might notice that the sequence spells out a mnemonic device... C. Hopkins Cafe, MightyGood. This helps some biology majors learn this sequence. For non-majors I'm likely to ask you only for some examples rather than the whole list, making this device probably not so helpful.

Macronutrients: CHO
air
water
PK(i)N
limited
in soil
S     Ca Fe     Mg
-------common in soil--------

Nutrients Needed: Micronutrients

The macronutrients are not sufficient to meet all of the needs for a plant. There are a number of other chemical elements needed for the life of any plant cell. These atoms are required in much less abundance but are still absolutely essential. Without the trace amount needed, a plant cannot survive. We call the group of trace minerals the micronutrients to indicate the small amount required for these critical elements.

Again most of these elements are metal ions taken from the soil water after release from soil particles. The amount needed is so very small becaue most of these metal ions play a role as cofactors for a few enzymes that are found in living plant cells. Because enzyme molecules function in very low concentration, only trace amounts of the metal cofactors are needed for survival. Most soils have sufficient micronutrients for plant growth, so a micronutrient fertilizer is generally not needed for crops or gardening. However Connecticut is a notable exception. Here we have granite as the bedrock which produces acids as it weathers, and we have acid rain thanks to sulfur dioxide and nitrous oxide pollutants released into the atmosphere in Ohio, Pennsylvania, and New York states. These precipitate as sulfuric acid and nitric acid when they combine with water in clouds and fall as rain. The pH of the rain here can be as low as 4 in Connecticut as the weather passes over these states and into ours. The acid soil and acid rain release the minerals from the soil particles en masse and they are carried down rivers and into Long Island sound. So it is often recommended to use a fertilizer with included micronutrients for our state.

The micronutrients include Co=cobalt, Mn=manganese, Cu=coppper, Zn=zinc, Si=silicon, Mo=molybdenum, B=boron, Al=aluminum, and Cl=chlorine. The sequence CoMn CuZn Si Mo B Al and Cl is often converted to "come on cousin, see Mo by Al and Cleo" for a mnemonic device by biology majors to help remember the entire list. For this course, I'll be satisfied if you at least remember a couple of examples as micronutrients.

I would be remiss if I did not remind you that as a college graduate, we expect you to use your mind and to think in ways that we might not expect for dropouts. I hope that as a graduate from my course, you will be able to think about the importance of doing a soil test before purchasing and applying any kind of fertilizer to our soil. So many people just buy and apply and do not think of consequences...and I hope you will think more deeply. If your soil does not need a particular nutrient, you might be providing an overdose for your plants in some fertilizer. It is worse than just wasting money on unneeded fertilizer. The excess you apply will percolate through the soil, into the local stream or pond, and may ultimately end up in Long Island sound. These excess fertilizers build up over time, foster the bloom of algae, that then shade each other out and die. The rotting algae robs the water of oxygen, the fish die, and so on. The dead bodies of algae and fish accumulate in the bottom of the body of water and fill it in...becoming a swamp. This process of aquatic environments becoming marshes is called eutrophication. People who do not do soil tests and just keep putting on more and more fertilizer add to this problem. In our lab we will be carrying out some soil tests and determining how much fertilizer to apply. You can bring a soil sample from your home lawn or garden to help you into making environmentally sound and financially conservative decisions about fertilizer.

Soil Water Is Important

The thin film of water around soil particles is the microscopic location of much of the important biology for the relationship between soil and plants. The water in these soil spaces is called capillary water. This water is tapped by the microscopic root hairs of the root for both water and minerals. A tree can go through hundreds of gallons of water in a short time and all of it comes from this thin film coating the soil particles and the root hairs. This water contains the dissolved soil minerals. These are carried into the xylem and up the plant stem to the leaf mesophyll. Here, the water evaporates to cool the leaf, and concentrating the soil mineral solution for the benefit of the leaf enzymes. So water is a vehicle...a carrier...for the mineral nutrients. Later in the course we will learn much more about water, but for now the critical features are that water is a great solvent for minerals and it dissociates.

The dissociation of water means, that in all situations water is not inert. It is in an equilibrium with different forms of itself. Water (H2O) can come apart (dissociate) to make a hydrogen ion (H+) and a hydroxide ion (OH-):

H2O ------> OH- + H+

So how much water is broken apart (dissociated) at any point in time? Well that depends upon conditions. In freshly distilled water (as pure as we can get it) the concentration of dissociated water is 10-7 M. The M means moles per liter and we are not talking about subterranean furry mammals or small human skin birthmarks. No, moles per liter is a number of molecules found in a liter. The 10-7 means that it is a LOW concentration of dissociated water molecules in that liter. To help us get rid of moles per liter and the exponents and so on, we just use pH as a measure of dissociation. pH is the proportion of Hydrogen ions in the water. It is the power of ten on the molarity, without the negative sign. So for distilled water the 10-7 moles per liter, converts to a pH of 7.

pH = - log [H+] = 7 for distilled water

Now what is dissolved in the water can influence how much is dissociated at any time. If we add an acid to the water, the concentration of H+ rises and the pH therefore gets lower. So the rain in Connecticut has acid dissolved in it and so the pH is not 7 but rather is closer to pH 4. If we add an alkalai to water, for example soap, then the acid is neutralized and the pH rises. Soapy water that you might wash your hands with is about pH 10. The range of pH values for water goes from 0 to 14.

100                 10-7                         10-14 M H+ Conc
0-----------------------7-------------------------------14 pH
acid
sour
vinegar
cranberries
neutral base = alkalai
bitter
soap
beans & asparagus

OK, so soil water has a certian pH...but what has that got to do with soil minerals? And, if my pH is wrong, is there anything I can do to fix it? The second question is easier to explain than the first one.

The pH of soil can be raised by adding limestone to the soil. So people in our state often lime their lawns and gardens each spring and fall. They spread ground limestone over the soil. Limestone is basically calcium hydroxide (CaOH2), and is therefore alkaline and raises the soil pH. 68 pounds of ground limestone can raise 1000 square feet of soil by one pH unit. Since lawn grasses prefer a soil pH of about 7.5, this will greatly enhance the lawn. In fact many Connecticut lawns have a lot of moss growing in them; moss is an indicator species because it can only survive in acid soils. So if your lawn grows a lot of moss, you REALLY need to lime your lawn. Athletes know the value of lime more intimately. The athletic fields are marked with powdered limestone, and the grass grows so much better around the yard markers than elswhere in the fields. It is greener, thicker, tougher!

OK so liming a lawn is great for Connecticut, but what if you need to go the other way? In the western states of the USA the soils tend to be alkaline (pH 8) and the lawns grow just great without any treatment. But what if you want to grow blueberries out west? Blueberries and cranberries which grow great out east without fertilizer or liming of any kind, prefer a pH of 4.5. So people who try to grow these fruits out west lose their plants unless they can lower the pH of the soil. They do this by adding a chemical that will release acid (H+). Once such chemical is aluminum sulfate. This compound combines with soil water and the sulfate converts to sulfuric acid. This lowers the pH.

Cation Exchange

Now we need to answer that harder question, how does the pH or acidity influence the availablity of soil nutrients? Before we can answer this directly we need to first know how the mineral ions are held by soil and released from soil to get into the plants.

In the soil, the clay particles are iron sulfides and other mineral particles. These generally have unshared electron pairs on their surfaces. In other words, the surface of a clay particle or a humus particle is coated with negative charges. Because items of opposite electrical charge attract each other, just like the north end of a magnet attaches and holds the southern pole of another magnet. The metal ions that are in our lists of macronutrients and micronutrients are mostly of positive charge. This means that as they are released from the weathering of bedrock, they are attracted to clay and humus particles. The metal ions are held electrostatically by the soil particles.

Now getting the particles back off from the soil particle is what a plant must do in order to mine the soil of its mineral nutrients. This job is carried out by the microscopic root hair cells of the root epidermis. These cells grow out from the root among the soil particles. The hair cells do not have cutin or suberin and so can take in water and minerals that they find in the capillary spaces in the soil. But root hairs are far more active in mineral uptake than just absorption! The root hairs secrete hydrogen ions (H+) into the water film. Now a hydrogen ion also has a positive charge and so it too is attracted to the clay and humus particles. But the mineral ions are already there! What happens next is caused cation exchange, because one positively charged ion (cation) is exchanged for another. The hydrogen ions from the root approach the clay particle, slip under the large metal ion, neutralize the charge holding the metal ion in place, and the metal ions breaks free going into the water film and from there into the root hair, the root xylem, and up the plant. The root hair thus has mined a mineral ion from the soil particle by cation exchange.

This exchange process reminds me of the defeat of the Spanish Armada. Perhaps you recall this from history class. The Brits and Dutch were in trouble for pirating and other anti-government activities while under Spanish rule. King Phillip was ticked off and sent a huge fleet of galleons (large warships) to England to put down the revolt. At the time Spain was rolling in the riches stolen from the native peoples of Central and Southern America. So it could afford huge ships loaded with cannon and horses, soldiers and weapons. They were big, tall, and heavy sailing ships. Quite intimidating to the Brit and Dutch sailors in their comparatively tiny boats. The outlook for the sea battle was grim. The Armada arrived in the English Channel. The Spaniards were planning on making their attack in the morning and were celebrating the future victory in their galleons. The brilliant Brit and Dutch sailors realized they were no match for the fire power of these ships, but realized their weakness and exploited it. At night, the rebel boats sailed to the sides of the galleons without a shot. Now as morning broke the rebels were underneath the Spanish cannons up on the high decks. Cannons cannot shoot down. But the cannons on the rebel boats could be fired at will into the hull of the galleon. The entire fleet was sunk at anchor in the channel! How embarrassing for the Spanish! Now what does this have to do with cation exchange?

Well the soil water is the English Channel, and its surface is where the ships are. The galleons are the huge metal ions; some have multiple positive charges (better guns) but have a weakness, the big size of the metal ion is a disadvantage for staying attached to the soil particle (surface of the water). So the metal ions (galleons) are awesome compared to the rebel boats (the Hydrogen ions), but the hydrogen ions can get below those positive charges on the big metal ions thanks to the small size of the hydrogen ion (smallest in the universe!). It neutralizes the charge underneath the metal ion, its attraction force (buoyancy) to the clay particle, and it is released from the surface (sinks) and joins the water film (the English Channel).

Well this cation exchange story is certainly interesting and it explains how plants normally accomplish their mining task at the root. But now what is the connection to soil pH? Well, if our picture of how cation exchange works is supplemented by acid rain, then the sinking of galleons is going to be massive! The infusion of much acid (H+) into the soil water film means that all of the minerals might be released at once into the film, cannot be taken up immediately and leach through the soil, into the stream, and out to Long Island sound.

So the cation exchange story tells us how acid rain negatively impacts plants because it turns what might have been "fertile" soil and impoverishes it suddenly and completely. This is why it is so important to control the soil pH before you do any fertilization. We need to make it possible for the soil to hang onto the minerals so that the roots can then release them and take them up into the plants. So by getting the soil closer to pH 7 we are improving the fertility of our soil. As rock weathering releases ions, then this soil can hang onto it until a plant can mine it.

So soil that is good for plants has a more-or-less neutral pH (near pH 7). Acidic soils (pH 4) or alkaline soils (pH 10) are either stripped of nutrients or will not release them to plants. How would one know the pH of soil? Well in laboratory we will use some chemical dyes to determine the pH of our soil samples. The original farmers from Europe had no way to do these kinds of test. They had no high-tech pH meters either. So they used the pH sensors that were evolution's gift to humans! Our tongues are loaded with pH sensors. The acid sensors are built into the tip of the tongue and acids give our brains that "this is sour" signal. Shock Tarts&tm; and Smarties&tm; give us these signals. Vinegar and other acids likewise give these signals. We like food with sour tastes. We like ketchup on our fries and dressing on our salads. The alkaline sensors are on the back of the tongue (toward the throat). These send signals of "this is bitter" to our brains. Again soap is an alkalai, and when I was a kid it was often administered to the tongue as a punishment for using unacceptable vocabulary. If you know that taste you know what alkalai is like. Another examples are the alkaloids in many medicines. An uncoated Tylenol&tm; tablet has this taste. You gag on it. We hate that bitter taste. So the pioneers from Europe would taste a pinch of soil. If it was the wrong texture or tasted sour or bitter this would not be a good place to settle. On the other hand if the texture was good and the taste was neutral (sweet), then this might be good farmland.

Cation exchange also helps explain why nitrogen and phosphorus are usually not found abundantly in soils. Nitrate, nitrite, and phosphate have negative charge and therefore are repelled by the negatively-charged soil particles. So these nutrients percolate to the waterways and are lost to the garden or field. Again, this is why it is not good to fertilize too much...the N and P of the fertilizer cannot be held by the soil particles...even if the soil pH is right! So what is it about potassium? Potassium has a positive charge, so why is it often depleted in soils? Well, it turns out that potassium ions are just about the largest ones on earth...and they possess only a single positive charge. In other words they are the hugest galleon in the armada and have just one cannon...it is the most easily sunk. Potassium is the cation that is easiest to release by hydrogen ions. So soils are often stripped of this ion.

Soil Horizons

So where to the leached ions go? Well the soil is formed in layers. The top layer (Horizon A) of soil is known as the topsoil. This is exposed directly to rain and generally leached of its nutrients by acid rain and the active growth of roots. The many living organisms and their side products and breakdown products make the topsoil very dark in color. Some would say it is black. It is certainly loaded with degrating organic particles. These help mitigate the leaching. So topsoil is sometimes considered "rich" but is really rich only if the pH has been kept corrected over a long period of time.

The next layer of soil (Horizon B) is where the minerals leached from topsoil accumulate. This layer is often called the subsoil. It is typically a red/brown color. It looks less rich, but in unmaintained soils is actually richer than the topsoil. When you are planting trees in a landscape we recommend digging a $50 hole for a $5 tree and mixing the recovered soil and sifting the gravel and rocks out of it, before filling in around the roots of the tree. This way you get the advantage of the organics in the topsoil along with the higher nutrient content of the subsoil.

The layer below the subsoil is called weathered bedrock. This is a very rocky layer and maybe packed or too deep for roots to penetrate. This is the source of new mineral ions however. So if the subsoil and topsoil are at correct pH, they can be replenished in minerals from Horizon C.

The lowest horizon (D) is the bedrock. Here in Eastern Connecticut this layer is granite. Our soil would be richer if our bedrock were limestone. In other parts of the country the bedrock is limestone and the crops thrive there!

blackHorizon Aleaching zone
organics here - most roots
these layers
lost by
erosion, esp
tropical soils
"red"Horizon Baccumulation zone
nutrients collect here
rockyHorizon Cweathered bedrock
solidHorizon Dbedrock

Obviously if we want good crops over time, we need to take good care of the pH and nutrient levels in soil. But there is one other precaution we must take too. We need to control loss of farmland in order to keep feeding the world populations. A lot of our soil is eroded by wind or rain and is lost to us. Mark Twain wrote of the Mississippi River which drains much of the US farmlands. In his novels he called the river the "Big Muddy" because of all the clay and silt that is flowing down the river into the Gulf of Mexico. If you have seen this magnificent river you know what he is writing about. The Louisiana delta is where some of the horizon A and B soils end up. The organics decay in the submerged sediments and produce natural gas and oil. We are now mining this from there with offshore rigs. But the loss of farmlands is destroying our agriculturally based economy. Just like the Egyptians not realizing that the Nile flooding was the source of its agricultural productivity in ancient times, we are somehow missing the root of our national prominence. We are letting it go down the river.

Obviously as educated people, we must develop ways to do our agriculture without laying the soil bare to wind and rain for long periods of time. We need to cultivate the soil cautiously. We need to use mulches or ground covers more effectively. We also need to develop agricultural crops that are perennial rather than annual. Corn requires yearly plowing leading to erosion because it is an annual crop. Its closest wild relative is a perennial. If we can get the perenniation genes from Zea diploperennis into Zea mays, then we can have corn fields that do not need to be plowed. The corn would come up year after year and just need weed control and harvesting! This would save much soil as the tangle of perennial roots would hold the soil firmly against wind and rain. It would save on fuel costs for plowing and planting. It would reduce pollution of rivers. Science is at work on such progress and we need the society to understand the need to fund research at the USDA and NSF to accomplish these important goals.

 

This page © Ross E. Koning 1994.

 

 

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