All soils have the origin story in geology. It is from rocks where soils originate and, over vast spans of time, in combination with the weathering effects of the sun, water, wind, and freezing temperatures rocks have been broken down into smaller and smaller pieces. This weathering process has also been influenced by biological processes like those carried out by fungi and lichens which secrete acids allowing for the rock to become bio-available for the organisms to eat. As the parent rock is broken down into smaller and smaller pieces it can transition from being a boulder, to a rocks and gravel, to sand, silt and finally clay. It is often spoken of sand being so small it will just slip through the gaps between one’s fingers, but sand particles can be 40 times larger than silt particles and silt particles can themselves be over 25 times larger than clay particles. These mineral particles compose about 45-50% of soil; in addition to the roughly 0-5% of biological matter, and another 50% of pore space filled with water and air.
The minerals that are contained within the soil results in the soils color and there are entire books like the Munsell Soil Color Charts, which contain color charts, like paint swaths, that are used to determine the mineral constituents and possibly how well draining the soil has been in the past. For example, a soil that had a reddish color could be an indication of high quantities of iron. These colors and ultimately the mineral composition of the soil will provide a basic understanding if the soil presents any excesses or deficiencies when beginning to grow plants. Where geological process move very slowly, with an understanding of what is needed in soils an individual is able to amend the soil with the correct quantity of necessary elements so that the plants and everything else thriving in the soil is able to have everything that is needed for life.
Just as with plants and animals it is appropriate to think of there being essential nutrients for healthy soils. A child might be able to run around for much of the day after eating a bunch of sugary foods and energy drinks, but this is not the recipe for lasting sustained growth. Similarly, plants are able to show a flurry of activity after the application of artificial fertilizers, but this is not going to result in sustained growth for the plants or the planet. There are three nutrients that have long received the focus as being the most important for healthy growth, this is what is seen on the sides of bags of fertilizer; and these are Nitrogen (N), Phosphorus (P), and Potassium (K). These three primary nutrients are very important, but there are others that are also very important such as the secondary nutrients Calcium (Ca), Magnesium (Mg), and Sulfur (S); and the seven micro-nutrients Zinc (Zn), Manganese (Mn), Iron (Fe), Boron (B), Chlorine (Cl), Copper (Cu), and Molybdenum (Mo). And where we might not yet understand how or why the other elements are important it has been suggested by soil scientist that through the long process of evolution that the soil biology has found a productive use for all of the elements.
Nitrogen is critical for plant growth. When the soil nitrogen levels begin to fall a plant’s leaves may turn pale green or yellowish around the older leaves which can be a sign that the plant is cannibalizing itself by stripping nitrogen from those other leaves so it is able to continue to grow. However, if too much nitrogen is added to the soil or added at the wrong time there can also be problems. Nitrogen is usable to plants in two forms ammonium (NH4+) and nitrate (NO3-). With ammonium being positively charged it is attracted to the negatively charged soil particles and as a result is more resistant to leaching and being washed out of the soil. Due to nitrate being negatively charged it is much more quickly flushed through the soil, especially when there is large quantities of precipitation, irrigation or if the soils are sandy. If the nitrogen is added in the fall, as is done on many farms in the East of the US in the form of cow manure, the cold temperatures may prevent the soil biology from metabolizing the nitrogen therein allowing it to be largely washed away before it is able to be put to use, leading to increasingly polluted waterways. If excess nitrogen is able to make its way into the active pathways of the plants it can actually do harm to the plant in a way resembling a burn.
Phosphorus is essential for root growth, fruit development and disease resistance. Phosphorus doesn’t move in the soil so once it is added to the soil a few time there is often no need to continue making additional applications, and because excess quantities in the soil can lead to problems for plants it is good to find out through testing if it is needed prior to application. In soils that are highly acid the phosphorus can get locked up, but through raising the soil pH with something like lime, the soil phosphorus can be released and therein available to the plants. Also, phosphorus is less available to plants when the soil temperatures are cool, so if amending with phosphorus, it can be helpful to apply it in the spring as a starter for herbs, flowers and vegetables. Because it does not move through the soil and is helpful for root growth it is useful to work the phosphorus amendments down into the soil instead of just leaving it on the surface. This is most easily done when a plant is first being planted that way it will be present in the plant’s root zone.
Potassium helps the plant become more resistant to disease, droughts and the cold. Potassium is also called potash due to wood ash being a good natural source, as well as kelp meal, potassium sulfate, and granite dust. More potassium is available to plants in arid climates than in rainy ones because without the precipitation the salts build up in the soil due to not being flushed out by the water. An excess of potassium in the soil can lead to there also being problems with an excess of salt in the soil.
Calcium is an important component of cell walls.
Magnesium is the core of the chlorophyll molecule making it essential for the process of photosynthesis to be performed.
Sulfur helps to build proteins and can be used to increase the soil’s acidity.
pH is also an important consideration when looking at the health of the soil and the organisms supporting and being supported by the soil. It is something that is easy and inexpensive for an individual to test with a simple test kit sold at nurseries or garden stores. pH refers to potential (p) Hydrogen (H), and reflects the electrical charge of something, here the soil particles or water. Hydrogen alone has only one electron in its outer shell, but that shell wants to be filled with two electrons not just one, so it is as if a single Hydrogen is missing an electron (which is negative -)and as a result is the single hydrogen having a change that is positive+.
It is often not necessary to understand all of the ways that pH effect soils, but it is worth noting that as the pH goes up and down the charge can become more positive or more negative. This change of being more positive or negative effects the other elements in the soil, because, like Hydrogen, they also have charges and can result in getting stuck in the soil due to the charges being opposite, be made bio-available to the organisms in the soils, or be somewhat repelled from the soil due to similar charges and as a result flushed out of the soil. If a soil test is taken it may show there is too much of an element but if the element is locked up, the plant might actually show signs of a deficiency. So if the pH of the soil (or the water going on to the soil) is slightly off, the plants might not be able to get the nutrient that they need, even if they have all been applied to the soil.
The pH scale ranges from 0-14 and is a logarithmic scale. This means that the each number on the scale is 10 times more alkaline or acidic then the next number on the scale. The number 0 represents something that is very acidic, 7 represents the point of being a neutral pH, and 14 represents something that is very alkaline or basic. When growing a particular plant it can be very helpful to understand the pH that the plant prefers. For instance blueberries like a very acidic soil (pH 4.5 -5.5), most vegetables thrive in a slightly acidic soil (pH 6-7), whereas members of the cabbage family prefer neutral to slightly alkaline soils (pH 7or slightly higher). For soils that are too acidic the pH can be raised through liming the soil with pelletized dolomite limestone. For conditions where the soils are too alkaline sulfur can be used as an amendment. In general, a good pH for growing most things will be between 5.8 and 6.8.
It could be noted that the nutrients that are mentioned as being necessary for healthy soil are those which are actually necessary for healthy plant. It is very much the case that for there to be healthy soil there needs to be healthy organism living in the soil. Soil is a dynamically alive ecosystem and therefore needs living organisms. Plants in essence feed on sunlight, and in combination with water and the carbon dioxide they breathe in, the plants are able to actively process these components in photosynthesis to produce plant sugars. It is these plant sugars that are one of the main sources which help promote and support life in the soil, and as a result contribute to healthy soils. Healthy soil well composed of necessary element helps result in healthy plants which are themselves nutrient dense; because, where plants are able to produce vitamins, fats, sugars, and proteins, if elements are not in the soils, it is not going to be in the plants.
Without there being some biological activity within the soil there would be merely dirt. Life, in fact, is one of the main components for there being soil, and the amount of life in soil is very significant. There are more living organisms in a teaspoon of soil than in all of the humans who have ever lived on Earth.
Soil is a dynamically active ecosystem populated with incredibly vast numbers of organisms. In healthy soils these organisms consist of bacterial, fungi, protozoa, nematodes, arthropods and worms, and collectively their numbers can be in the trillions in just a cup of soil. The role of these organisms is very important for maintaining the health of the soil, and their interactions are often reflected through the term ‘soil food web’. The ‘soil food web’ moves the thinking beyond the outdated notion of a food chain and recognizes that everything is a means of food for something else. Being that most things make their way to the ground, with all the diversity of life, there is something that can consume nearly everything natural including bacteria, viruses and many pollutants that wash down drains and into oceans. The essence of this soil food web relationship can be identified by considering the sources of input into the soil system.
The soil biology works in a reciprocal relationship with the plants, specifically the roots but also the leaves, fruit and eventual dead tree that falls to the ground, to share resources gathered from the sun with resources gathered from the Earth. Plants feed on sunshine, however that is not all they need. Through sunshine, water and the carbon dioxide that is taken in from the air, the plants convert the carbon, oxygen and hydrogen in to carbohydrates or plant sugars. More than half of the carbohydrates that are produced by a plant are actually given off by the roots like a waste product.
These precious resources produced by the plant are not merely wasted however; the sugars are emitted by the roots and serve as food for soil microorganisms. Fungi in the soil take the carbon from the plant sugars and form strands that can stretch 250 yards forming connections to different plants. The ability of the fungi or mycorrhizae ('myco' - fungi and 'rrhizae' - root) to extend out is so vital for plants as it can expand the surface area of the plant's roots by over 50 times due to the sharing that takes place around the rhizosphere (the miniature ecosystem right around the plants roots). With this increase in surface area the root is able absorb far more water and nutrients than it would be able to with just its roots alone. The fungi tip or hyphae can produce digestive substances which it uses to break down parent rock material. The fungi do not have a mouth so it breaks up the minerals to the point that they are able to be absorbed through its cell walls. These minerals are then able to be transported anywhere throughout the strands of the mycorrhiza. The mycorrhiza receives a good meal from plant roots, but if the plant is not healthy it will not receive a good meal; so the mycorrhiza in detecting a lack of some soil nutrient around a plant will transfer the minerals to plants so as to keep the system in balance.
The concept of companion planting and plant communities also utilizes some of these reciprocal relationships and in some cases it has been observed how some nutrients are even able to be shared from one plant to another over 100 feet away through the transport relays set up by the mycorrhiza. And as all of these process go on in the soil they are being supported largely by the plant sugars coming as root exudate, and it is through this process that carbon originally taken from the air is able to be locked up by the plants and sequestered in great quantities into the soil.
So, for healthier plants and cleaner air and water it is important to understand how the countless numbers of organisms are functioning under every bit of Earth.
This understanding of the relationships influencing these organisms is also important because similar to beautiful animals going extinct on the land and in the seas, there are also whole soil systems or series which are going and have recently gone extinct. Unlike a single animal which may go extinct, when a soil series (a series is a classification for a soil in an area) goes extinct it can mean that tens of thousands of different species that have formed a unique adaptive relationship are going extinct. When an animal is threatened with extinction it can be brought to a zoo or some sanctuary and cared for, however with a soil series they cannot be removed from their unique environment. If there is real interest in saving animals from extinction, there needs to be attention paid to ensuring the animals have food to eat to survive. When traced back to their source photosynthesizing plants are the base of nearly all food, and for nearly all plants to survive there needs to be healthy soils. So to help stop species loss, it is important to help protect soils and ensure that the rates of soil loss are reduced as much as possible and stopped as soon as possible.
Depletion, Erosion and Death
There is a direct relationship between the growth that is seen in a plant and what is being extracted from the ground around the plant. Therefore as the plant increases in size the surrounding soil decreases in the quantity of nutrients. If this was the whole story it would not be good for the plant; however, evolution has supplied ways for keeping this from becoming a real issue. In a natural setting there are typically several plants growing together, often called a plant community. People over time have modeled naturally forming plant communities with specifically chosen plants and came up with beautiful beneficial plant groupings, such as, the ‘three sisters’ combination of corn, beans and squash. These plants, in this example, are planted together and work together. The corn grows tall and provides a vertical support for the beans to climb upon. The large squash leaves cover the ground to help retain moisture in the soil and prevent weed seeds from germinating. While, the beans fixing nitrogen help fertilize the soil. In nature, similar arrangements have naturally worked themselves out and what is seen is a mostly contained loop where the plants grow in the area and some part or all of the plant falls back to the ground and acts to continue feeding the rest of the members of the group.
Historically many of the world’s most fertile regions for agriculture have been in river valleys. Similarly, the fertility of these areas is the result of plants falling to the ground and breaking down into food for soil microorganisms and therein for plants. The unique feature about these river valley areas is that the nearby rivers would pick up all of these nutrient accumulations from the ground and deposit them along the flooded areas near the rivers. Unfortunately, many of these rivers have been diverted or damned for irrigation or navigation, and therefore, have been substantially diminished due to changes in their head waters, furthermore, this has prevented the regular deposit of nutrients by the river. This situation leads to a slow decline of the soil, because the plants continue to take material out of the soil when the river no longer deposits nutrients back into to the soil. The depletion of nutrients in the soil prevents it from maintaining optimal health, and therefore, does not allow the soil to support growing healthy plants.
Some of these degrading soils can be artificially supported for some time by bringing in and adding fertilizers, but these fertilizers often diminish the health and quantity of the soil microorganisms, which result in the need for more and more fertilizer to generate the same level of output. To a greater consequence, in the areas where the soils are the most severally damaged due to unsustainable agriculture or industrial disregard, we are seeing vast numbers of cases where soils are going extinct. This is a very alarming situation, because soils are harmonious collections of thousands to millions of different species of organisms; so as a soil series goes extinct it is not just reflecting the loss of a single species, but the loss of untold numbers of species. The formation of soils is cultivated through the long processes of evolution. Currently, we are unable simulate this natural evolutionary process to grow these soils in labs outside of their native habitat, even if there was a desire to support and preserve these important natural resources and health allies.