How Soil Affects Plant Growth
Biochar Update 1
TLDR of What I learned
1. How nutrients get to the plant.
Nutrients are they are broken down into ions (splitting of a molecule into atoms with positive and others with negative charges) which happens by getting into the water.
Since sand and silt have little to no charge they attract little water and nutrients. Clay and Organic Matter have a negative charge which helps them in attracting the positive hydrogen ions in water.
How roots receive this water is based on a process called cohesion where the positive hydrogen of one H2O molecule is attracted to the negative of an oxygen atom of another H2O molecule. This means that the movement of the molecule in 1 direction would move the second molecule in the same direction.
Since toots can only grow in sand or silt since they have large enough pores to hold the root. The root would absorb the limited water in the sand and silt. Through cohesion, As a water molecule is pulled into the root, it pulls more water into the same direction it moved because of the strong hydrogen bonding of the water.
2. How pH affects plant growth
The reason this is important is that this has been the main concern for biochar (from what I have seen so far). Biochar is usually has a pH of over 7.5 where plants are good around the 6 to 7 range. Decreasing biochar pH is possible so it should not be a problem in the future.
The pH scale measures the number of hydrogen ions in a liquid (in this case water). The issue with pH is that it affects the nutrient levels by making changing them into a useable form for plants (not an ion).
Ex. Imagine a soil that has pH 7 and plenty of nutrients for plants. If we add some hydrogen ions, the pH will become more acidic (decreasing in pH. value). At this lower pH, phosphorus reacts with aluminum and precipitates, reducing the number of both ions in the soil solution. Plants now can’t get enough of either one to grow properly. There is still lots of phosphorus and aluminum in the soil, but it is now in a form that plants can’t use.
Ideal Soil Breakdown
Types of Minerals in Soil (with Organic Matter)
Sand (Large pores) — almost no charge
Silt (Medium pores) — almost no charge
Clay (Small Pores) — Negative charge
Organic matter — Negative charge
Mechanism of How Water Moves Through Soil
Water (H2O) with one end being positive (H) and the other negative (O). You can think of water molecules as being small magnets, with one end being attracted to the other. This sticky nature of water is called cohesion.
How Water Interacts with Minerals and Organic Matter
Hydrogen has a positive charge (since it has a lower electronegativity than Oxygen) while clay and organic matter have negative charges attracting the positive charges of the hydrogen from the water. Creating a strong bond makes it harder for gravity to pull the water down. While Sand and Silt almost have no charge meaning their pores are not able to hold water as long as clay
How Root Gets Water
Since roots grow into large and medium pores (sand and slit). The surface of the root comes in contact with the water layer in the pore. It is then able to absorb limited water from the sand and silt.
As a water molecule is pulled into the root, it pulls more water into the same direction it moved to strong bonding of the water positive and negative charges. This results in a constant flow of water, from areas away from the root to areas toward the root. It is the cohesive nature of water that allows roots to pull water from small pores, even though they are too big to fit into the spaces.
Aggregates
The smaller pieces of sand, silt, and clay have been mixed with OM to form larger structures called aggregates. When the soil has aggregate it will grow lots of plants and vice-versa.
There is a direct relationship between the size of aggregates and the fungal biomass. The microbial biomass decomposes plant and animal residues and soil organic matter to release carbon dioxide and plant-available nutrients. The larger the aggregate, the more plants grown, the more animals eat, the more waste created by both plants and animals.
Clay, iron oxide, and organic matter also act as cement in forming aggregates, but microorganisms provide the best binding agents (used to build up aggregate).
The key to why more aggregate = more yield is through is the release of carbon dioxide into the plants.
Soil pH
The pH scale measures the number of hydrogen ions in a liquid given as a number between 0 and 14. With a ph of 7 as neutral. Anything above 7 is an alkaline low level of hydrogen ions, and below 7 is acidic. Biochar normally seems to be a bad alkaline. Most plants grow best between 5.5 and 7.0 pH. Soil pH ranges from 3.5 to 10.5, but these are extremes; most are between 5 and 8.5.
How pH is measured — Not 100% important, but important to Know
This is common knowledge (the above), but what most people don’t know is that pH is measured on a logarithmic scale. (Do you remember high school math?) This means that a pH of 8 is 10 times more alkaline than a pH of 7; a pH change of 1 unit is an acidity change of 10 units. A change of 2 numbers, for example, 5 to 7, is a change of 100, which is a significant change.
pH Does Not Directly Affect Plants
Plant growth is primarily affected by the level of hydrogen ions (H+ from the water molecule).
pH affects the nutrient levels in the soil solution and therefore influences plant growth.
Nutrients + water = main method of growth of plants.
Types of Minerals
Two main categories:
On-mineral — make up 96% of a plant and consist of oxygen, hydrogen, and carbon. The plant absorbs CO2 from the air, which provides most of the carbon and some of the oxygen it needs, and takes in water and oxygen through the roots.
Mineral Nutrients — although minerals like nitrogen, phosphorus, and potassium, account for only 4% of a plant’s weight, they are vital to its growth.
How Plants Use Minerals
We talk about plants using iron, calcium, and magnesium, but actually, plants can’t use any of these. Putting an iron nail in the soil does nothing to feed plants. They can use these metals only once they are converted into something called ions.
Ex. When calcium is exposed to air, a chemical reaction with oxygen forms a type of rust, producing a white powder, calcium oxide (CaO). When CaO dissolves in water, calcium, and oxygen separate into charged particles that are called ions. The calcium ion has a positive charge (a cation), and the oxygen ion has a negative charge (an anion).
Based on water cohesion, one end of the water molecule has a positive charge, and the other has a negative charge, and it is this property that allows the calcium oxide to separate into two charged particles. The calcium, being positive, is attracted to the negatively charged end of the water molecule, and the oxygen is attracted to others. Once the calcium is in the form of an ion, plants can absorb it through the roots and use it inside the plant.
Ions are created from being dissolved in water. The nutrients that a plant gets come from the water which comes from pores in the soil.
Salt
Definition: The general public uses the term salt to mean table salt, which is sodium chloride. Chemists, soil scientists, and this book use the term to refer to any compound that is made up of ions. Sodium chloride is one of many different types of salts. In water, it breaks up into sodium ions (Na+) and chloride ions (Cl-).
Soil Myth: Salt Kill Soil Microbes
Many believe that the salts in synthetic fertilizer harm soil life, but that is not true. They dissolve in water, forming ions, the same as the ions released from organic material like manure or compost. They are essential for the growth of microbes and plants. Any chemical, no matter how useful, can become toxic at high levels. Provided fertilizers are used in appropriate amounts, they do not harm soil life.
Movement of Nutrients in Soil.
Sand and silt particles have almost no electrical charge on their surface, so ions don’t stick to them very well. When nutrient ions come into contact with these particles, they just keep moving along with the water. For this reason, rain easily washes nutrient ions out of sand and silt into the subsoil layers, which explains why such soils have low natural fertility.
Clay and organic matter have both negative and positive charges, but they are mostly negatively charged. Both anions and cations stick to clay and OM very tightly, preventing water from washing them away. When rain flows through clay soil or soil that contains a lot of organic matter, it does dislodge some nutrients and moves them deeper in the soil, but the effect is minor. Most nutrients remain stuck in place. The net effect is that nutrients move much more slowly in these soils than in sandy soil.
Most movement happens as a result of ions moving into small clay pores which are connected to medium (silt) and large (sand) pores which are where the roots are connected. Over time this supplies water with ions (or salts) that help the plant grow.