Soil Chemistry

Soil chemistry is the study of the chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and environmental factors. Soil is the unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants. The unconsolidated mineral or organic matter on the surface of the Earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro and micro organisms, conditioned by relief, acting on parent material over a period of time. Potting soils are usually mixtures of all these native ingredients mixed together in optimal ratios for plant growth.

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Management of crop nutrients

Organic soil fertility management is based on feeding the soil a rich, complex diet of plant residues, animal manures, and compost. In contrast, conventional agriculture simplifies crop nutrient and soil fertility management by feeding the plant soluble nutrients. Thus, crop nutrition in conventional agriculture can be managed with three pieces of information:

  1. The amounts of nutrients used by the crop during the growing season. 
  2. The amount of plant-available nutrients in the soil.
  3. The amounts of fertilizer nutrients that need to be added to account for differences between crop nutrient needs and available soil nutrients.

Nutrient management in organic systems is more complex. Organic inputs cannot easily be added to the soil to provide the exact balance of nutrients needed by the plant for at least three reasons.

  1. First, many organic inputs (such as cover crops, crop residues, weeds, and compost) are added to the soil for reasons other than fertility management, yet they contribute to the pool of nutrients in the soil.
  2. Second, most organic materials, including compost and manure, have only a small component of soluble nutrients; most of their nutrients must be transformed through biological processes before they become available to plants.
  3. Thirdly, most manures and composts do not have a consistent nutrient content (such as the 10%N-10%P-10%K found on bags of synthetic fertilizers). They also contain a ratio of nutrients different from that needed for optimal plant growth. For example, dairy manure has a nitrogen to phosphorus ratio of approximately 2:1. However, hybrid corn requires eight times more nitrogen than phosphorus.(10)
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One of the most important components of soil is the pH. The pH of soil can be modified by adding different chemicals. Soil pH indicates how acid or alkaline the soil is. The pH scale ranges from 0 to 14. Any substance with a pH near the lower end of the scale is very acidic. Substances in the upper range of the scale have a high alkalinity or are very basic.

The pH of a soil is crucial because crops grow best in a narrow pH range which can vary among crops. For example, blueberries and a few types of flowers grow best when the pH is 5.5 or less. Potatoes, a more familiar crop, grow best with a soil pH range of 5.5 to 6.0. Most garden vegetables, shrubs, trees and lawns grow best when the soil pH is over 6.0 or 6.5. The range between 5.5 and 7.5 is favorable for two reasons. It allows sufficient microorganisms to break down organic matter. It is also the best range for nutrient availability.

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cation exchange capacity

Cation exchange capacity (CEC) is a measure of the soil’s ability to retain and supply nutrients, specifically the positively charged nutrient ions called cations. These include the cations calcium (Ca2+), magnesium (Mg2+), potassium (K1+), ammonium (NH4+), and many of the micronutrients. The ability of soil to hold positively charged nutrients from being leached and lost from soil is important to maintaining soil fertility. Clay and organic matter have a negative charge. They allow the soil to hold these nutrient cations due to the attraction of charges. Soils with high clay or organic matter content will have a higher CEC. Sandy soils tend to have a lower CEC. Low CEC soils are more susceptible to cation nutrient loss through leaching.

CEC is reported in units of milli-equivalents per 100 grams of soil (meq/100 g) and can range from below 5 meq/100 g in sandy, low organic matter soils to over 15meq/100 g in finer textured soils and those high in organic matter. When base saturation is well outside of these ranges it is typically associated with deficient or excessive potassium or very acidic or alkaline soil conditions. Contact Following the fertilizer recommendations provided with your soil report will typically result in base saturation values within normal ranges.

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buffering capacity

This is the ability to withstand rapid pH fluctuation. The greater the buffering capacity, the greater the quantity of acid or base which must be used to alter the pH. Soil types having low buffering capacities include sandy soils with little clay or organic matter. Soils with a higher buffering capacity would have large quantities of mineral clay and organic matter. Therefore, a thick rich soil with a high buffering capacity would require more lime in order to raise the pH.

Iron Deficiency from elevated soil pH.

Iron Deficiency from elevated soil pH.

Azomite, Iron, Sulfur and Dr. Earth round out this farmers addition to his soil chemistry profile

Azomite, Iron, Sulfur and Dr. Earth round out this farmers addition to his soil chemistry profile

improving the chemical properties of your soil

Producers describe soil chemical properties in terms of soil fertility, salinity, and acidity or alkalinity(soil pH). Soil fertility can be subdivided into nutrient availability for plant uptake, the soil’s nutrient-holding capacity, and the balances among plant-available nutrients. You can evaluate your soils for good chemical conditions by monitoring the following characteristics:

  • Soils have a near-neutral pH (unless the soil is used to grow acid-loving crops, such as blueberries or cranberries). Near neutral pH for cannabis has shown to cause problems for nutrient lockout for elements like iron and zinc.
  • Sufficient nutrients are available for productive crop growth, but not in excess that causes plant toxicities or contaminates nearby streams or aquifers.
  • Soil nutrients are in forms available for plant uptake, but they are held sufficiently enough not to be easily leached or carried away by runoff.
  • The availability of plant nutrients is sufficiently balanced to promote healthy plant growth and microbial activity.
  • This objective recognizes that a plant is only as healthy as its most limiting nutrient allows.
  • Soils do not contain toxins or heavy metals in concentrations high enough to inhibit plant growth or the growth of beneficial soil organisms. However, many inorganic nutrients in the cannabis industry carry many different types of heavy metals which can accumulate in the soil and often end up in the finished flowers.
Sulfur deficiency from a highly acidic soil.

Sulfur deficiency from a highly acidic soil.

Soil pH testers are great tools for a farmer

Soil pH testers are great tools for a farmer

Calcium deficiency in a heavily acidic soil.

Calcium deficiency in a heavily acidic soil.

Elemental sulfur can be added to highly alkaline soils to help lower pH.

Elemental sulfur can be added to highly alkaline soils to help lower pH.

Sulfur prills and other amendments can help plants regulate the soil pH through their root zone.

Sulfur prills and other amendments can help plants regulate the soil pH through their root zone.

This grower tried to overwatered and overfed to try and compensate for observed deficiency and made matters worse.

This grower tried to overwatered and overfed to try and compensate for observed deficiency and made matters worse.

Diagnosing Soil Conditions

Problems Caused by Alkaline Soils

The availability of many plant nutrients in soils, including iron, zinc, copper, and manganese, is reduced at high pH values. Iron chlorosis in plants, caused by inadequate iron, is a common problem in alkaline soils.

Phosphate, a macronutrient, may also be limited in these high pH soils due to its precipitation in the soil solution.

The pH of a soil can be readily and inexpensively tested by a soil laboratory. County Extension Agents can give advice on how to sample soil and where to have the samples analyzed. Soil pH test kits may also be purchased and will give an estimate of the soil pH. Third party laboratories can also provide more detailed consulting analysis and reccomnedations for a particular condition.

Treatment of High pH Soil (Alkaline Soils)

Fertilizers and chelates can be added to soil to increase concentrations of plant nutrients. It is important to note that addition of phosphate fertilizer alone will further reduce the availability of other nutrients.

Lowering the pH of alkaline soils, or acidifying the soil, is an option. Elemental sulfur can be added to soil as it forms sulfuric acid when it reacts with water and oxygen in the presence of sulfur-oxidizing bacteria. Iron and aluminum compounds can be added to soil, as they cause the release of hydrogen when they react with water. Sulfuric acid may also be added directly.

Additions of appreciable amounts of organic matter will help to acidify the soil as microbes decompose the material, releasing CO2 which then forms carbonic acid. Organic acids are also released during humus decomposition. Peat and peat moss are highly acidic forms of organic matter but can be costly.

Application of acidifying fertilizers, such as ammonium sulfate, can help lower soil pH. Ammonium is nitrified by soil bacteria into nitrate and hydrogen ions.

Soils naturally containing carbonates, or lime, are very difficult to acidify, and it may take years before a significant change in soil pH is seen. Even then, the carbonatic parent material will continue to weather, producing more soluble carbonate and buffering the soil solution pH.

Problems Caused by Acidic Soils

Soil acidity can be caused by a number of factors:

  • Soils in areas with large amounts of rainfall tend to be acidic because the water leaches basic cations (calcium, magnesium, sodium, and potassium) out of the soil profile, and these cations are then replaced by acidic cations (hydrogen and aluminum).
  • Carbonic acid formed from carbon dioxide and water acidifies soils in high-precipitation areas.
  • Acidic soils tend to be high in iron and aluminum oxides, as they are the slowest minerals to weather in soil. Aluminum in these increasingly acidic soils is solubilized and will combine with water to release additional hydrogen ions (acidity).
  • The soil parent material (or mineral types from which the soil developed) can be a source of acidity in soils.
  • Nitrification of ammonium fertilizers yields hydrogen ions.
  • Acid rain contains nitric and sulfuric acid.
  • Added elemental sulfur oxidizes to form sulfuric acid.
  • Plants take up, and thus remove, basic cations from the soil.
  • Plant roots excrete hydrogen ions in exchange for nutrients in the soil.

Treatment of Low pH Soil (Acidic Soils)

Soil acidity can be ameliorated and the pH of the soil increased by the addition of lime/limestone (calcium carbonate) and similar compounds that have been ground fine for use. Types of lime-like amendments include:

  • Dolomitic limestone
  • Quicklime
  • Hydrated lime
  • Chalk
  • Oystershells
  • Wood ashes

Each lime-like amendment has its benefits and drawbacks, such as effectiveness, price, and purity. Lime is most effective at neutralizing acidity when it is incorporated/tilled into the soil to the full depth of the plow layer or root zone.

Problems Caused by Water Logged Soils

Johnson (2015) states that in flooded soils, the oxygen concentration drops to near zero within 24 hours because water replaces most of air in the soil pore space. Oxygen diffuses much more slowly in water filled pores than in open pores. Roots need oxygen to respire and have normal cell activity. When any remaining oxygen is used up by the roots in flooded or waterlogged soils, they will cease to function normally. Therefore, mineral nutrient uptake and water uptake are reduced or stopped in flooded conditions (plants will often wilt in flooded conditions because roots have shut down). There is also a buildup of ethylene in flooded soils; an excess of this plant hormone can cause leaf drop and premature senescence. Lack of root function and movement of water and calcium in the plant will lead to calcium related disorders in plants.

In general, if flooding or waterlogging lasts for less than 48 hours, most vegetable crops can recover. Longer periods will lead to high amounts of root death and lower chances of recovery.