Macronutrient Elements

There are nine essential macronutrient elements needed by plants to thrive. Three (carbon, hydrogen, and oxygen) are typically available in abundant supply and are derived from the air and water. The remaining six (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur) are much less abundant and are derived from the soil.

Primary Nutrient Elements

Nitrogen (N), phosphorus (P), and potassium (K) are often referred to as primary nutrient elements.

Nitrogen

Nitrogen is used by plants in large quantities, and a nitrogen deficiency is most often responsible for limiting the growth of plants. Nitrogen stimulates growth in plants, increases the number of seeds and fruit produced, and improves the quality of crops grown for leaves and forage.

Chlorophyll is the compound found in plants that reflects green-yellow light, giving most plants their green color. It is made of carbon, oxygen, hydrogen, nitrogen, and magnesium. Carbon, oxygen, and hydrogen are available to plants in abundance from water and air. However, nitrogen and magnesium are not so abundant in soil. A deficiency in nitrogen is a common cause of yellowing of leaves in plants, though a deficiency in magnesium can cause similar effects.

Photosynthesis is the process by which green plants turn light energy into chemical energy. Plants absorb light, water, and carbon dioxide, and produce oxygen and carbohydrates (glucose or sugars) used as food by the plants to stimulate growth. Chlorophyll (and therefore nitrogen) play a key role in this process as it is the chlorophyll that allows the plants to absorb light energy.

Most soils simply cannot provide enough nitrogen to plants. However, nitrogen levels in soil can be supplemented using fertilizers, though this should be done in a responsible manner to minimize environmental pollution.

Nitrogen can occur in the soil in three forms: organic nitrogen, inorganic nitrogen, and elemental nitrogen.

Organic Nitrogen

About 98% of all nitrogen in the soil occurs in the form of organic nitrogen. About 5% of the overall weight of organic matter in the soil (humus) is from this organic nitrogen. Though plants cannot directly consume it in its organic form, organic nitrogen gets converted to inorganic ammonium (NH₄⁺) during the decomposition of organic matter through a process called mineralization. The inorganic ammonium produced during decomposition can then be absorbed by plants to provide them a source of nitrogen or released back into the atmosphere as nitrogen gas (N₂).

The reverse can also happen, where inorganic nitrogen compounds (ammonium and nitrate) gets converted to organic nitrogen through a process called immobilization. This occurs when new organic matter is added to soil and there is not enough nitrogen in that organic matter to support the production of proteins within the microorganisms which decompose the matter. When this occurs, the microorganisms consume the inorganic nitrogen compounds from the soil, lowering the amount of inorganic nitrogen in the soil for consumption by plants. When the bacterial microorganisms responsible for decomposition of the organic matter eventually die and decompose themselves, much of the nitrogen is returned to the soil.

Organic nitrogen can be added to the soil by the application of organic fertilizers like manures or sewage sludge produced by wastewater plants. These are often popular because the mineralization process that converts the organic nitrogen to plant-usable inorganic nitrogen (ammonium) is a relatively slow process.

Inorganic Nitrogen

Most fertilizers used for agricultural crops contain inorganic nitrogen in the form of ammonium (NH₄⁺), nitrate (NO₃^-), and urea (CO[NH₂]₂). Urea is naturally produced by many animals as a metabolic byproduct (for example, it occurs in organic manure), and this type of urea is considered an organic fertilizer. But it can also be synthetically produced and purchased by farmers as fertilizer. Though it is technically an organic compound (i.e. contains carbon), it quickly turns into inorganic nitrogen and “acts” as if it is an inorganic nitrogen fertilizer. Urea has a very high capacity to provide nitrogen to plants and is the most powerful fertilizer.

In soils that are warm and moist where the pH is about 6, most ammonium in the soil is quickly converted to nitrate by soil organisms. Plants primarily consume inorganic nitrogen in the form of nitrates, though they do consume ammonium nitrogen if it is in the soil solution.

Nitrates are negatively charged anions and therefore cannot be adsorbed by the surface of soil particles, so it exists in the soil solution trapped in the pores between soil particles. Because nitrates dissolve quickly in the soil solution, it is easy for plants to consume nitrates. However, nitrates in the soil are easily lost from leaching when there is lots of rain or irrigation. Leaching can also be a problem in coarse sandy soils. Nitrates can also be lost through a process called denitrification, consisting of a series of chemical reactions in the soil that end in the release of nitrogen oxide (NO) or elemental nitrogen (N₂) gas.

Elemental Nitrogen

Elemental nitrogen gas (N₂) is found in the air between soil particles. While it cannot be directly consumed by plants, elemental nitrogen can be converted to other nitrogen compounds capable of being consumed by plants through a process called fixation. For example, bacteria often live on and inside plant roots which can pull elemental nitrogen gas from the air between soil particles and convert it to nitrogen compounds usable by plants.

Phosphorus

Phosphorus is very important to photosynthesis in plants. It is necessary for the transfer of light energy to chemical energy. Phosphorus is needed in the active portion of the plant. For young plants, it is translocated from older tissue to new tissue so that it can be concentrated in the areas of new growth. As plants mature, phosphorus is concentrated in the plant’s seeds and fruit.

Phosphorus is used by plants to help process starches and sugars, food for plants produced through photosynthesis. It is also responsible for many cell-level activities such as the formation of cell nuclei, cell fusion and multiplication, and cell organization. It is a vital component of DNA, the genetic code of all living things. Phosphorus is also a vital component of RNA, the compound that interprets the DNA code to build proteins and other compounds essential for plant structure. It has been shown to stimulate root development, improve stalk and stem development and strength, and to improve flower formation and seed production. Phosphorus increases the nitrogen-fixing capacity of legumes and increases the resistance of plants to diseases. Once inside plants, phosphorus is mobile and can be translocated from older tissue to new tissue as needed.

Plants absorb phosphorus as phosphate ions (PO₄^3-), which are dissolved in the soil solution. Plants only need the soil solution to contain a few phosphate ions per million parts of water to stimulate adequate plant growth. As plants remove these phosphate ions from the soil solution, they are replenished through the breaking down of rocks into soil minerals, the decomposition of organic matter, and the application of fertilizers containing phosphorus.

Phosphorus is most available to plants at pH levels ranging from 6 to 7. It is important that you mix it in well when adding it to soil. This should be done before planting so that it is available for absorption by the roots of the plants. Below is a chart showing phosphorus fertilizers and their percentage of phosphorus pentoxide (P₂O₅), the anhydride of phosphoric acid:

Fertilizer	Percentage of P₂O₅	Speed of Release of P
Ammoniated phosphates	46+	Rapid (water soluble)
Triple superphosphate	46	Rapid (moderately water soluble)
Ordinary superphosphate	20	Rapid (moderately water soluble)
Rock phosphate	30	Very slow
Colloidal rock phosphate	20	Very slow
Bonemeal	24	Slow

Phosphorus Fertilizers and Characteristics

Potassium

Potassium (K) is absorbed by plants in larger quantities than all other essential elements except nitrogen, and in some cases calcium. While nitrogen and phosphorus are found in organic combination with plant tissues, potassium is not. Potassium is necessary in adequate amounts for reactions related to several metabolic processes in plants. It is necessary for plants to metabolize carbohydrates (sugars and starches) created through photosynthesis thus converting them to chemical energy. Potassium also increases the resistance of plants to diseases and helps make some plants more drought tolerant.

Potassium occurs in soil solution as a cation (K+). The amount of potassium that is retained by the soil particles through adsorption depends on the cation exchange capacity of the soil. If the CEC is low, then under conditions of high rainfall, potassium can be leached from the soil.

Fertilizer	Percentage of K₂O	Speed of Release of K
Potassium chloride	60	Rapid (water soluble)
Potassium sulfate	52	Rapid (water soluble)
Potassium magnesium sulfate	21	Rapid (water soluble)
Potassium nitrate	44	Rapid (water soluble)
Wood ashes	5-10	Rapid (water soluble)
Seaweed	6	Rapid (water soluble)
Farm manures	2	Rapid (water soluble)

Potassium Fertilizers and Characteristics

Secondary Nutrient Elements

Calcium (Ca), magnesium (Mg), and sulfur(S) are often called secondary nutrient elements. These elements are called secondary because while they are required for proper plant growth, they are typically needed in lesser amounts.

Calcium

Calcium is important to plants for many reasons, but there is a delicate balance that must be maintained between the amounts of calcium, magnesium, and potassium in plants. As a soil supplement, calcium reduces soil salinity and improves water penetration. It is necessary for plant growth and development. Once absorbed by plants, calcium in the form of calcium pectate becomes an essential part of cell wall structure, holding them together and giving them strength. It increases resistance to disease and is important in in several aspects of photosynthesis such as activating certain needed enzymes and proteins.

Calcium is absorbed by plants from soil solution where it occurs as a positively charged cation ion (Ca²⁺). It can be supplied to soil as naturally occurring soil minerals, organic matter, fertilizers, and liming materials like calcitic and dolomitic limestone. Cation exchange sites on most soil particles have a strong affinity for Ca²⁺ cations, making it the most common cation retained in most soils with a pH of 6 or more.

In acidic soils where the pH needs to be raised, adding liming materials like calcitic and dolomitic limestone that contain calcium not only increases the pH, but also supplies calcium to the soil. If the soil’s pH is within range but suffers from a calcium deficiency, then gypsum (CaSO₄[H₂O]₂) can be used to supply calcium to the soil without changing its pH level. Ordinary superphosphate and triple superphosphate can also be used.

Magnesium

Magnesium is one of the elements that combine to make chlorophyll. As such, its presence is important for photosynthesis to occur. It also helps activate many enzymes in plants necessary for growth. Magnesium is absorbed from the earth as the divalent cation Mg²⁺. It is a relatively mobile element within plants and is easily moved from older to younger parts of the plant where new growth is occurring.

Magnesium is applied to soil most commonly in dolomitic limestone, which also contains calcium. Because dolomitic limestone contains both calcium and magnesium, it is great for neutralizing acidic soil.

Sulfur

Sulfur is primarily absorbed by plants in the form of sulfate (SO₄^2-) and is used by the plant to create various organic compounds. Sulfur is released into most soils when organic matter like humus and crop residues decompose. Because sulfur is an anion instead of a cation, it is held in the soil solution until it is absorbed by the plant. This makes it subject to leaching and microbial immobilization.