The ABCs and NPKs of Plant Nutrition
Plant Elements from Air and Water
There are 16 elements necessary for plant growth. About 80 to 90 percent of the fresh weight of a living plant is water, H2O. Thus, hydrogen and oxygen are two of the elements needed in largest quantities. Without water, life as we know it could not exist. It stands to reason that providing a plant with adequate moisture is the first and most important step in having a healthy plant.
If you dried a living plant to remove most of the free water, about 95 percent of the plant’s dry weight would be composed of carbon, hydrogen, and oxygen. About half of the dry weight would be carbon, derived from carbon dioxide (CO2) in the air. Green plants combine water and CO2 in the presence of chlorophyll and light energy to form carbohydrates, a process known as photosynthesis.
Plant Elements from the Soil
The other 5 percent dry weight contains the 13 essential elements for plant growth obtained from soil. More commonly called essential plant nutrients, these are divided into three groups based on the relative quantity of each nutrient plants need and the relative occurrence of deficiencies worldwide.
- Primary macronutrients are needed in relatively large quantities, often exceeding 100 pounds per acre per year for a vegetable crop.
- Secondary smaller nutrients are typically needed in much smaller quantities.
- Micronutrients are just as important to total plant nutrition as the others, but a little bit goes a long way.
An acre of crops usually takes up less than ½ pound of each micronutrient. Soil fertility problems are more often related to primary and secondary macronutrients or soil pH interactions. Most Alabama soils contain adequate quantities of micronutrients.
Primary Plant Nutrients
Nitrogen for Green, Leafy Growth
Nitrogen is the growth element that promotes green, leafy growth. As a primary component of proteins, nitrogen is part of every living cell. Therefore, this element is usually more responsible for increasing plant growth than any other nutrient. Inside the plant, nitrogen is converted into amino acids, the building blocks for proteins. Because all enzymes are proteins, nitrogen is necessary for enzymatic reactions in plants. As part of the chlorophyll molecule, nitrogen is directly involved in photosynthesis. It helps the plant produce and use carbohydrates. It is a part of plant DNA. The percentage concentration of nitrogen in fertilizers is the first number listed on a fertilizer bag or box.
Inadequate nitrogen causes light green or yellowish foliage; slower, stunted growth; and shedding of older leaves in some plants. The yellowing appears first on the oldest leaves, then on younger ones as the deficiency becomes more severe. A deficiency can be easily corrected with nitrogen fertilizers. Overfertilizing with nitrogen can cause excessing vegetative growth, lodging (falling over), and poor flowering and fruit set in many plants.
Nitrogen is very mobile in the soil, and nitrogen in the form of nitrate has a tendency to leach away from the root zone. Nitrogen can also be lost to the atmosphere. Certain fertilizers, particularly urea, are easily converted to volatile forms of nitrogen if they are watered or incorporated into the soil after application. Soil testing for nitrogen is not reliable, because it is so mobile in the soil. Timing of nitrogen fertilizer application should be crop-specific Excessive nitrates in drinking water is an environmental concern—another reason to maximize nitrogen utilization by plants and minimize excessive applications.
Phosphorus for Energy Transfer
Phosphorus is essential in energy transformations in the plant. Without adequate phosphorus, carbohydrates manufactured in the leaves could not be transported to the flower or developing fruit, or stored in roots or bulbs. Phosphorus is usually associated with flowers, fruiting, and carbohydrate storage in roots, tubers, and bulbs.
Phosphorus deficiency is difficult to detect in most plants because it results in an overall stunted plant, mimicking other health problems. With severe deficiency, dead areas may develop on the leaves, fruit, and stems. Older leaves will be affected before younger ones as phosphorus moves to the growing part of the plant. A purple or reddish color may be seen on deficient corn plants. Because phosphorus is not mobile in the soil, correcting a deficiency once the plant is growing is often not practical. Proper phosphorus fertilization at planting time avoids phosphorus nutritional problems during the season.
Unlike nitrogen, phosphorus is not very mobile in soil. This makes phosphorus an easy nutrient to analyze in the lab. If it has built up in the soil, don’t add any more. If it is low, add some.
High phosphorus in the landscape can cause surface water pollution problems. Excess phosphorus in surface waters promotes excessive growth of algae and other aquatic vegetation. As this vegetation dies, decomposition takes oxygen out of the water, which can cause fish kills. This process, called eutrophication, is the result of excessive phosphorus runoff into lakes and streams, especially where soil erosion is also a problem. This is the main reason for concern about over fertilization with phosphorus-containing and organic fertilizers.
Potassium for General Health
Potassium is essential for photosynthesis, as regulation of cell turgidity, respiration, and water movement in the plant. It also controls the opening and closing of the plant’s stomata. Adequate potassium fertilization helps plants cope with drought stress, increases disease resistance, improves winter hardiness, and improves crop quality.
One of the most common potassium deficiency signs is yellowing along the leaf margins of older leaves. Plants deficient in potassium grow slowly and have poorly developed root systems and weak stalks. Therefore, lodging is common in potassium-deficient plants. Usually, by the time a deficiency is observed on annuals, potassium fertilization is of little value for the current season. Fertilizer potassium is water-soluble, and the plant will take it up in proportion to that available in the soil. Because potassium is taken up as a cation, excess potassium may compete with the uptake of other cations, such as magnesium or calcium.
Potassium is taken up as a cation and is held by the soil’s cation exchange capacity. Soils with high clay or organic matter have a higher cation exchange capacity and can retain more potassium. Potassium is relatively easy to test in the lab.
Secondary Plant Nutrients
Calcium stimulates root and leaf development. It forms compounds that are part of cell walls and strengthens plant structure. Calcium helps reduce plant nitrates by setting in motion several enzyme systems that neutralize organic acids in the plant. Calcium also promotes root growth, molybdenum availability, and uptake of other nutrients. It indirectly promotes yields by reducing the toxicity of aluminum and manganese in the soil.
Poor root growth is a common sign of calcium deficiency. In severe cases, the growing point dies. Calcium-deficient roots often turn black and rot. (Soil nematodes, diseases, chemical damage and aluminum toxicity can cause similar problems.) Because calcium is not translocated in the plant like nitrogen, phosphorus, and potassium, young leaves and the growing points of shoots develop characteristic symptoms. New tissue needs calcium pectate for cell wall formation. Generally calcium deficiency will cause gelatinous leaf tips. In peanuts, calcium deficiency is commonly observed as “pops.”
Magnesium is part of the chlorophyll molecule, so it is actively involved in photosynthesis. This secondary plant nutrient also aids in phosphate metabolism and plant respiration and sets in motion several enzyme systems.
Deficiencies generally appear first on the lower, older leaves because magnesium is translocated within the plant. Older leaves show a yellowish, bronze, or reddish color, while leaf veins remain green. Soils that have been limed with dolomitic limestone (6 percent or more magnesium) rarely have magnesium-deficient plants. Sometimes an imbalance between calcium, potassium, and magnesium may increase a magnesium deficiency. Excess potassium fertilization may induce a magnesium deficiency when the soil contains borderline levels of magnesium.
Sulfur, like nitrogen, is essential in protein formation because it is an essential component of three amino acids: methionine, cysteine, and cystine. Organic sulfur compounds are also found in some plants such as garlic, onions, and members of the cabbage family, contributing to the characteristic odor and taste of these vegetables.
Because sulfur is an essential component of proteins, deficiency symptoms are similar to those of nitrogen. Deficient plants show a pale green color, generally appearing first on younger leaves. Eventually, the entire plant can take on a pale green appearance. Sulfur deficiencies show up most frequently in sandy soils in the early spring. Reddish colored soils and clayey soils tend to hold sulfate anions (SO42-). Sulfate anions leach through sandier soils just like nitrate anions (NO3-). Like nitrogen, sulfur can be mineralized from soil organic matter. In cool soils, mineralization is slow because microorganism activity in the soil is slow.
Sulfur deficiencies usually develop shallow-rooted crops during cool weather, such as corn and winter legumes. It is also deposited in rainfall from natural sources and pollution estimated at 5 to 10 pounds per acre per year.
Micronutrients are just as essential as the primary and secondary nutrients, but are needed in much smaller amounts. Most micronutrients are essential in the specific enzymatic reactions in the plants. Concentration of plant-available micronutrients is sufficient in soils as long as soil pH is maintained, so deficiencies are rare.
Boron (B) is critical for cell wall structure and function in the plant. Deficiency symptoms can appear as deformed fruits/flowers or thick, short petioles. Excess boron can be toxic to plants. Boron should be applied according to soil test recommendations for specific crops.
Copper (Cu) is present in several enzymes and certain plant proteins. No copper deficiencies have been observed in mineral soils in Alabama.
Iron (Fe) is necessary for the maintenance of chlorophyll in plants. An iron deficiency results in chlorotic, yellow tissue between the veins of new leaves. Iron deficiencies are usually induced by high soil pH (above 7.0), poor root growth due to soil compaction or disease, excessive phosphorus fertilization, or soil drainage. Fertilization with iron may be only partially effective in correcting an iron deficiency because other soil properties cause the condition, not the iron content of the soil itself. Foliar fertilization with iron will temporarily correct the deficiency.
Manganese (Mn) activates many enzymes. Deficiencies of iron and manganese are similar, and both are usually induced by high soil pH (above 7.0). Manganese toxicity is more common than deficiencies in Alabama’s acid soils. Maintaining the soil pH between 5.5 and 6.5 will assure adequate manganese availability and avoid toxicity.
Molybdenum (Mo) is required for the normal assimilation of nitrogen in plants. Visual molybdenum deficiency symptoms are similar to nitrogen deficiencies. Molybdenum deficiency is typically observed only in legumes in very acid soils (pH below 5.5). Liming will usually correct a molybdenum deficiency.
Zinc (Zn) is also essential in selected enzymatic reactions. Deficiencies are most common on pecans/fruit trees and on corn in the early spring in Alabama. In corn, green and yellow broad striping on new leaves in the whorl (an arrangement of three or more plant parts in a circle around the same point) characterizes zinc deficiency. It is sometimes called white bud of corn. Deficiencies occur primarily in sandy soils with a pH above 6.5 in the early spring or in soils that have recently been limed. Fruit trees and pecans have special zinc needs. However, the continued, indiscriminate use of zinc fertilizers can result in an excessive buildup to the point where zinc toxicity could be a problem on legumes, which are sensitive to high soil zinc levels.
Chlorine (Cl) is known to be an essential nutrient, but deficiencies are difficult to find in nature because chlorine exists in soils, most fertilizers, rainfall, and the atmosphere.