Plant Nutrients

The plant nutrients taken up by crops during the growing season may come from many sources, including soil reserves, added fertilizer or manure, and crop residues. Nutrients such as nitrogen, phosphorus, and potassium are required in large quantities, while others are required in very small quantities. Nutrients such as sulfur, calcium, and magnesium are required in intermediate quantities. Soil supplies for nutrients required in small quantities (micro-nutrients) need not be large, but the nutrients must be available to the plant.

All the nutrients listed in Table 1.2-2 are essential for plant growth. The roles and deficiency symptoms of the primary nutrients are discussed below. The symptoms are rarely clear cut, so it is important to use both soil and plant analyses when trying to diagnose a suspected nutritional problem.

Table 1.2-2. Essential plant nutrients
Primary nutrients	Secondary nutrients	Micro-nutrients
Nitrogen (N)	Sulfur (S)	Iron (Fe)
Phosphorus (P)	Magnesium (Mg)	Manganese (Mn)
Potassium (K)	Calcium (Ca)	Boron (B)
		Chlorine (Cl)
		Zinc (Zn)
		Copper (Cu)
		Molybdenum (Mo)
		Nickle (Ni)

Nitrogen (N) is a critical component of proteins, which control the metabolic processes required for plant growth. It also is an integral part of the chlorophyll molecule and thus plays a key role in photosynthesis. An adequate supply of nitrogen is associated with vigorous vegetative growth and a plant’s dark green color. Nitrogen deficiency is characterized by reduced plant growth and a pale green or yellow color. This yellowing generally begins at the tip of the leaf and goes down the middle of the leaf. If the deficiency is severe, the affected area eventually turns brown and dies. Since nitrogen is mobile in the plant, older leaves show the first symptoms of nitrogen deficiency.

Phosphorus (P) is a critical component of nucleic acids, so it plays a vital role in plant reproduction, of which grain production is an important result. Considered essential to seed formation, this mineral is often found in large quantities in seed and fruit. Phosphorus is essential for the biological energy transfer processes that are vital to life and growth. Adequate phosphorus is characterized by improved crop quality, greater straw strength, increased root growth, and earlier crop maturity. Phosphorus deficiency is indicated by reduced plant growth, delayed maturity, and small fruit set. These symptoms may be accompanied by a purple coloring, particularly in young plants. Like nitrogen, phosphorus is mobile in the plant; therefore, any deficiency symptoms show up first on older leaves.

Potassium (K) is not an integral part of any major plant component, but it does play a key role in a vast array of physiological processes vital for plant growth, from protein synthesis to maintenance of plant water balance. Potassium deficiency is characterized by reduced plant growth and a yellowing and/or burning of leaf edges. Since potassium is mobile in the plant, the symptoms appear on the older leaves first. Another indication of potassium deficiency is reduced straw or stalk strength, which results in lodging problems, reduced disease resistance, and reduced winter hardiness of perennial or winter annual crops. The secondary nutrients, calcium, magnesium, and sulfur play a variety of roles in plants.

Calcium (Ca) is an integral part of plant cell walls. Calcium deficiency is rare among agronomic crops under Pennsylvania conditions. When a soil is properly limed to maintain an optimum pH level, calcium is usually adequate for agronomic crops.

Magnesium (Mg), a key component of chlorophyll, plays a critical role in photosynthesis. Magnesium deficiency is characterized by white stripes between the leaf veins. Magnesium is best supplied by a limestone that contains this nutrient.

Sulfur (S) is a common component of proteins and vitamins. Sulfur-deficient plants have a general yellowing and are very spindly. Symptoms of sulfur deficiency are similar to those of mild nitrogen deficiency, except that they appear sooner in new growth than in old growth, since sulfur is not mobile in the plant. Under Pennsylvania conditions, sulfur deficiency is not common. Rainfall supplies significant amounts of sulfur, and it is recycled efficiently through the manure applications that much of the cropland in the state receives.

Micro-nutrients are key players in many of the processes important for plant growth. Few micro-nutrient problems exist in Pennsylvania, because (1) the heavier loamy texture of our soils helps to maintain adequate levels of micro-nutrients (sandy textured soils, by contrast, often show micro-nutrient deficiencies); (2) the slightly acidic nature of our soils helps to maintain micronutrient solubility; and (3) Pennsylvania agriculture is largely animal based, so much of our cropland gets periodic applications of manure, a good source of micro-nutrients.

Of the micro-nutrients, boron (B) and zinc (Zn) are usually the only two that occasionally are deficient in the state. Boron deficiency can be overcome by periodically applying boron when topdressing alfalfa. Zinc deficiency is sometimes observed on corn, particularly when phosphorus levels become excessive from over application of phosphorus fertilizer. Symptoms on corn appear as a rough stripping on either side of the midrib of the corn leaf. Zinc deficiency can be corrected with a periodic broadcast application of zinc. Routine application of zinc is not recommended.

The lack of general response to micronutrients has made it difficult to calibrate a micronutrient soil test for use in Pennsylvania. In addition, it is not possible to rely on calibrations developed for other soil, climatic, and cultural systems. The best tool for evaluating a plant’s micronutrient status is plant tissue analysis.

Additional considerations

Some nutrients, such as nitrogen and phosphorus, are present in the soil in large amounts but are made available to plants only very slowly. Others, such as potassium, are both present in the soil in large amounts and readily available.

Nutrients also behave differently in the soil. One important behavior characteristic of a nutrient in soil is its relative mobility. This is illustrated in Figure 1.2-2.

In general, as nutrient mobility increases, fertilizer placement becomes less critical, but the potential for nutrient loss becomes greater. Thus, nitrogen placement relative to the plant is not very critical for uptake, but the potential for loss of nitrogen once it has been applied to the soil generally is very high, and little available nitrogen will accumulate in the soil.

Conversely, as nutrient mobility decreases, fertilizer placement becomes more critical, but the potential for nutrient loss lessens. At this extreme, phosphorus placement near the plant is very critical for uptake, because phosphorus does not move more than ¼ inch to get to a root.

The loss of phosphorus from soils usually requires erosion of the soil itself, however, so phosphorus therefore accumulates in the soil.

Some recent evidence indicates that, in spite of its low solubility, significant amounts of soluble phosphorus can be lost in runoff from fields with very high or excessive soil test phosphorus levels. Therefore, from an environmental perspective, it is best to manage phosphorus to avoid excessive soil test phosphorus levels if possible. The behavior of most micro-nutrients is similar to that of phosphorus. The other nutrients fall somewhere between these two extremes in mobility behavior.