2.3 Nutrient Mobility in Soil and Plant Deficiency Symptoms

Diagnosing nutrient issues in the field requires an understanding of how nutrients move in the environment. Mobility must be considered in two entirely different contexts: mobility in the soil and mobility in the plant. These two concepts are completely independent of one another. For example, nitrogen (as nitrate) is highly mobile in both the soil and the plant, whereas phosphorus is largely immobile in the soil but highly mobile within the plant.

Nutrient Mobility in the Soil

Nutrient mobility in the soil dictates how a plant root encounters the nutrient and how easily the nutrient can be lost to the environment via leaching.

1. Mobile Nutrients in Soil: These nutrients exist primarily as dissolved anions in the soil solution. Because soil colloids are negatively charged, they repel these anions, meaning they are not held tightly by the CEC. Mobile nutrients move with soil water via mass flow.

   • Examples: Nitrate (NO3-), Sulfate (SO4^2-), Chloride (Cl-), and Boric Acid (H3BO3).
   • Management Implications: Because they move with water, these nutrients are highly susceptible to leaching below the root zone, particularly in sandy soils or under heavy rainfall/irrigation. Fertilizer applications of mobile nutrients are often split or timed closer to maximum crop uptake to minimize environmental loss and improve use efficiency. Roots do not need to proliferate extensively to find mobile nutrients; the water brings the nutrients to the root.

2. Immobile Nutrients in Soil: These nutrients are held tightly by soil colloids or precipitate into insoluble mineral complexes. They do not move easily with soil water.

   • Examples: Phosphorus (H2PO4- / HPO4^2-), Potassium (K+), Calcium (Ca2+), Magnesium (Mg2+), and most metallic micronutrients (Fe, Zn, Mn, Cu).
   • Management Implications: Immobile nutrients are rarely lost to leaching (except in extremely sandy or highly saturated soils). Instead, they are primarily lost through soil erosion or crop removal. Because they do not move toward the root, the root system must actively grow toward the nutrient, acquiring them via diffusion or root interception. Banding these fertilizers near the seed row (starter fertilizer) is highly effective because it places a concentrated zone of nutrients in the path of early root growth.

Nutrient Mobility in the Plant

Once a nutrient is absorbed by the roots and enters the plant's vascular system, its mobility dictates how the plant handles a localized or whole-plant deficiency. Plant mobility refers to the plant's ability to translocate a nutrient from older tissues to actively growing tissues (the sinks) when external supplies run low.

1. Mobile Nutrients in the Plant: These nutrients can be easily remobilized via the phloem. When the plant senses a deficiency in the soil, it will sacrifice its older, mature foliage by stripping it of mobile nutrients and sending them to the growing point to sustain new life and reproductive organs.

   • Examples: Nitrogen (N), Phosphorus (P), Potassium (K), and Magnesium (Mg).
   • Deficiency Symptoms: Because the plant scavenges from the bottom, deficiency symptoms for mobile nutrients always appear first on the older, lower leaves.

2. Immobile Nutrients in the Plant: These nutrients, once incorporated into plant tissue, are locked in place. They cannot be easily extracted and moved via the phloem to new growth.

   • Examples: Calcium (Ca), Sulfur (S), Iron (Fe), Zinc (Zn), Manganese (Mn), Copper (Cu), and Boron (B).
   • Deficiency Symptoms: Because the plant cannot move these nutrients from old reserves, the new growth must rely entirely on immediate root uptake. If the soil supply is low, the new tissue suffers. Thus, deficiency symptoms for immobile nutrients always appear first on the newer, upper leaves or growing points.

Diagnosing Common Deficiency Symptoms

By combining knowledge of plant mobility and specific physiological roles, agronomists can visually identify nutrient deficiencies in the field.

Symptoms on Older Leaves (Mobile Nutrients)

• Nitrogen (N): Presents as a general, uniform yellowing (chlorosis) of older leaves. In corn, N deficiency shows a classic V-shaped yellowing starting at the leaf tip and moving down the midrib.
• Phosphorus (P): Older leaves appear dark, stunted, and often develop a purplish or reddish discoloration, particularly along the margins. This purple coloration is due to the accumulation of anthocyanin pigments when sugars build up due to restricted ATP production. It is most commonly seen early in the season during cold, wet conditions that restrict root growth.
• Potassium (K): Shows as yellowing or tissue death (necrosis) along the outer margins (edges) of the older leaves. In alfalfa, it appears as white or yellow spots along the leaf margins.
• Magnesium (Mg): Causes distinct interveinal chlorosis (yellowing between the veins) on older leaves. The veins remain green while the tissue between them turns yellow, sometimes progressing to a reddish or purplish tint.

Symptoms on Newer Leaves (Immobile Nutrients)

• Sulfur (S): Similar to nitrogen deficiency (general uniform yellowing), but it appears on the new, upper leaves first. The whole plant may eventually turn pale green or yellow, often characterized by spindly, stunted growth.
• Iron (Fe): Presents as sharp, distinct interveinal chlorosis on the newest leaves. In severe cases, the newly emerging leaves may be completely white (bleached) due to a total lack of chlorophyll production.
• Zinc (Zn): Causes interveinal chlorosis on new leaves, but is also uniquely characterized by shortened internodes, leading to a "rosetting" effect (leaves clustered tightly together). In corn, it can cause broad white or yellow bands on either side of the midrib on new leaves.
• Calcium (Ca): Severe deficiencies result in the death of the terminal bud or growing point. Because Ca is vital for cell wall structure, deficiency in fruits causes localized tissue collapse, famously resulting in "blossom end rot" in tomatoes and peppers, or "bitter pit" in apples.
• Boron (B): Like calcium, boron deficiency often kills the terminal bud. It also causes brittle leaves, hollow stems in brassicas (like broccoli), and poor pollination leading to barren ears in corn or empty pods in soybeans.