7. Determination of Plant Nutrient Status
The nutritional status of a plant can be "determined" on two temporal levels:
  • Currently/Diagnostically: Is the plant sufficiently supplied with nutrients at present?
  • Prospectively/Predictively: Will the plant's nutrient supply be sufficient (in the future)?
It is desirable to answer both questions as accurately as possible. The first is significant when problems arise directly (symptoms of disorders observed at plant parts), while the second is crucial for the long-term planning of measures (e.g., fertilizer application based on soil analysis results).
Underlying most methods is the relation between nutrient sypply and plant growth and plant development
The curve typically plots plant yield (or growth metric) on the Y-axis against nutrient supply/availability (or applied fertilizer rate) on the X-axis. It's often an S-shaped or sigmoidal curve, but can be simplified into three critical phases.
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Yield
  ^
  |                                                                           Phase 3: Luxury Consumption
  |                                                          *  ***********************               & Toxicity
  |                                           *                                                   *
  |                                     *                                                                *
  |                               *                                                                            *
  |                          *                                      |                                                *
  |                     *                                  Phase 2: Sufficiency
  |                    *         Linear Response
  |                 *  
  |               *   
  |             *
  |          * Phase 1: Deficiency
  +-----------------------------------------------------------------------------------------------------> Nutrient Supply
    (Low)                                                                          (High)

The Three Critical Phases
Phase 1: Deficient Zone
  • Characteristics: Yield increases very slowly or not at all with added nutrient. The plant is severely deficient, and other factors (e.g., lack of another nutrient, water stress) may be limiting growth. This is the "minimum" or "base" level.
  • Key Point: The critical deficiency concentration (in tissue) or critical supply level (in soil) lies at the transition from Phase 1 to Phase 2. Below this point, deficiency symptoms are visible.
Phase 2: Sufficiency Zone (Linear Response Zone)
  • Characteristics: Yield increases linearly and sharply with each additional unit of nutrient. This is the phase of maximum efficiency, where the added nutrient is the primary limiting factor. Plant uptake and utilization are optimal.
  • Key Points: The slope of this line represents the agronomic efficiency of the nutrient (e.g., kg of grain per kg of N applied).
  • The Critical Nutrient Concentration (CNC) or Critical Level is the point at the end of this linear phase, where the yield increase per unit of nutrient begins to diminish. This is the single most important agronomic value—it defines the minimum nutrient level for near- maximum yield.
Phase 3: Luxury Consumption and Toxicity Zone
  • Sub-Phase A: Luxury Consumption: Yield plateaus at the maximum yield potential (biological limit). Additional nutrient is absorbed by the plant ("stored") but does not increase yield. This represents economic inefficiency and potential environmental risk (leaching, runoff).
  • Sub-Phase B: Toxicity: With very high supply (often for micronutrients like boron, copper, or zinc, or under saline conditions), yield declines due to direct toxic effects or induced deficiencies of other nutrients (antagonism)

Factors That Influence the Curve
  • Nutrient Type: Macronutrients (N, P, K) typically show broad sufficiency zones, while micronutrients have a much narrower range between deficiency and toxicity.
  • Crop Species and Cultivar: Different plants have different nutrient demands and internal efficiencies (e.g., nitrogen use efficiency).
  • Soil Properties: pH, CEC, organic matter, texture, and interactions with other ions (e.g., high P can induce Zn deficiency) can shift the curve.
  • Climate and Management: Water availability, temperature, and planting density all interact with nutrient response.
  • Other Inputs: The response to one nutrient depends on adequate supply of all others.
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