Mobility of Nutrients, Deficiency Symptoms and Plant Tissue Analysis

After reading this article you will learn about the mobility of nutrients, deficiency symptoms and plant tissue analysis.

Nutrient Mobility in the Plant:

Location of deficiency symptom is related to plant nutrient mobility.

1. Mobile Nutrients:

N, P, K, Mg and Zn. Deficiency symptoms occur in old leaves.

2. Immobile Nutrients:

Fe, B, Mn, Cu, Ca, Mo, S. Deficiency symptoms occur in young leaves

Deficiency Symptoms:

Drawbacks of using nutrient deficiency symptoms to identify nutrient problems are as follows:

1. Visual symptom may be caused by several nutrient deficiencies. Example – N deficiency symptoms may be identified, although S may also be deficient and its symptoms may not be readily apparent.

2. Deficiency of one nutrient may be related to excessive quantity of another. Example: Adding large quantities of Fe to a marginally Mn deficient soil may induce Mn deficiency. Plants growth on a medium soil test P will not require as much N compares with higher soil test P —> increasing N rate can cause P deficiency (Liebig’s law of the minimum).

3. Other plant stresses (diseases, insects, herbicide damage etc.) can be difficult to distinguish from nutrient deficiencies. Example – Leaf hopper damage can be confused with B deficiency in alfalfa.

4. Visual symptoms may be caused by more than one factor. Example – Sugars in corn combine with flavones to form anthocyanins (purple, red, and yellow pigments), and their accumulation may be caused by low P availability, low soil temperature, insect damage to the roots, or N deficiency.

5. Nutrient deficiency symptoms appear after the crop has been nutrient stressed; therefore it may be too late to correct the deficiency without yield loss.

6. Element Toxicities – Toxicity of some nutrients and other elements can produce visual symptoms similar to nutrient deficiencies.

Plant Tissue Analysis:

Reasons for tissue tests and plant analysis:

(i) To help in determining the nutrient supplying power of the soil.

(ii) To help identify the deficiency symptoms and even more important, to determine nutrient shortages days or weeks before they appear.

(iii) To determine the effect of fertility treatment on the nutrient supply in the plant.

(iv) To study the relationship between the nutrient status of the plant and crop performance.

(v) To survey large areas.

(vi) To create interest among peoples about sound soil testing programmes.

Balance of Nutrients:

One of the problems in the interpretation of plant analyses is that of balance among nutrients. Ratios of nutrients in plant tissue are frequently used to study mineral balances in crops.

Commonly used ratios which often reflect some known nutritional antagonism are N/S, K/Mg, K/Ca, Ca + Mg/K, N/P, and so on. Correlation of these ratios with crop yields has been only partially successful because they have been misunderstood or misinterpreted.

Sumner at the University of Georgia has explored the meaning of such nutrient ratios in order to define and understand their relationship with crop yields. He points out that when a nutrient ratio has an optimal value, any yield is possible, the actual level being determined by the factors contributing to yield.

When a ratio is too low, a response to the element in the numerator will be obtained if it is limiting. If the element in the denominator is excessive, a yield response may or may not occur depending on the level of other yield factors.

When the ratio is too high, the reverse is true. These conclusions are supported by the following examples based on the assumption of an optimum range for the N/S ratio in a particular plant part within which the crop yield is maximized.

When the N/S ratio is in this optimum range or balanced it will be identified by a horizontal arrow (→). Ratios above the optimum will be recognized by an upward vertical arrow (↑), and those below it will be assigned a downward vertical arrow (↓).

In situations with N/S = → or in its optimal range, three possibilities exist:

It is not possible from the ratio alone to determine which of the situations above represent what is actually taking place in the plant. All that can be said is that the two nutrients are in relative balance.

Where the N/S ratio is either above or below the optimal range, two possibilities exist in each case:

With N/S above the optimal range, a response to sulfur will be obtained only if sulfur is lacking. If nitrogen is excessive and sulfur normal, additional sulfur may not necessarily be beneficial. The same is true with respect to nitrogen when the N/S ratio is below the optimal range. This analysis demonstrates why, when a ratio has a given value outside the optimum range, a yield response is not always obtained.

Fresh Tissue Tests:

Determination of nutrient in fresh tissue.

General methods:

(a) Tissue is either extracted or simply squeezed to produce sap (e.g. corn stalk NO₃ analysis)

(b) Rapid test, but can be highly variable (affected by time of sampling and plant part)

Total Analysis:

Determination of total nutrient concentration in dried plant samples through laboratory digestion.

i. Critical nutrient content must be available to interpret results.

ii. Must test correct plant part (Table 28.2 in text).

iii. Time of testing-plant tissue concentration changes with age, therefore the time of sampling is important

iv. Interpretation of the test

(a) Indication of general performance and plant vigour (semi-quantitative).

(b) Can be affected by the level of other nutrients in the plant.

(c) Can be affected by soil and climatic conditions.

v. Adequate nutrient concentration is correlated to maximum yield.

Chlorophyll Meters for Plant N Status

i. Quick, non-destructive analysis.

ii. Good correlation with yield.

iii. Must be calibrated for specific crops and growing conditions.

iv. Does not indicate which nutrient is deficient.

Grain Analysis:

i. Good correlation with yield

ii. Postmortem test

iii. Monitoring over several years can help develop N recommendations for a specific field.

Remote Sensing:

Determining the normalized difference vegetative index (NDVI) using on optical sensor:

1. NDVI is calculated from the visible and near-infrared light reflected by vegetation.

2. Dense, dark green vegetation (left) absorbs most of the photosynthetic active radiation or visible light and reflects most of the near infrared light.

3. Sparse, light green or yellow vegetation (right) reflects more visible light and less near infrared light.

4. Dense vegetation will reflect much greater near infrared than visible radiation (high NDVI values), whereas sparse vegetation reflects near infrared and visible light equally (low NDVI values).