Methods for used for evaluating soil fertility and predicting plant nutrient needs

1. Visual symptoms of nutrient deficiencies:

• Maize is indicator plant for Nitrogen and Potassium, cauliflower is indicator plant for Boron etc.
• Some nutrient deficiency symptoms are easily identified while others may become difficult.
• Deficiency of element does not produce directly but with an accumulation of certain organic compounds and that produce abnormal condition recognized as symptoms and has definite relation to shortage of elements.

Occurrence of nutrient deficiency on plant cause

a) Complete crop failure at seeding stage. E.g. Khaira disease in rice (Zinc deficiency).

b) Severe stunting of the plants.

c) Delayed or abnormal maturity.

d) Yield differences with or without leaf symptoms.

e) Poor quality of crops as in proteins, oils and starch content or keeping or storage quality is reduced.

f) Poor root growth.

g) Specific leaf symptoms.

Advantages and disadvantages of visual method

Advantages

a) There is no need of any equipment.

b) It is cheaper method.

c) It becomes supplement to other diagnostic techniques for further analysis.

Disadvantages

a) Great reduction in yield has already been occurred as symptoms appear.

b) Experience is needed for the surety of the deficiency symptoms.

Problems/precautions

a) Expression of particular mineral deficiency may differ from crop to crop.

b) Identification of multiple deficiency symptoms becomes difficult.

c) Symptoms may be the results of more than one cause e.g. accumulated sugar in corn may combine with flavones to form anthocyanin (purple, red and yellow pigments). Sugar accumulation may be due to: low supply of Phosphorus; cool nights; insect damage to roots; Nitrogen deficiency.

d) Damage done by insects, diseases and mechanical injury may resemble with nutrient deficiency e.g. leaf hopper damage is very similar to Boron deficiency in alfa alfa.

e) Toxicity of certain elements or damages done by herbicides, impeded drainage etc. may be mistaken for some cases of the nutrient deficiency.

2. Plant and tissue analysis:

A. Fresh tissue analysis:

• It refers to quick test or spot test performed in the fields to estimate the amounts of nutrients present in fresh plant tissue.
• It usually involves simple chemical tests which produce color that reflects qualitatively the
• amount of a particular nutrient present in plant sap. Results showing a high color intensity usually indicates a high content of the nutrient.
• Green plant tissue can be tested for several elements including nitrate – N, P, K, Mg, and Fe.
• Following conditions can be expected from fresh tissue analysis:

a) General performance and vigor of plant

b) Level of other nutrient in that plant

c) Incidence of insects and diseases

d) Soil conditions

e) Soil moisture

f) Climatic condition

Advantages and disadvantages of fresh tissue analysis

a) Advantages

• Easy to handle.
• Small numbers of equipment are sufficient.
• More tests can be made within a few hours.
• Immediate action (suggestion to farmers) and reliable.
• It becomes supplement for visual nutrient deficiency method.

b) Disadvantages

• Difficult to interpret, experience is needed.
• Field kit is needed which becomes expensive than visual method.

B. Total plant analysis:

• It refers to the quantitative analytical determination of essential elements in plant tissue by chemical procedures in a laboratory.
• It is performed on the whole or some plant parts. A Precise analytical test is performed in various elements after plant dried, grounded and ashed.
• The instrument used are atomic absorption, flame photometer, emission spectroscopy etc.
• This method of soil fertility evaluation is based on the assumption that there is, within certain limits, a positive correlation between the amount of any nutrient supplied, the amounts of this nutrient that is contained in the leaf and the yield.

Advantages and disadvantages of Total Plant analysis

a) Advantages

• Many elements are analyzed at a time.
• Accurate amount can be detected because of use of very precise/sensitive equipments.
• Apply for all nutrients except C, H and O.
• Can determine other non-essential compounds i.e. Al, Na levels that directly affect the physical and chemical conditions of soil.

b) Disadvantages

• Take more time for different processes.
• Experienced personnel are needed.
• Instruments and equipment are very expensive e.g. Atomic absorption spectrophotometer (AAS), flame photometer etc.

Use of plant analysis

• To verify suspected deficiency, toxicity or mineral imbalance in plants.
• To identify hidden hunger.
• To evaluate effectiveness of fertilizer treatments, its adequacy an does not leave hidden hunger.
• To asses nutrient amount removed by crops.

Critical nutrient concentration (CNC) or critical nutrient point (CNP)

• Level of nutrient below which crop yield or quality or performance is unsatisfactory e.g. In case of maize.

3. Biological test

I. Uses of higher plants:

a) Fertilizer trials in fields

• Field trials are conducted both at the framers field and experimental/research stations to test the yield response of crops due to the application of various doses of fertilizer materials.
• It measures crops response to added nutrient or fertilizers. In order to make field trials more effective and fast it should be carried out in conjunction with laboratory and the greenhouse support and should include the correlation and calibration of soil and plant tests.
• Treatment selection, randomization and replication are done during field trial. Variation in
• land fertility, irrigation, and slope are considered.

Advantages and disadvantages of use of higher plants

a) Disadvantages: Climatic conditions and other limiting factors are uncontrolled; it is time consuming and costly than that of visual diagnosis technique.

b) Advantages: This is used to provide fertilizer recommendation that will optimize crop yield, maximize profitability and minimize environmental impact of nutrient use.

b) Laboratory and Greenhouse trial

• More rapid biological techniques, small amounts of soil used and very simple to conduct.

Advantages and disadvantages of greenhouse trials

a) Advantage of greenhouse trial over field trial

• Less time is required than field trial and can be conducted at any time of year;
• Less chemicals are used, more positive results;
• All the factors except the one under investigation can be controlled easily
• Large number of treatments can be tested at relatively low cost and in a short time.

b) Disadvantages of greenhouse trials

• Environment is artificial
• Cropping intensity is much higher than that of field.
• Bulk density of soil and plant root volume is much more different than that of field.
• Greenhouse results may have little direct relevance to crops grown in the field.

II. Microbiological methods:

• The microbiological methods are usually rapid, simple and require little space.
• In this method the growth of or nutrient uptake by microorganism is measured and correlated with nutrient status of soil.
• Certain microorganism shows a behavior similar to that of higher plants. E.g., Azotobacter is used for Ca, P and K. Aspergillus niger: to determine P and K.
• Small amounts of soils are incubated for a period of 4 days in flask containing the appropriate nutrient solution.
• The weight of mycelia pad or amount of K absorbed by these pads is used as a measure of nutrient deficiency.

4. Chemical Soil Testing:

• It is a chemical method for estimating the nutrient-supplying capacity of soil.
• The main objectives of soil testing is to measure soil nutrient status and lime requirements to make fertilizer and lime recommendation for profitable farming.
• In this, soil test values must be calibrated against nutrient rate experiments in the field and in the greenhouse.

Objectives of soil testing

• To build and/or maintain fertility status of a given field.
• To predict the probability of obtaining profitable response to lime and fertilizer.
• To provide a basis for recommendation on the amount of lime and fertilizer to apply.
• To evaluate the fertility status on a district, soil area or state-wise basis by the use of soil test summaries. Such summaries (e.g. soil fertility map) are helpful in developing plans for research and education work.

Soil test includes

1. Collection of soil sample
2. Sample preparation
3. Laboratory analysis
4. Calibration
5. Interpretation of the test
6. Fertilizer recommendation