Productivity of crops depends on adequate supply of nutrients from the soil. Generally, the quantity of nutrients removed by the crop (nutrient uptake) is much greater than the quantity of applied nutrients resulting in poor fertility status of soils.
Hence, to improve the fertility status of soil and to sustain the productivity of crops, soil fertility evaluation is inevitable. This can be done through assessment of nutrient requirement of crops and pre and post-harvest analysis of soil.
The under mentioned techniques are commonly employed to assess the fertility status of soil.
1. Nutrient Deficiency Symptoms:
Plants exhibit deficiency symptoms as a direct consequence of physiological malfunction, which is due inadequate supply of essential nutrients from the soil. This is also associated with mobility of mineral ions inside the plant system.
These deficiencies can be alleviated through visual diagnosis, which help identifying deficiency symptoms of nutrients. But, certain precautionary measures are to be adopted in visual diagnosis for determining the specific nutrient deficiency.
i. Plants may exhibit symptoms due to multiple nutrient deficiencies.
ii. One nutrient deficiency may resemble other nutrient, which is present in higher concentration (nutrient toxicity).
iii. It is difficult to distinguish deficiency and disease symptoms.
iv. Many factors are associated with symptoms.
The following symptoms are usually observed in plants as they are good indicator of soil fertility:
i. Marginal scorching of leaves.
ii. Stunting of plants.
iii. At the seedling stage, complete crop failure occurs.
iv. During a particular season, specific leaf symptoms appear at varying times.
v. Internal abnormalities.
vi. Abnormal maturity.
vii. Obvious yield differences.
viii. Poor quality of crops.
2. Quick Plant Tissue Tests:
The roots of plants absorb nutrients from the soil and these are transported to other parts of the plant where they are required. The concentration of the nutrients in the cell sap will evince the availability of nutrients in the soil while performing the tissue test. By knowing the deficiencies of nutrients, remedial measures can be adopted for proper growth and development of crops.
Objectives of tissue test:
a. To identify nutrient deficiency symptoms.
b. To assess the nutrient supplying power of the soil.
c. To determine the shortage of nutrients before they appear as symptoms.
d. To find out the effect of applied nutrients.
e. To study the relationship between the nutrient concentration of the plant and yield.
The colour developing reagents are added to the plant sap (leaf tissue) and the resulting colour intensity is compared with standard chart which indicates the concentration of nutrients. This nutrient concentration in the plant sap is used as a measure of the supply of nutrient.
a. Tissue Test for Nitrogen:
Colour developing reagent -1-2 drops of 1% diphenylamine.
b. Tissue Test for Phosphorus:
Colour developing reagent – 8 g of ammonium molybdate in 200 ml of water and add 126 ml of conc. HCl and 74 ml of distilled water. This reagent is diluted four times before use.
c. Tissue Test for Potassium:
Colour developing reagent – 5g of sodium cobalt nitrite and 30g sodium nitrite in 60 ml of water and add 5 ml of glacial acetic acid, make up the volume to 100ml.
d. Plant Parts to be Tested:
It is essential to test the part of the plant that will give the best indication of the nutritional status. In general, the recently matured leaf is used for testing and the immature leaf in the top of the plants is avoided. The leaf samples should be taken during blooming to the early fruiting stage for testing.
e. Total Analysis:
The plant samples are collected and kept in the hot air oven at 105oC till the constant weight is achieved. After drying, the plant samples are ground into fine powder in a Wiley mill and used for chemical analysis to find out the concentration of nutrients. This nutrient concentration in the plant sample will give the measure of supply of nutrients.
3. Biological Tests:
The fertility status of the soil is evaluated either by growing plants or by using microorganisms. It involves field tests, strip tests and laboratory greenhouse tests.
a. Field Tests:
Application of graded levels of nutrients in the particular field where the crops are grown. The rate of growth and nutrient uptake of crops will show the fertility status of soil.
b. Strip Tests:
Narrow field strips with proper nutrient management practices will help in assessing the fertility status of soil. These tests are conducted in farmer’s fields.
c. Greenhouse Tests:
This test is conducted by using field soils under green house.
i. Mitscherlich Pot Culture Method:
Mitscherlich predicted the plant nutrient reserved (native fertility) as the percentage of increases in the yield expected from the addition of a given quantity of fertilizers.
ii. Jenny Pot Culture Tests:
He grouped the plants into 3 categories viz.:
(a) Definite deficiency
(b) Probable deficiency and
(c) Uncertain deficiency based on the percentage of yield.
iii. Neubauer Seedling Method:
It is based on the uptake of nutrients by a large number of plants grown on a small quantity of soil. The roots of plants occupy the entire soil column and exhaust the available nutrients supply within a short time. The total nutrients removed by the crops are quantified. This quantification of nutrients will be utilized for achieving satisfactory yields of various crops.
iv. Sunflower Pot Culture Techniques for Boron:
Deficiency is classified as marked deficiency, moderate deficiency and not or little deficiency based upon the number of days after which boron deficiency is observed.
v. A Value Technique:
Fried and Dean (1952) calculated the available nutrients in the soil by using radioactive isotopes. A value is defined as that amount of nutrients in the soil which behaves in a similar way as the applied fertilizers nutrient doses.
a. Aspergillus flavus test.
b. Connicomella test.
4. Soil Analysis:
The pre and post-harvest soil analysis help in evaluating soil fertility. Proper nutrient management strategies to crops can be given through fertility assessment which helps to sustain productivity of crops. This is the most reliable method.
a. To provide an index of nutrient availability.
b. To predict probability of obtaining profitable response.
c. To provide a basis for fertilizer prescription.
d. To evaluate the fertility status of soils.
The following points should be taken into consideration while sampling of soils:
i. Selection of area for sampling.
ii. More number of samples to be taken and make it composite soil sample.
iii. Depth of sampling.
iv. Time of sampling.
Standard Soil Tests:
Soil Test Interpretations:
< Critical level = Deficient
> Critical level = Sufficient