soil fertility evaluation

SOIL FERTILITY EVALUATION 

Several techniques are commonly employed to assess the fertility status of a soil.  Any evaluation procedure would give an idea about the nutrient supplying power of soil. There are various diagnostic techniques employed to evaluate the soil fertility

I. Nutrient deficiency symptoms of plants

II.Plant Analysis

1.Plant tissue tests or rapid tests analysis

2. Total Analysis

3. Biochemical methods

III.Biological tests in which the growth of higher plants or certain microorganisms is used

a)Use of higher plants

i) Field tests using higher plants

               Simple field experiments

               Complex field experiments

               Use of indicator plants

ii) Laboratory or greenhouse tests

Mitscherlich pot culture

Lettuce pot culture

Neubauer seedling method

Sunflower pot culture technique

Foliar sprays

b) Microbiological methods

Azotobacter method

Sackett and Stewart technique

Aspergillus niger method

Mehlich’s Cunninghamella plaque method

Carbon di oxide  evolution method

IV.Chemical soil tests

Soil analysis – Total analysis

                      Tests for available nutrients

I. Nutrient deficiency symptoms of plants:

The appearance of deficiency symptoms on plants has been commonly used as an index of nutrient deficiency. This is basically a good method because the plant acts as an integrator of all growth factor. This method of evaluation of soil fertility is very simple, inexpensive and does not require elaborate equipments. But it becomes difficult to judge the deficiency, if many nutrients are involved. Also it requires an experienced eye to make proper judgment

II. Plant Analysis

         The basis of plant analysis for diagnostic purposes is that the amount of given nutrient in a plant is an indication of the supply of that particular nutrient and is directly related to the quantity present in the soil. The normal growth of plant is determined by the supply of the nutrients.

1. Plant tissue tests or rapid tests analysis:

This is a rapid test of the cell sap of the growing plants. The sap from the ruptured cells is tested for unassimilated N, P, potash and other nutrients.

         The concentration of these nutrients in the cell sap is a good indicator of the supply of nutrients from the soil. These tests are qualitative in nature and are very rapid and show clearly the needs of the growing plants.

         The cell sap from plants, usually the leaves, is extracted and the sap is tested for N, P and K by the use of specific reagents which develop the colour. The intensity of the colour is compared to set standards and thus, it gives a qualitative index of the content of the nutrient in question. Tissue tests are getting popular because of the convenience of handling and the small number of equipments needed for the test. The tests can be made in few minutes. Time of testing, plant parts to be test and interpretation are very important.

2. Total Analysis: It is used extensively in research work as this gives a quantitative indication of the level of nutrients in plants. However, it should be remembered that the determination of total analysis gives both the assimilated and unassimilated nutrients. Many nutrients such as N, P, K, Ca, Mg, micronutrients can be determined by this method. Usually, the mature plants are selected for this testing.

         The plant material to be analysed is dried, powdered and ashed; and tested either by micromethods or by routine tests after dissolving it in acid. Total analysis can be used as a means to correct the deficiency of the current crop; and it is a useful tool in research work too for indicating nutrient uptake, etc.

Crop Logging: Plant analysis is used in the production of sugarcane in Hawai, USA. It is called “Crop logging”. The crop log, which is a graphic record of the progress of the crop, contains a series of chemical and physical measurements. Indicate the general condition of the plants and suggest changes in management that are necessary to produce maximum yields. 

         Plant tissues are sampled every thirty-five days and analysed for moisture, weight of the young sheath tissues, N and sugar content. The analysis of P and potash is made at critical periods and adjustments are made accordingly in the management practices. The moisture content is an index for regulating irrigation particularly during the ripening period. This helps to achieve maximum yield.

Methods employed based on tissue testing/plant analysis

i) Critical nutrient concentration (CNC)

         The CNC is used in interpreting plant analysis results and diagnosing nutritional problems. 

The critical soil test level, concept advanced by Cate and Nelson (1965) isthe level of the nutrient below which a reasonably satisfactory economic responseshould be expected from the application of that particular nutrient and above whichthe probability of such response is low. In order to apply this concept, the soil iscollected from each field are analysed, field experiments are conducted withapplication of graded doses of nutrient and response curve is fitted. A scattereddiagram of percentage yield on Y-axis and soil test value on X-axis is then plotted.It is divided into four quadrants positioning the lines in such a way that the numberof points in the upper right and lower left quadrants is maximum. The point wherethe vertical line crosses the X-axis is defined as the critical soil test value.

Graphical method Cate & Nelson (1965)

                                             Yield at 0 level nutrient

  % yield      =                       ----------------------------------   x 100

                                             Highest yield from nutrient

ii) Chlorophyll meter

Also called as SPAD (Soil and Plant Analysis Division, Japan) is a simple diagnostic tool to measure the chlorophyll content of leaves.  Meter readings are given in Minolta company defined SPAD values which indicate the relative amount of chlorophyll present in plant leaves

iii) Nutrient Ratios

Ratio of nutrients in plant tissue is frequently used to study nutrient balance in crops.  For example, N/S, K/Hg, K/Ca, Ca+Mg/K, N/P and other ratios are commonly used. Ex: When 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, 3 possibilities exist

N →            N↑               N↓

------- or    --------   or    -------

S →             S↑                S↓

It is not possible to determine from the ratio alone which of the 3 situations is occurring in the plant.  All that can be said is that the two nutrients are in relative balance

         N                 N→             N↓

    -------   = ↑ ---------   or -----------

         S                  S↑                S →

S insufficiency        N excess

N                 N→             N↓

     -------   =   ↓   ---------   or -----------

         S                  S↑                S →

  S excess     N insufficiency

 iv) Diagnosis and Recommendation Integrated System (DRIS): DRIS is a system that identifies all the nutritional factors limiting crop production and, thus, increases the chance of obtaining high crop yields by improving fertilizer recommendations.  To develop a DRIS for a given crop, the following requirements must be met whenever possible

Establishment of DRIS Norms. A survey is first employed in obtaining the data required to establish DRIS norms.  A large number of sites where a crop is growing are selected at random in order to represent the whole production area of a country, state, or district.  At each site, plant and soil analyses for all essential nutrients are conducted.

         Second, the entire population of observations is divided into two populations (high and low yielders) based on vigor, quality and yield.  Using N-P-K, in corn leaves as an example, the significant ratios have been found to be N/P, N/K, and K/P.  Determination of Relative N-P-K Requirements.

3. Biochemical methods: The general functional nutrients is to activate various enzymes, eg. P and Mg activate a number of enzymes, whereas other nutrients activate a few specific enzymes only. Therefore, the activity of enzymes may be correlated to the presence of specific minerals in plants.

         Biochemical methods to determine the soil fertility require costly equipments, but offer great opportunities for research work.

III.                Biological tests in which the growth of higher plants or certain microorganisms is used

a)Use of higher plants

i) Field tests using higher plants: Treatments are selected depending upon the problem that the experimenter wants to investigate. The treatment is applied in random manner with replications so that the result obtained has statistically significance. These experiments are implemented in the State Experimental Stations and assist in making general recommendations. These experiments are expensive and time consuming. However, they are not well adapted for use in determining the nutrient status of a large number of soils. In India, simple filed experiments as well as complex field experiments are popular.

  Simple field experiments: Many experiments conducted on farmer’s fields have revealed the deficiency of nutrients at various levels. These experiments have to be simple in nature with 0, N, P, K, NP, PK, NK, NPK as the treatments. These simple field experiments on farmer’s fields are very educative and effective for the farmers, as they themselves see the deficiencies and the response of the nutrients. These trials are useful for advising the correct type and amount of fertilizers.

   Complex field experiments: Complex field experiments allow the testing of many factors at a time and permit a study of interaction among various nutrients. Complex fertilizer trials help to determine the correct kinds of fertilizer, amount and the method of application for each of the soil zones. These experiments are complicated, expensive and can be done only by experienced people.

 Use of indicator plants: There are certain plants that are more susceptible to the deficiency of a specific nutrient and develop clear symptoms if grown in that nutrient deficient soil. Hence, these plants are called indicator plants. Plants suitable for indicator plants are

N-Cauliflower, cabbage

P- Rape

K-Potato

Ca-Cauliflower, cabbage

Fe-Cauliflower, cabbage, potato

Cu-Wheat

Zn- Maize

Mg-Potato

Na-sugar beet

Mn-Sugar beet, oats

B-sunflower

Mo - Lucerne

ii) Laboratory or greenhouse tests: Simple and fast acting biological techniques involving

  Mitscherlich pot culture: In this method, oats are grown to maturity in pots holding 6 lb of soil. The yield of NP, NK treatment is expressed as a percentage of the yield due to NPK treatment. With these percentage yields the plant nutrient reserve in the unfertilized soil can be found in lb/acre from the yield trials of Mitscherlich. These values help to predict the % increase in the yield expected from the addition of a given amount of a nutrient. Mathematically, Mitscherlich’s equation can be expressed as dy/dx = C1(A-Y)

         dy/dx = the rate of increase in yield produced due to the factor x

         A =  Maximum possible yield due to increase in x under given conditions

         Y = Actual yield

         C1 = Efficiency factor

Lettuce pot culture: Jenny modified Mitscherlich’s technique to determine N, P, K, S and lime supply in soils using lettuce. The plants were grown to six weeks and harvested. The criterion of treatment effect is the percentage yield values. These values were placed under three heads

i)          Definite deficiency

ii)         Probable deficiency

          iii)        Uncertain deficiency 

IV. Chemical soil tests: Biological method of soil testing while possessing certain advantages suffer from the disadvantage of being time consuming; and therefore, are not well suited specially if a large number of samples are to be tested. Under such conditions chemical soil tests should be resorted to because they are rapid. Further, soil tests are advantageous to conduct vis-à-vis the usual deficiency symptom methods and the plant analysis methods for the diagnosis of soil fertility, since the soil tests can be done before the plants are grown.

 Total analysis: Strong acids were used to extract the total nutrients from the soil. However, it was soon realized that these strong acids extracted lot more nutrients than could be made available to the plants for growth. Consequently, strong acids were replaced by weaker solutions which extracted the same amounts of nutrients as could be extracted by the roots under the conditions present in soil.

Available nutrients: Thus, estimation of total nutrients was replaced by estimation of available nutrients which are determined in soil testing laboratories. Soil testing provides a rapid method of determination of available nutrients and provides a basis for fertilizer recommendation.

Soil Testing

A Generic Term – includes measurements made on soils for various purposes

         Although plant analyses are extremely valuable in diagnosing nutrient stress, analysis of the soil is essential in determining the supplemental nutrient requirement of a crop .A soil test is a chemical method for estimating the nutrient supplying power of soil . Compared to plant analysis, the primary advantage of soil testing is its ability to determine the nutrient status of the soil before the crop is planted.