Measuring the Fertility of Soil: 10 Biological Tests

This article throws light upon the top ten biological tests used for measuring the fertility of soil. Some of the biological tests are: 1. Field Tests 2. Strip Tests on Farmers’ Fields 3. Laboratory and Greenhouse Tests 4. Mitscherlich Pot Culture 5. Neubauer Seedling Method 6. Short-Term Method 7. Microbiological Methods 8. Sackett and Stewart Technique and Others.

Biological Test # 1. Field Tests:

The field-plot method is one of the oldest and best-known of the biological tests. The series of treatments selected depends on the particular question the experimenter wishes to have answered. The treatments are then randomly assigned to an area of land, known as a replication, which is representative of the conditions.

Several such replications are used to obtain more reliable results and to account for variations in soil and management.

These experiments are helpful in the formulation of general recommendations. When large numbers of tests are conducted on soils that are well characterized, recommendations based on such studies can be extrapolated to other soils with similar characteristics. Field tests are expensive and time consuming, and one is unable to control climatic conditions and other limiting factors.

They are valuable tools, however, and are widely used by experiment stations, although they are not well adapted for use in determining the nutrient status of large numbers of soils. Rather, they are used in conjunction with laboratory and greenhouse studies as a final proving ground and in the calibration of soil and plant tests.

Biological Test # 2. Strip Tests on Farmers’ Fields:

Strips of fields are being treated with fertilizer by extension, industry, and growers alike to check on recommendations based on soil or plant tests. The results of these tests must be interpreted with caution if they are un-replicated and use of two or three replicated is recommended. Repetition of strip tests at several locations is also helpful.

Biological Test # 3. Laboratory and Greenhouse Tests:

Simpler and more rapid biological techniques which still involve higher plants but utilize small quantities of soil have been developed. These methods have met with wide acceptance on the European continent and have frequently been employed in the United States.

Biological Test # 4. Mitscherlich Pot Culture:

In this method oats are grown to maturity in pots containing 6 lb of soil. A total of 10 pots are used. The yields of the N-P and N=K treatments are expressed as a percentage of the yield from the complete N-P-K treatment. For example, if the N-P-K treatment yielded 80 g and the N-K treatment, 60 g, the yield would be 75%.

With these percentage yields, the plant nutrient reserve in the unfertilized soil can be read in pounds per acre from yield tables prepared by Mitscherlich, and from these same tables predictions of the percentage increased in yield expected from the addition of given amounts of nutrients can be obtained.

Biological Test # 5. Neubauer Seedling Method:

The Neubauer technique is based on the uptake of nutrients by a large number of plants grown on a small amount of soil. The roots thoroughly penetrate the soil, exhausting the available nutrient supply within a short time. The nutrients removed are usually determined quantitatively by chemical analysis of the entire plant.

In some procedures, however, the tops and the roots are harvested and analyzed separately. Tables have been set up to give the minimum values for satisfactory yields of various crops. The Neubauer method has been used for the availability of several nutrients including phosphorus, potassium, calcium, micro-nutrients, or fertilizer materials.

Biological Test # 6. Short-Term Method:

The short-term method helps to bridge the gap between chemical extraction and greenhouse pot methods. Plants deficient in the element under study are grown in sand contained in a cardboard carton with the bottom removed. A dense mat of roots is formed at the bottom in 2 to 3 weeks.

The roots are then placed in contact with soil, or soil plus fertilizer, contained in a second carton. An uptake time of about 1 week is allowed, after which the plants are analyzed. Uptake is generally well correlated with uptake in conventional pot tests that require several weeks.

Biological Test # 7. Microbiological Methods:

Winogradsky was one of the first to observe that in the absence of mineral elements certain micro-organisms exhibited a behavior similar to that of higher plants. It was shown that the growth of Azotobacter served to indicate the limiting mineral nutrients in the soil, especially calcium, phosphorus, and potassium, with greater sensitivity than chemical methods.

Since that time several techniques which employ different types of micro-organisms have been developed and a few are described. In comparison with methods that utilize higher plants, microbiological methods are rapid, simple, and require little space.

Biological Test # 8. Sackett and Stewart Technique:

The Sackett and Stewart technique is based on Winogradsk’s work and was used to study the phosphorus and potassium status of Colorado soils. A culture is prepared of each soil, phosphorus is added to one portion, potassium to another, and both elements to a third portion.

The cultures are then inoculated with Azotobacter and incubated for 72 hours. The soil is rated from very deficient to not deficient in the respective elements, depending on the amount of colony growth.

Biological Test # 9. Aspergillus Niger:

To determine phosphorus and potassium small amounts of soil are incubated for a period of four days in flasks containing the appropriate nutrient solution. The weight of the mycelial pad or the amount of potassium absorbed by these pads is used as a measure of the nutrient deficiency.

Mehlich devised a more refined technique in which the mycelial pad is also analyzed for potassium. An example of the criteria used is tabulated.

Biological Test # 10. Mehlich’s Cunninghamella-Plaque Method for Phosphorus:

The soil is mixed with the nutrient solution; a paste is made, spread uniformly in the well of a specially constructed clay dish, inoculated on the surface in the center of the paste, and allowed to incubate for 4½ days. The diameter of the mycelial growth on the dish is used to estimate the amount of phosphorus present.