III. Biological methods

III. Biological methods

    It is conducted for calibrating the crop responses to added nutrients. Different methods are adopted for evaluating fertility status of soil.
  1. Field tests:
      Field tests are conducted on different fertilizers and crops with treatments impositions in replications. The treatment which gives highest yield will be selected. These experiments are helpful for making general recommendations of fertilizer to each crop and soil and we can also choose right type and quantity of fertilizer for various crops. It is laborious, time consuming, expensive but most reliable method. They are used in conjunction with laboratory and greenhouse studies as final proving technology and in the calibration of soil and plant tests. Thy widely used by experiment stations.
  2. Indicator plants:
      These are plants that are more susceptible to the deficiency of specific nutrients and develop clear deficiency symptoms if grown in that nutrient deficient soil. Hence these are called as indicator plants.
      Some indicator plants are;


      Indicator plants

      N, Ca

      Cabbage, Cauliflower



      K, Mg



      Cauliflower, cabbage, potato, oats



      Na, B

      Sugar beet


      Sugarbeet, oats, potato





  3. Microbiological test:
      By using various cultures of microorganisms soil fertility can be evaluated. These methods are simple, rapid and need little space. Winogradsky was one of the first to observe in the absence of mineral elements certain microorganisms exhibited a behavior similar to that of higher plants. Microorganisms are sensitive to deficiency of nutrients and could be used to detect the deficiency of any nutrient. A soil is treated with suitable nutrient solutions and cultures of various microbial species (bacteria, fungi) and incubated for a few days. Then observing the growth and development of organisms in terms of weight or diameter of the mycelia pad, the amount of nutrient present in the soil is estimated.
        Ex: a. Azotobactor method for Ca, P and K.
               b. Aspergillus niger test for P and K
               c. Mehlich’s Cunninghamella (Fungus)- plaque method for phosphorus
               d. Sackett and Stewart techniques (Azotobacter culture) to find out P and K status in the soil.
  4. Laboratory and Green house Tests:
      These are simple and more rapid biological techniques for soil fertility evaluation. Here, higher plants and small amounts of soils are used for testing. All these techniques are based on the uptake of nutrients by a large number of plants grown on a small amount of soil. It is used to assess availability of several nutrients and they are quantitatively determined by chemical analysis of the entire plant and soil. Some common methods are;
        E.g.: a. Mitscherlich pot culture method for testing N,P, K status in oat 
                b. Jenny’s pot culture test using lettuce crop with NPK nutrients
                c. Neubauer seedling method for NPK d. Sunflower pot-culture technique for boron
Last modified: Wednesday, 7 December 2011, 5:39 AM