Soil fertility Testing

 

Soil fertility Testing

Fertile soil is a mixture of well-balanced minerals, high organic matter, humus, humic, fulvic and carbonic acids having good aeration and a rich microbial population. Soil fertility is the ability of a soil to sustain plant growth by providing essential plant nutrients and favorable chemical, physical, and biological characteristics as a habitat for plant growth. Fertilizers are chemical or natural substance or material that is used to provide nutrients to plants, usually via soil, but sometimes to foliage or through water. The main function provided by a fertile soil is the provision of food and thus have an economic impact and is related to economic growth and the fight against poverty.

Soil fertility comprises three interrelated components: physical fertility, chemical fertility and biological fertility.

Physical fertility is the physical properties and processes of soil that affect soil fertility by altering water movement through soil, root penetration of soil and waterlogging. Important physical properties that affect fertility include soil structure and texture.

Chemical fertility means the presence and concentration of plant nutrients which include the macronutrients such as nitrogen, phosphorus and potassium, sulfur, calcium and magnesium and Micronutrients like Iron, cobalt, selenium, boron, chlorine, copper, iron, manganese, molybdenum and zinc.

Biological fertility includes the organisms that live in the soil, the population of which varies greatly depending upon various environmental conditions and it is highly complex and dynamic. It is the least well-understood fertility component. There may be hundreds of millions to billions of microbes in a single gram of soil. The most numerous microbes in soil are the bacteria, fungi, algae, protozoa and virus.   

Soil organisms improve soil fertility by performing a number of functions such as releasing nutrients from organic matter, Fixing atmospheric nitrogen, Increasing phosphorus availability, Degrading pesticides, Controlling pathogens and improving soil structure. In addition to soil fertility, soil microorganisms play essential roles in the nutrient cycles that are fundamental to life on the planet.

Releasing nutrients from organic matter - Soil microorganisms are responsible for most of the nutrient release from organic matter. When microorganisms decompose organic matter, they use nutrients in the organic matter for their own growth and release excess nutrients into the soil and these can be taken up by plants. If the organic matter has a low nutrient content, then microbes will take nutrients from the soil.

Fixing atmospheric nitrogen - Nitrogen fixation is a significant source of available nitrogen for plants in soil. Nitrogen fixing bacteria such as Azotobacter, Azospirillim, Rhizobia, etc fix nitrogen gas from the atmosphere and make it available to the plants.

Increasing phosphorus availability - Phosphate solubilizing bacteria, fungi, Mycorrhiza etc are capable of solubilizing inorganic phosphorus from insoluble compounds and can increase phosphorus uptake by the plant. Mycorrhizal fungi can provide phosphorus to plants.

Degrading pesticides - Some microorganisms are capable of producing enzymes that can break down agricultural pesticides or other toxic substances added to soil.

Controlling pathogens - Some microorganisms and soil animals infect plants and thereby decrease plant yield. There are many microbes in in the soil that can control the spread of pathogens. For example, some pathogenic fungi in soil destroyed by certain protozoa, they consume the pathogenic fungi.

Improving soil structure - Some bacteria and fungi produce substances during organic matter decomposition that chemically and physically bind soil particles into micro-aggregates. The hyphal strands of fungi can cross-link soil particles and maintain aggregates. It is observed that a single gram of soil can contain several kilometres of fungal hyphae. Soil micro fauna increase pores by tunneling through the soil thereby improving the soil porosity and they also increase aggregation by ingesting soil.

Soil microbiology is the study of organisms in soil, their functions and how they affect soil properties. Soil microorganisms can be classified as bacteria, actinomycetes, fungi, algae, protozoa and viruses. Each of these groups has different characteristics that define the organisms and different functions in the soil it lives in.

Soil testing/soil analysis/ Soil fertility evaluation

This help one to know the nutrient supplying power of soil to the crop.  Soil testing is used to determine both the amount of different types of nutrients that is immediately available and that can become available during the growth of a crop.  A complete soil test includes: ​soil pH, alkalinity, organic matter, macro elements, microelements, ​cation exchange capacity (CEC), and cation saturation. A foliar test includes all the major and micro elements.

Objectives of soil testing

·         To accurately determine the status of available nutrients in soils (P, K, Mg, pH, Zn, B)

·         To clearly indicate the seriousness of any deficiency or excess of any nutrient

·         To form the basis on which fertilizer needs are determined in such a way to permit an economic evaluation of the fertilizer recommendation

Techniques commonly employed to assess the soil fertility are

1. Soil testing

2. Analysis of tissues from plants growing on the particular soil to be analysed 

3. Biological tests in which the growth of higher plants or certain micro-organisms is used as a measure of soil fertility

4. Nutrient deficiency symptoms of plants

1. Soil testing:

Soil testing is the chemical analysis that provides a guideline for addition of fertilizer to soils.  The primary advantage of soil testing is determining the nutrients status of the soil before the crop is planted as compared to the plant analysis.

Objectives of Soil testing

·         Soil fertility evaluation for making fertilizer recommendation

·         Prediction of likely crop response to applied nutrients

·         Classification of soil into different fertility groups for preparing soil fertility maps of a given area

·         Assessment of the type and degree of soil related problems like salinity, sodicity, acidity etc., and suggesting appropriate reclamation / amelioration measures

Steps involved in soil fertility analysis

1.      Soil Sampling

2.      Preparation of samples

3.      Analytical procedure

4.      Calibration and interpretation of the results

5.      Fertilizer recommendation

1. Sampling:

This is the most vital step for analysis since a very small fraction of the huge soil mass of a field is used for analysis and used as an index of the whole area in hectare basis. So it becomes extremely important to get a truly representative soil sample from the field.  This sample should be a true representative of the farm/plot/field.

A rough map of the farm is to be made dividing it into sampling units.  A minimum of five to six samples in different zones of the area being tested is to be obtained.  Sample the soil from the surface to about 15 cm (6 inches) depth with a sampling tube.   Any surface organic material has to be removed from the shovel and then the soil samples are mixed together to create a single sample.  

The best tool is the metal tube called “sampling tube”. If this is not available, cutlass, shovel, hand trowel and auger could be used. Do not use brass, bronze, or galvanized tools because they will contaminate samples with copper and/or zinc.

2. Preparation of sample:

Drying, grinding and sieving of the soil sample is done according to the need of analytical procedure.

3. Analytical procedure:

All analytical instruments used should be properly calibrated either using different concentration of standards or with a known concentration of standard.  Spectroscopic methods are used for analyzing inorganic molecules and chromatographic methods are used for organic compounds.

A suitable method is one which satisfies the following three criteria.

·         It should be rapid

·    It should give accurate and reproducible results of a given Samples

·         It should have high predictability

Soil test kit is a compact soil testing equipment with full complement of devices and reagents for the determination of the pH, electrical conductivity, nitrogen, phosphorus and potassium concentration, fertilizer and water. Carbon, which is a good index of organic content in soil, can also be determined. The equipment is robust and cost saving in terms of laboratory space.

Chemical methods used for determination of different nutrients

Analysis of inorganic compounds

Analysis of anions

Colorimetric techniques are used for the analysis of Chloride, nitrate, nitrite, sulphate, phosphate, sulphide and ammoniacal nitrogen.  Generally, an automated spectrophotometer which can perform multi element analyses are used.  There will be an autosampler, and a system for sequentially adding the appropriate chemicals to develop the colour, and then colour developed will be read and used to calculate the analyte concentration. These automated analysers are very efficient and can run up to two hundred samples per hour.

Analysis of Metals

Metals are usually measured by Inductively Coupled Plasma, or ICP analysis or by atomic absorption spectroscopy.

Analysis of Sulphur compounds

·         Total sulphur can be measured using an induction furnace, but as sulphur can exist in many forms, it will not give much information regarding soil fertility

·         Elemental (or free) sulphur is analysed using a solvent extraction, followed by HPLC

·         Total sulphides can be analysed by acid digestion and ICP

Analysis of Nitrogenous compounds

·         Nitrate and nitrite will be analysed using water extract of the soil by colourimetric spectroscopy

·         Ammoniacal nitrogen test include both ammonia (NH3) and ammonium (NH4), and again is either done on a water extract  or distilled as exchangeable ammonia

·         Kjeldahl nitrogen gives measure of the ammoniacal nitrogen and organic nitrogen, and is analysed by a distillation and titration method

Analysis of Available Micronutrients

·         DTPA extractable test is done which extract and analyze complexed, chelated and adsorbed form of Fe, Mn, Zn, Cu from soil

Analysis of Organic materials

The amount of organic matter in soil is relatively high and changes rapidly over a short period of time. Hence, sampling to monitor changes in soil organic matter should be done regularly.  There should not be any roots or added organic materials in the sample.

The most common method used to estimate the amount of organic matter present in a soil sample is by measuring the weight lost by an oven-dried (105°C) soil sample when it is heated to 400°C; this is known as 'loss on ignition', essentially the organic matter is burnt off.

Chromatography techniques such as HPLC – High performance (or high pressure liquid chromatography), GCFID – Gas chromatography with flame ionization detection, GCMS – Gas chromatography with mass spectroscopy detection are also used.

4. Calibration and interpretation of the results:

For the calibration of the soil test data, , particular nutrient is selected and the test crop is grown with varying doses of particular nutrient and basal dose of other nutrients. By plotting the soil test values against the percentage yield will allow to calculate the relationship between soil test values and per cent yield response

 2. Plant Testing:

1. Analysis of tissues from plant growing on the soil

Plant tissue analysis is the determination of the concentration of an element in a plant sample taken from a particular portion of a crop at a certain time or stage of morphological development.  The plant analysis has been used as a diagnostic tool or complementary to soil testing because

(i) In many situations, the total or even the available content of an element in soil fails to correlate with the plant tissue concentration or the growth and yield of crop due to many reasons including the physico chemical properties of the soils and the root growth patterns.

(ii) On the other hand, the concentration of an element in the plant tissue is positively correlated with the plant health. Therefore, the plant analysis has been used as a diagnostic tool to determine the nutritional cause of plant disorders/diseases.

Steps

1.      The collection of the representative plant parts at the specific growth stage,

2.      Washing, drying and grinding of plant tissue

3.      Oxidation of the powdered plant samples to solubilize the elements,

4.      Estimation of different elements,

5.      Interpretation of the status of nutrients with respect to deficiency / Sufficiency /toxicity

3. Biological tests in which the growth of higher plants or certain micro-organisms is used as a measure of soil fertility

Neubauer seedling Method

·         The Neubauer seedling technique is based on the uptake of nutrient by growing a large number of plants on a small amount of soil.

·         The seedlings exhaust the available nutrient supply within short time.

·         The total nutrients removed are quantified and tables are established to give the minimum values of nutrients available for satisfactory yield of various crops.

Microbial methods

·         In the absence of nutrients, certain microorganisms exhibit behavior similar to that of higher plants.

·         For example, growth pattern of Azotobacter or Aspergillus niger reflects nutrient deficiency in the soil.

·         The soil is rated from very deficient to not deficient in the respective elements, depending on the amount of colony growth.

·         In comparison with other methods that utilize higher plants, microbiological methods are rapid, simple and require little space.

4. Nutrient deficiency symptoms of plant

·         The plant requires seventeen essential nutrients for their optimum growth and development.

·         When a plant nutrient is below critical concentration in plant, it shows deficiency symptoms

·         It is good tool to detect deficiencies of nutrient in the field

·         These symptoms are nutrient specific and show different patterns in crop.

Limitations:

·         The visual symptoms may be caused by more than one nutrient or may be due to an excess quantity of another nutrient.

·         Deficiency symptoms in the field may be due to disease or insect damage which can produce certain micronutrient deficiencies.

·         Nutrient deficiency symptoms are observed only after the crop has already suffered an irreversible loss.

Deficiency indicator plants

Plant                                                    Nutrient deficiency

Oat                                                      Mg, Mn and Cu deficiencies

Wheat and barley                                Mg, Cu and some times Mn deficiencies

Sugar beets                                           B and Mn deficiencies

Maize                                                  N, P, K, Mg, Fe, Mn and Zn deficiencies

Potatoes                                               K, Mg and Mn deficiencies

Rape                                                    N, P and Mg deficiencies

Brassica species                                  K and Mg deficiencies

Celery and sunflower                          B deficiency

Cauliflower                                         B and Mo deficiencies

Flax                                                     Zn deficiency