SOIL SAMPLING

SOIL SAMPLE COLLECTION - COMPOSITE AND CORE

Soil is analyzed to know the nature of soil, to classify and advocate to farmers about the peculiarities of the soils and how much of fertilizers should be applied for better crop production.

A complete soil, of course, cannot be moved into a laboratory. The value of the laboratory work depends upon care in sampling. Each soil sample needs to be a fair representative of the specific area or horizon worth sampling.

If the sample is to be a representative of an area, it is necessary to take large numbers of samples spread over the area, pool them and sub-sample it so as to get a sample of desired size. For soil survey work, samples are collected from a profile typical of the soil of the surrounding area.

Materials required

             (i) Spade 

                    (ii) Khurpi   

                     (iii) Auger (screw or posthole type) 

                     (iv) Core sampler  

                     (v) Soil testing tube (for wet soil)   

                     (vi) Sampling bags and 

                     (vii) Plastic basin or Bucket.

Collection of soil samples from the field

• Normally each field may be treated as a sampling unit. But two or more fields, which are similar in appearance, production and past-management practices, may be grouped together into a single sampling unit. Samples should be collected separately from areas, which differ in soil colour or past-management practices such as liming, fertilization, cropping pattern etc. During collection of soil, avoid dead furrows, old manure or lime piles, wet spots, areas near trees, manure pits, compost pits and irrigation channels. The sampling should be done in a zig zag pattern across the field to get homogeneity. A wise collecting agent is one who collects samples in the presence of the owner or cultivator of the land who is the best judge in deciding which area of his farm should be sampled separately.

• Scrap away the surface litter and insert the sampling auger to plough depth (15 cm). Take at least 15 samples randomly distributed over each area and place them in a clean bucket. If a sampling auger is not available make a ‘V’ shaped cut to a depth of 15 cm using a mammutty or spade and remove 1.0 to 1.5 cm thick slice of soil from top to bottom of the exposed face of the ‘V’ shaped cut and place in a clean bucket or basin.

• Thoroughly mix soil samples taken from 15 or more spots of each area. Remove foreign bodies such as plant roots, stubbles, pebbles, stones or gravels. By quartering, discard all but ½ to 1 kg soil. Quartering is done by dividing the thoroughly mixed soil into four equal parts and discarding two opposite quarters. Remix the remaining two quarters and again divide into four equal parts and reject the opposite two. Repeat this procedure until about ½ to 1 kg of soil is left. Instead of quartering, compartmentalization method can be followed. For this, spread the soil on a clean hard surface and mark lines from both the sides and create number of compartments. Take a little quantity of soil from each compartment and put into a clean container. Repeat the process of collection until the required quantity is collected. Store the soil in a clean cloth bag or container with proper labeling for further analysis.

Note:   

                   i) Sampling should be done after the harvest of the crop.

        ii) In case sampling is necessary during crop growth, sample between the lines of the growing plants.

           iii) Avoid storing soil samples in fertilizer bags.

Collection of soil samples from a profile

After the profile has been exposed, clean one face of the pit carefully with a spade and note the succession and depth of each horizon. Prick the surface with a knife or edge of the spade to show up structure, colour and compactness. Describe the profile as per the standard terminologies. Use the Munsell colour chart for noting the colour and find out the texture by feel method.

Collect samples from each horizon by holding a large basin at the bottom limit of the horizon while the soil above is loosened by a khurpi. The sample is mixed and transferred to a bag after labeling.

Preparation of soil sample for analysis

The soil sample received at the laboratory is air dried in shade and spread on a sheet of paper after breaking large lumps if present, with a wooden mallet. It is further ground by pounding with a wooden mallet in such a way that the aggregate particles are broken down to ultimate soil particles. The soil thus prepared is sieved through a sieve with round holes, 2 mm in diameter. The material on the sieve is again ground and sieved till all aggregate particles are fine enough to pass through and only stones and organic residues remain on the sieve. Mix well the ‘fine soil’ got by sieving and store in a suitable bottle or container with one label on the outside and another inside the container.

For determination of organic carbon, powder and sieve the soil through 0.5 mm sieve. For the soil samples meant for micronutrient analysis, iron, brass, copper and zinc containers must be avoided for collection and storage of soil samples.

Sub-sampling for analysis

The soil in the bottle is emptied on a clean thick sheet of paper and evenly spread with a sampling knife. It is heaped into a cone by raising the four ends of the paper. It is again mixed well and evenly spread on the paper as before. The process is repeated 3 or 4 times to ensure uniformity and then finally spread evenly on the paper again. Now it is divided into four equal quarters and small quantity of soil is taken from various points in each quarter to get a representative sample for analysis.