How to sample your soil for soil testing

By George Munene

Soil testing is the first step toward improving yields and profitability as it gives valuable information that enables farmers provide the necessary nutrients for crops improving agricultural productivity.

A soil test is however only as good as the sample you take is representative of your land. Hence it is very important to conduct a representative sample of your land.

• Tools required for soil sampling:

1. Notebook
2. Soil sampling containers (clean paper bags, cloth bags)
3. Two clean buckets
4. Soil sampling tool ( panga, spade, soil auger)
5. Ruler

Make a sketch map of the field/plot to be sampled, indicating the difference in soils that you recognise. Each soil sample shouldn’t represent more than two hectares. Though speaking to Kenya Agricultural and Livestock Research Organization (KALRO) representatives during the institute’s field day held last month this can be pushed to one sample every five hectares. These may be whole fields or sections of fields depending on the following situations:

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1. Let each soil sample represent not more than two hectares. For any field or soil larger than this a correspondingly larger number of samples must be taken.
2. Irrespective of the field size, let a separate soil sample represent parts of the field that differ in:

• Soil colour
• Soil texture (sand, loam, or clay)
• Drainage
• Slope (if contour formed, sample contour area separately)
• Crop performance (crop quality or symptoms indicating varying degrees of nutrient deficiencies)
• Management practices e.g, mulched vs unmulched

On the sketch map list your field or block designation, and sample numbers and indicate/mark the approximate distribution of the borings (spots where a sample was taken). Remember to record your address in the soil sampling information sheet (or notebook). 

Sampling Operation

1. Topsoil– Take a soil core or cut it to a depth of about 20cm and transfer it into a bucket. Repeat this at least 12 times in 12 sampling points so as to cover the farm. Mix thoroughly and put half a kilo of soil into the sample bag/cloth.

-Topsoil sampling can be adequate for growers of shallow-rooted crops such as horticulture crops.

2. Subsoil– At every other boring (where top soil was taken) take a subsoil sample from about 20 to 50 cm. Place into a second bucket and proceed as in (a) above.

-This is done for deep-rooted crops such as fruits, bananas, arrow roots, etc.

Label the sample containers (clean paper bags, cloth bags) giving the field or block designation “TOP” or “SUB”, depth in cm, date, and samplers name.

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NOTE

• When sampling soil from fruit crops, coffee, e.t.c., take samples within the most active feeding zones, i.e, just within the leafy canopy.
• Keep samples from mulched and unmulched areas separate and indicate this on the information sheet (notebook)
• Do not sample hot spots e.g ant hills, knolls, fertiliser bands, terrace channels, dead furrows, areas with lime, manure or fertiliser have been in a pile or spilled, areas where brush or trash have been burned, or any other such area from the filed as a whole.
• Do not sample when too wet

NB:  Have your soil re-tested after two to three years when carrying out conventional farming.

Soil sampling methods on a uniform field with no apparent differences:

Photo courtesy: Harvesto Group

Photo courtesy: Ecological Organic Agriculture – EOAI-AFRICA

Example of soil sampling in a non-uniformed field, i.e, different soil types, manuring levels, topography, different crops, e.t.c.

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Photo courtesy: Darryl Warncke, MSU

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Photo Courtesy: Sokoine University of Agriculture