Importance of soil sampling in agricultural decision-making

By Bob Aston

The Agricultural Sector Development Support Programme (ASDSP)-Laikipia, held a training workshop for members of Laikipia Produce and Marketing Cooperative Society on soil sampling. The training on February 9, 2016 at Ng’arua Maarifa Centre in Ol-Moran Ward, Laikipia West Sub County, looked at the importance of soil sampling in decision making on crops to plant and fertilizer use.

Speaking during the training, Mr. James Kamau, Ol-Moran Ward Agriculture Officer said that soil sampling ensures proper management of all crop inputs for optimal yields and quality. He said that soil are analyzed for macro and micro elements such as nitrogen, calcium, phosphorous, magnesium, potassium, Sulphur, iron, copper, manganese and Zinc.

He urged farmers to do soil sampling before preparing their farms. This will help to indicate what nutrient reservoir the soil would provide at the beginning of a crop.

In addition, doing this would permit its analysis and allow fertilizer purchase and addition in the field for early planting.

He said that before taking a representative soil sample, one needs to consider soil type or texture, soil conditions, colour of top soil, topography, crop appearance, and trouble spots. He said that soil sampling and analysis looks at soil PH, citation exchange capacity, soil structure, and nutrient deficiencies.

Mr. Kamau highlighting a point during the training

“The most suitable soil reaction for majority of crops is in the range of 6.0-7.0 with a certain tolerance to a more acid or slightly alkaline condition,” said Mr. Kamau.

He said that the best tools for soil sampling are panga, spade, shovel, or soil auger. Soil bags and plastic buckets are also required. It is ideal to collect the samples in plastic bags.

He noted that accurate analysis, identification of nutrients deficiencies and building the humus and microbial diversity, helps in ensuring correct sampling techniques.

He took the farmers through simple random, stratified, and systematic sampling. He noted that simple random sampling is more precise and less subject to the bias of the sampler although it is time consuming.

The stratified random sampling has good precision, as it is relatively faster and less cumbersome than the simple random sampling method. In addition, the method is preferred in routine soil testing programmes.

Systematic sampling ensures better coverage of the field as selected sampling spots are at a regular interval away from each other in one or two dimensions, thereby forming a grid.

He said that heterogeneity of the sample itself and seasonal variation usually affect the representativeness of a soil sample.

“The composition of soil may change under the influence of diverse processes over time like leaching, microbial activity, precipitation, and mineralization,” said Mr. Kamau.

He said that topography, colour, field conditions, and other types of analysis dictate samples per unit. A depth of up to 30 cm for top soil and up to 45 cm for sub soil is ideal for fertility evaluation samples.

He noted that dry samples keep better while immediate analysis of wet samples is important. In addition, samples should not be stored for more than one week because of microbial activities. He said that the recommended sample for 5 acres is one while it is important to separate sampling for different soil types and topographical characteristics in the farm.

 “It is important to mix soil cores to form homogenous samples then dividing into four quarters and taking the end 2 samples and mixing them thoroughly before forwarding soil for analysis,” said Mr. Kamau.

He noted that achieving and maintaining appropriate levels of soil fertility, especially plant nutrient availability, is of paramount importance if agricultural land is to remain capable of sustaining crop production at an acceptable level.