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Saturday, 24 July 2021 17:15

Is your Soil up for the Challenge?

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With mounting pressure to produce food, feed and even fuel for an increasing population, farm businesses are now asking more from the soil. Soil health, or the capacity of the soil to function, is now critical to the sustainability of farm businesses. The key starting point in tackling this challenge is soil testing.


Soil testing

The introduction of high yielding varieties has resulted in increased demand for nutrients which cannot be achieved from the inherent soil fertility. 


Inputs such as fertilizer must be used to get optimum yields. The cost of such inputs is prohibitive while their rational use is imperative. There is need for soil testing and analysis to determine nutritional requirements for maximum productivity of a given crop. Many crop farmers neglect this important pre-farming activity which affects the total farm output. It is an essential tool in assessing soil fertility, suitable crops and fertilizer requirements. When done regularly, it also assesses the extent to which farm activities are affecting soil fertility. The potential for increased yields and profits has been the obvious motivator for the keen interest in soil testing.


Fertilizers are the major cost driver in the production of most crops. The aim is to maximize marketable yields and not to harm the environment, which usually is a result of excessive use. Blind use of fertiliser can also lead to inefficiency and unnecessary production costs. A soil testing program is a valuable tool in determining efficient and economic crop production. 


Chemical soil analysis determines the content of basic plant nutrients; nitrogen (N), phosphorus (P2O5), potassium (K2 O), pH, humus content, total CaCO3, available lime, organic matter, total sulphur (S), trace elements, and other physical characteristics (capacity, permeability, density, pH – value)


Why is Soil Testing Important?

Soil testing is the best way to assess soil fertility. It helps you to make important farm decisions on what to grow and soil correctional measures required for maximum crop production.  Soil testing is importing in the following key areas: 

Optimizes crop production. Soil testing allows for determining the macro and micronutrient requirements of your crop.  Applying too little fertilizer results in low yield whereas high yields can be obtained when adequate fertilizer is applied. Soil testing measures soil acidity and alkalinity usually known as soil pH.  

Reduces contamination by leaching of excess fertilisers. 

Reduces production costs by limiting fertiliser applied to only what is required by the plant

Helps in decision making.


What is soil pH?

Soil pH can be viewed as an abbreviation for Power of concentration of Hydrogen ions in a solution. The pH of a soil is a measure of hydrogen ion activity or concentration ([H+]) in the soil solution. As the H+ activity increases, soil pH decreases.  As the soil pH decreases, most desirable crop nutrients become less available while others, often undesirable, become more available and can reach toxic levels. 


From the table above one unit decrease in pH implies 10 times increase in acidity. A soil with a pH of 5.0 is 10 times more acid than a soil with a pH of 6.0 and 100 (10 x 10) times more acid than a soil with pH 7.0



What are the steps involved?

  • Soil sampling
  • Soil testing/ Analysis
  • Interpretation
  • Implementation


Soil sampling

Soil sample is the first and foremost component of soil testing that decides ultimate value of the service to the farmers. As a small amount of soil in the form of sample represents the entire delineated area, a poor soil sample becomes the primary source of error in soil testing and interpretation of results for nutrient recommendation or any other intended purposes. The following are points to consider when taking samples:

Taking soil samples for analysis is done after harvesting of crops and before any fertilizing, at optimum soil moisture.

The soil samples should be true representative of the whole land.

Divide the land into similar areas depending on their colour of soil or past management practices with respect to liming, fertilizing or cropping. 

The best time for sampling is after the rains and when the soil has dried out e.g. April and May.

A minimum of about twenty sub samples should be taken with a trowel for each demarcated area. Each sub-sample should be trowel full of soil to a brush of litter or loose soil on the surface.

A sample is taken to a depth of normal tillage i.e., to rooting of plants, for field crops 0-30 cm, for permanent crops (orchards and vineyards) 0-30cm and 30-60cm. 

Samples must be taken at regular intervals of between 10-20m apart in the field.

The sub-samples from the same area should be placed in a clean container free from contamination and thoroughly mixed.

The resultant mixture should be spread on a clean sheet of paper, air dried and divided into 4 equal parts. 

Small portions of soil should be taken from each quarter until about 1-2kgs of soil has been obtained. This represent the composite soil sample that should be placed in a khaki bag or soil carton but not in plastic bag.

Each composite sample should be clearly labelled and if possible, the farmer should draw a sketch map of each land indicating from where the samples were taken.

Unusual areas such as those in the vicinity of termite mounts, drains, wet spots and contour ridges must be avoided. 

The accuracy of soil analysis will depend on the thoroughness of the sampling technique.

A complete history of land i.e. virgin land, fallow or riveted, density of cover and whether incorporated or burnt off, time of ploughing, past fertilizer history including liming must be submitted with each sample so that the soil testing laboratory can provide more reliable report.


Soil analysis

Soil testing is a time-tested tool for soil fertility evaluation and monitoring. It also helps restoration of depleted soils by offering soil-test based recommendations on plant nutrients and amendments. Soil testing operates on the principle of probability, meaning, if all other factors of productivity are at the optimum, there is high probability to obtain more profitable response to applied nutrients based on soil testing than to those applied on ad hoc basis. So far, the results have been impressive, provided the recommendations are formulated suiting the specific crops grown. 



After collecting your samples, you bring them to ZIMLABS laboratory where you fill in the attached form for submission together with your samples. 






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