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Lime Testing in the Field or in the Lab? Why Laboratory Figures are Higher

The measurement of soil pH is a vital routine - now more than ever. 

Soil acidity is the biggest single cause of lowered yields and yet the actual measurement of soil pH sometimes varies. 

The three commonly used methods for determining pH are,

  • BDH Soil Testing Kit
  • Electronic pH meters
  • Laboratory testing under controlled conditions.  

Confusion has existed for many years over differing results; laboratory tested soils frequently result in higher readings than in-field testing, how can this be?  The answer is simple; in the laboratory soils are oven-dried, finely ground, and soaked in distilled water prior to electronic measurement.  This ‘thorough’ process provides a ‘thorough’ result.

However field conditions are not so exact, so which is accurate? 

Both are, but crops do not grow in fields in soil that is dried and ground to a fine powder, no crop has yet been farmed in the laboratory.  Although field pH assessment is crude in scientific terms it is nearer to nature.  So we have the following conundrum; lab testing is dead accurate, but field testing is the better guide!

When comparing these two methods bear in mind ALL the discoverable pH will be revealed by the laboratory; but field testing clearly identifies true soil pH values. 

 

As a farmer you may be presented with two different readings, one from the laboratory at pH 6.8, and a BDH field reading of pH 6.2 - here is what to do;

  • Examine the crop, look up the liming history of the field, regard the BDH test as the one closest to reality – Retest – Looking for patchy acidity.
  • Take remedial action in accordance with the ALA Lime calculator -  http://www.aglime.org.uk/lime_calculator.php.   
  • Maintain and update your soil management and field records plans regularly, build up liming histories, field test regularly, avoiding recent organic/inorganic fertilizer applications. 

Dilligence in testing will be wasted if poor quality coarse lime is spread.

The Fertiliser Regulations 1991 control the sale and legal declarations specific to Liming Products.

Regulation can be viewed at:  http://www.legislation.gov.uk/uksi/1991/2197/schedule/1/made.

CHECK:

  • Determine the name and grade of the material your merchant supplier is proposing, check with the regulation – is this suitable? 
  • Ask for a product specification datasheet, check the % passing the Regulation gradings especially 150 microns – ideally > 40%   
  • Check the declared Neutralizing Value NV especially on the delivery ticket, dependant on geological location values can vary between  NV 44%  to NV 57%, -  a 30% difference!  
  • Reactivity Value RV, this determines the product speed of reaction in the soil and is directly related to rock type and the degree of milling - the higher the % passing 150 microns the quicker the product will be absorbed in the soil solution to reduce and neutralize acidity.  

 “Time spent in reconnaissance is never wasted – assumption is the mother of all mistakes”

 Fine grinding is necessary, chippings will neutralize your bank balance, but not your acidity, ideally 40% of the liming material should pass through a 150 micron sieve.  If not, your efforts are wasted, the soil testing is wasted, the spreading is wasted, the whole programme of pH management is wasted.

Still, it WAS a bit cheaper !

 

For further information please contact Stephen Hill via ala@mineralproducts.org

 
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