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The Bigger Picture With Soil Analysis


  • Have you ever wondered about the results of your soil analysis?
  • Have you heard that different labs get different results?
  • Have you ever compared results from different laboratories?
  • Has this given rise to the thought that perhaps South African labs are unreliable?
  • Have you wondered how this could affect your fertilizer bill?


This article will hopefully allay your fears and put your mind at rest.



In South Africa a number of different agricultural organisations have been involved with the selection and standardisation of analysis methods and their verification for advisory purposes for at least the last fifty years.

The precursors of the ARC-Institute for Soil, Climate and Water, and different crop institutes, prior to the establishment of the ARC in 1992, played an important role. So did the Fertilizer Society of South Africa, especially when it had an active centralised research component, organisations such as the current SASRI and a number of private laboratories. Much of the early work was institutionalised but attempts to co-ordinate more broadly eventually culminated in the publication of the Soil Analysis Handbook under the auspices of the Non-Affiliated Committee on Soil Analysis.



Most South African soils laboratories belong to the Agri Laboratory Association of Southern Africa (AgriLASA).

AgriLASA is a non-profitable umbrella body, founded in 1993, for laboratories with agricultural relevance.

AgriLASA’s unique strength lies in providing a forum for member laboratories to:

  • share technical knowledge and experience
  • address common challenges
  • involve dedicated scientists in the continual improvement of the art of agricultural analyses
  • benchmark equipment, most of which is imported


Membership is open to any Southern African agricultural laboratory. A comprehensive inter-laboratory comparison scheme ensures that all fertilizer, water, soil, plant and animal feed analyses are constantly monitored and evaluated.


Ask to see the annual performance certificates issued to member laboratories by AgriLASA. This will ensure that you can rely on the analysis results that you receive.



One of the challenges relating specifically to soil analysis is the choice of suitable extractants for different elements. The aim is to simulate the ability of the plant to obtain the necessary nutrition during the growing period, by selecting an appropriate chemical extractant. This is a unique challenge, as it is inappropriate to use “total” values of elements present in soil, the availability of which is dependent on temporal transformation that is difficult, if not impossible, to simulate.

Soil analysis differs, in this respect, from most other agricultural analyses, where total values are normally determined.

Much work has been done by many organisations on calibrating soil analysis data with plant response and yield in different climatic zones. In South Africa this important aspect has also received considerable attention. Experienced field consultants and agriculturalists apply this information, tempered by their own data, correlations and insights, into practical implementable recommendations.


What this means in practical terms is the following:

  • A particular extractant (method) dissolves a certain proportion of the element being analysed. The method therefore needs to be stated in all cases.
  • The amount of element extracted depends on the procedure and chemicals used for the extraction.
  • The values obtained for different extracts are mostly different from each other and cannot be compared directly.



Some specific criticisms that have been levelled at laboratories also need to be addressed:

  • Laboratories generally do a standard range of analyses, the choice of which is largely dictated by available calibrated recommendation guidelines for specific areas and crops. If you require a specific method that differs from these, the laboratory should be able to oblige. For example, most labs would be able to do a Mehlich 3 analysis, but not much calibration has been done on it locally.
  • Laboratories are sometimes experienced as impersonal. Personal contact between clients and laboratories is encouraged. Build a personal relationship that is mutually advantageous and rewarding.
  • Long turn-around time. The condition in which many samples are received requires time for drying, sifting and general preparation. Turn-around times should be negotiated with the laboratory.



  • The levels of the element determined need to be related to the plant availability of that element, as determined through plant and soil analyses, pot experiments and field trials by qualified agriculturalists.
  • Fertilizer recommendations are based on these correlation and calibration studies, with local experience gained over time together with personal and institutional memory. Not only are the analyses taken into account according to the particular method, but also crop, yield, climatic, economic and management considerations under local conditions.
  • This means that the most appropriate recommendations are made, saving the farmer unnecessary input costs and contributing towards sustainability in the farming enterprise.




Contribution supplied by R.O. Barnard – Chief Technical Advisor: Fertasa

From information compiled in collaboration with AgriLasa


Picture: AgriLASA Website.


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Fertasa | Soil Analysis: The Bigger Picture