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The Value of Soil Testing for Fertilizer and Lime Recommendations
About soil testing ...
Laboratory and other tests are commonplace in the age in which we live. A doctor may take a blood or urine sample for testing to diagnose the patient's illness. The motor mechanic tests the car's ignition system or exhaust gases to identify why there is a loss of power. Similarly, tests may be carried out on the soil to evaluate the soil's nutritional status.
Soil testing is not a recent development. In European countries as well as in the U.S.A., soil testing services have been in operation for over 50 years. Soil testing is now an integral part of agricultural advisory services in most developed countries.
At Cedara, a soil testing and fertilizer advisory service has been offered since the early 1970's. The Soil Fertility and Analytical Services Section (Part of the Technology Development and Training Directorate of the KwaZulu-Natal Department of Agriculture) now analyses more than 15 000 soil samples for farmers every year. The section also offers other analytical services including the testing of soils and waters to assess their suitability for irrigation, and the elemental analysis of plant material, composts and manures.
The value of soil tests
Soil tests provide the only objective means of assessing the fertility status of soils. There is no alternative to soil testing. Soil tests indicate the amounts of nutrients in the soil which are available for crop growth. They also reflect soil acidity levels. The following considerations emphasize the value of soil testing in crop production.
- By taking samples several months before planting, it is possible to correct problems for the current season, before yield losses are incurred.
- Since soil tests provide specific information on the availability of individual nutrients as well as on soil acidity levels, more efficient use of inputs is permitted. Thus, for example, on a land where a soil test has indicated that only phosphorus (P) is in short supply, an NP fertilizer rather than an NPK fertilizer may be used. Unnecessary expenditure on K is thus avoided.
- Where capital for purchasing fertilizer and lime is in short supply, soil test results enable the farmer to direct expenditure to those factors which are most limiting, and in this way maximum efficiency in the use of capital is ensured.
- Where soil tests are based on ongoing comprehensive field trial research, as is the case with the Cedara Service, the farmer, by regularly submitting soil samples, is put in the position of constantly applying the most up-to-date research information on his farm.
Soil tests carried out
The Cedara Soil Testing Service, and indeed most soil testing services, include the following tests in evaluating soils for their fertility status:
- Calcium (Ca), magnesium (Mg) and potassium (K)
- Extractable acidity - this includes exchangeable aluminium (Al ) and hydrogen (H)
- Soil pH (KCl)
- Plant-available phosphorus (P), zinc (Zn) and manganese (Mn)
The Cedara Service uses the units mg/L in reporting the concentrations of Ca, Mg, K, P, Zn and Mn, while Al + H is reported in cmolc/L. Ca, Mg, K and Al + H are the exchangeable cations, while the sum of these (in cmolc/L) is the total cations, also sometimes referred to as the effective cation exchange capacity. The ratio of extractable acidity to total cations, expressed as a percentage, is referred to as the soil’s acid saturation. The ‘KCl’ listed after soil ‘pH’ indicates that it is measured in a suspension of soil in a 1 M KCl solution; this influences the magnitude of the pH reading.
A soil test is of little value if crop requirements (the ‘optimum’ values) are not known. Research must be carried out with different crops to determine optimum values of the major nutrients and soil acid saturation. This research takes the form of field trials in which crops are grown in small plots with varying amounts of fertilizer and lime. Because optimum values differ for the various crop species, and furthermore, differ between soil types, this research is very time consuming and expensive, and not all soil testing services have research results at their disposal. One of the strengths of the Cedara Soil Testing Service is that it is based on decades of field research, in which optimum P, K and acid saturation values have been determined for a number of crops.
For fertilizer and lime recommendations based on soil analysis, submit soil samples to the Soil Fertility and Analytical Services Section at Cedara.
Sample submission forms and sample boxes are available from the Soil Science Reception Office at Cedara and from Agricultural Extension Offices.
Prices for the standard soil fertility test:
First sample in batch: R10.00
Subsequent samples: R25.00
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Soil Fertility and Analytical Services
KwaZulu-Natal Department of Agriculture
Private Bag X9059
Pietermaritzburg
3200Contacts
Zandra Smeda, Ruby Punwasi or Vino Naidoo
Telephone: (033) 3559 455/6
Facsimile: (033) 3559 454
