by Frikkie Koegelenberg, Pr Eng and Hendrik Jordaan
In the last edition of this series, we take a quick look at the calibration of instruments and the importance thereof.
We thank the ARC Agricultural Engineering in South Africa for making their manual on soil water sensors available to the readers of ProAgri Zambia.
Calibration is essential for the success of any method. All the indirect measuring methods measure one or other relation influenced by water content. This relationship is dependent on the composition of the soil matrix and is not unique for all soil types.
If the actual water content for a specific soil must be determined, the relationship for the specific soil must be calibrated. Figure 1 shows the relationship between soil water tension and available soil water for different soil types. Soil water tension is mostly measured by means of a tensiometer.
It is clear from the above figure that the soil type cannot be ignored. Similar relations exist for all the other methods. Soil type is not the only parameter, and the salinity and compaction of soils also have an influence on some measuring methods. The calibration is usually done with a direct measuring method such as gravimetric water determination.
If the instrument is not calibrated for a certain soil, the measurements can still be applied to monitor the tendency of water change in the soil. This is however not the actual water content of the soil.
1. Jordaan. H. J. 2001. Grondvogsensors. ARC-Institute for Agricultural Engineering. RSA.
2. Piaget, J. 1991. Tensiometers: Beskrywing, voorbereiding, opstelling en gebruik. Elsenburg Agricultural Development
Institute for the Winter Rainfall Area. RSA.
3. Van Zyl, J. L. 1981. Waterbehoefte en besproeiing. In: (Reds. Burger, J. and Deist, J.). Viticulture in South Africa.
ARC-Infruitec-Nietvoorbij, Stellenbosch. RSA.
Next month we shall start a new series on irrigation principles. Visit www.arc.agric.za for more information.