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Soil water sensors reveal subsoil conditions

Part 1: The feel method, gravitational soil water determination and electric resistance

by Frikkie Koegelenberg, Pr Eng and Hendrik Jordaan

Every farmer knows: If you apply too little water, your crops suffer; if you apply too much water, your bank account suffers. The decision depends on accurate knowledge of your soil water status.

Fortunately, there are a variety of ways in which irrigation scheduling can be applied, that is with which to determine when to irrigate. The decision of which irrigation scheduling technique should be used, depends on the farmer’s choice. Scheduling aids will only be to his benefit if at least two readings can be taken per irrigation cycle. It is recommended that the readings be obtained from the different scheduling aids and plotted on graphs to identify tendencies in water consumption and possible problems with the irrigation management in time.

We thank the ARC Agricultural Engineering in South Africa for making their manual on soil water sensors available to the readers of ProAgri Zambia.

The most practical method is to follow a programme calculated by means of the historic evapotranspiration and adapted by soil water measuring at strategic points in an irrigation block. The calculation of evapotranspiration can be simplified by the use of the models. Continuous soil water measuring is recommended for especially open hydroponics systems where irrigation is applied daily. For further reading, the handbook, “Class notes for the Soil and Water Management course” of the University of Stellenbosch in South Africa can be obtained, as edited by Dr JE Hoffman.

The quantity of water to be applied depends on how empty the soil water reservoir is at that specific stage.

Feel method

Determining soil water by means of feeling and observation is one of the oldest methods used for determining the soil water status. This is a simple method, but practice and experience is necessary to determine this exactly. Soil samples taken at different depths in the root zone are collected with a soil auger, after which they are thoroughly studied and felt. The soil is classed on the basis of the observation and the soil water can be determined with the aid of tables. With practice, the soil water can be determined within 10 to 15% accuracy. Table 1 shows the relation between soil water shortage, texture classes and appearance. This method of scheduling is not recommended.

Water determination by means of the feel and observation method. Firmly squeeze the soil in your hand to form a ball. Rolling the soil between thumb and index finger forms a the sausage.

Gravimetric soil water determination

Gravimetric soil water determination is a direct method by which soil samples are collected at different depths with a soil auger. The soil water is determined by establishing the wet mass of the sample, after which it is dried in an oven for 24 hours at 104°C. The dry mass is then established. For quicker results, the soil sample can be dried in the microwave oven for approximately 20 minutes. In both an oven and a microwave some of the organic materials can be burned at high temperatures, which will then result in a lower mass. If the soil has a high organic content, the maximum temperature must preferably not exceed 65°C. The drying period must then be maintained until there is no further reduction in mass. The percentage soil water per dry mass (gravimetric water content (Pw)) can be determined with the following equation:

Pw = ((Wet soil mass – Dry soil mass) / Dry soil mass)

The volumetric water content, the water content per volume (θv), can be determined by multiplying the gravimetric water content with the relative dry mass density, (BD) (dry mass per undisturbed soil volume) of the soil. If the value is multiplied by 1 000, it gives a value in mm/m, therefore mm water per metre soil depth. Remember however, that all this water is not available to the plant, since a portion of the water is being held too strongly by the soil matrix for the plant to take up.

Θv = Pw x BD

Note: The values in brackets indicate the estimated soil water requirement (in mm/m) to field capacity for uniform soils.

Benefits:

  1. It is very accurate, at least for the relevant soil sample used.
  2. It is an objective method and the personal judgement of the person making the decision is not applicable.
  3. It is a cheap method and a large number of samples can be handled simultaneously.
  4. The salt content of the soil does not influence the test.

Disadvantages:

  1. It requires laboratory equipment, together with a balance scale and drying oven, which means the determining cannot be done in situ.
  2. It is time-consuming, requiring 18 hours of drying time plus additional cooling time and a result can only be obtained at least a day after the sample was taken.
  3. The method is destructive in the sense that a sample is physically removed from the soil and a hole remains.
  4. Since it is destructive, it is impossible to do a follow-up determination on the same spot. Measurements done over time therefore include an element of inaccuracy (because of the spatial variation present in any landscape when moving from one position to another).
  5. In order to convert the gravimetrical reading to a volumetric reading, it is also necessary to determine the bulk density. Many soils become dense during sample-taking, which gives a false image of the bulk density.
Apparatus required for the gravimetric method.

Determining the soil’s electric resistance

The electric resistance of a volume of soil depends, among others, on the soil water content. If the electric resistance of a soil is determined, such a reading can, after calibration, be converted to soil water content. An example of a measuring instrument, which is based on this principle, is the gypsum block (Picture 5).

The instrument consists of a porous gypsum block in which two electrodes are placed, which are connected to two electric cables. When the block is buried in the soil, the water in the gypsum block will equilibrate with the soil water. Water will move through the pores of the block until the matrix potential (soil water tension) inside and outside the block is the same. The resistance that the electric current experiences in flowing between the two electrodes, can then be determined by means of an ordinary resistance bridge. The resistance is equal to the prevailing soil water tension, but the resistance can also be calibrated against the soil water content (gravimetric or volumetric).

Picture 5: Gypsum block. Photo: ictinternational.com

An example of a calibration curve is shown in Graph 1. Soil water tension can also be directly determined, while the absolute soil water content can be read indirectly from a soil water characteristic curve. The two electrodes can also be placed in porous nylon or fibreglass blocks.

Benefits:

  1. The gypsum blocks are relatively cheap and the resistance can be determined with any commercially available resistance bridge.
  2. The blocks function over the entire soil water spectrum (from dry to wet), but the accuracy and sensitivity is better in the dry area than in the wet area of the spectrum. The sensitivity of nylon blocks in the wet area is better than that of the gypsum blocks.
  3. After installation of the blocks, the soil water content can be determined on the same spot every time. The blocks can also be buried at any soil depth.
  4. The apparatus can be connected to an automatic register.

Disadvantages

  1. Each block must be calibrated individually, since even small differences in the dimensions of the blocks will cause a change in the resistance reading.
  2. The calibration curve also changes with time, especially in the case of the gypsum blocks.
  3. Soil characteristics other than the water content also influence the resistance reading. Especially dissolvable salts in the soil solution play a role here, because the more dissolvable salts the soil contains, the better the soil will conduct an electric current. The changes in resistance reading are therefore not necessarily only because of a change in the soil water content.
  4. Special set-ups (current circuits) are necessary when more than one block (at different depths in the same place) is connected to a data register.
Graph 1: Calibration curve that indicates the relation between electrical resistance of a gypsum block and soil water content.

Next month we shall discuss the neutron soil moisture meter and the tensiometer. Published with acknowledgement to the ARC Agricultural Engineering for the use of their soil water sensors manual. Visit www.arc.agric.za for more information.

Gypsum blocks are used with resistance meters. Photo: slideplayer.com

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