Soil plant water relationships Cranfield University

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Any direct attempt to study problems of plant water use can only be successful if adequate soil water data are obtained to supplement water measurement of precipitation and evapotranspiration. Where the soil water measurements required are few, these can be found by sampling and oven drying. More frequently, however, water data have to be taken at regular intervals over a long period of time. In this situation, the oven drying method is time consuming, it causes disturbance which is important in small plots, and also the same volume of soil cannot be sampled twice. One alternative method for monitoring soil water content is to use electrical resistance units which can be buried in the soil at different depths. The resistance readings can be recorded continuously without soil disturbance. There are disadvantages with resistance blocks, in that they may be affected by salts and also calibration is uncertain due to soil water hysteresis effects.

The principle of the electrical resistance method is that when an absorbent is buried in contact with soil, the amount of water held in the pore spaces will vary with changes in the water potential in the surrounding soil. If the absorbent is itself a virtually perfect non-conductor, its water content may be calibrated in terms of electrical conductivity. The closely woven nylon used fulfils these requirements very well and it is used to separate two stainless steel electrodes across which the resistance is measured. The passage of electricity between the electrodes depends on the dissolved salts in the soil water. In Britain, the salt concentration is small, and so, changes in resistance reflect changes in water status. In saline soils, the above does not hold and in order to obtain a relationship between resistance and water content, it is necessary to artificially increase the total conductivity of the soil solution to a higher but stable level. This is achieved by ensuring that the water around the electrodes is saturated with calcium sulphate. In practice this is done by casting a block of gypsum in contact with the nylon fabric.

Inner electrode 65 x 12 mm with 3 strands protruding 12 mm at one end

Outer electrode 65 x 40 mm with 3 strands protruding 12 mm at one end.

1. Cut electrodes to size and connect the stainless steel wires protruding from the gauze and the copper wires of the flex using the crimp connectors.

NOTE: Use only the crimping tool provided and ensure it acts on the metal inner tube. Test the joint for strength.

2. Now, fill the ends of the crimp connectors completely with araldite to strengthen and seal off the joint from all water.

3. Allow resin to harden (approximately ½ hour)

4. Wrap the inner electrode tightly with the nylon woven fabric so that each side of the electrode is covered by six thicknesses of material. Since the electrode is 65mm long there will be 5 mm overlap at each end.

5. Place the covered inner electrode in the middle of the outer electrode. Fold the outer electrode around the inner electrode so that the seam is central.

1. Follow steps 1-6 as in construction of resistance units.

2. Place rubber bung in the end of the PVC mould and press to ensure a good seal.

3. Position the resistance unit so that it lies just beneath the surface of the PVC mould making sure it does not touch the bottom or the sides.

4. Mix 44 g of Calcium sulphate (Plaster of Paris) and 36 ml of water. Stir well to rid the mixture of lumps. Quickly pour this mixture into the PVC mould to cover the resistance unit.

5. Allow the Plaster of Paris to harden for at least ½ hour.

6. Once the Plaster of Paris has hardened, place the mould into the extruder and, using the adapter plate and the blank, extrude the plaster block.

1. Mark the flex so that the nylon and gypsum units can be identified when buried

2. Take an empty box large enough to take approximately one litre of soil and weigh the box with the units, flex, etc.

3. Take enough soil to fill the box and saturate it with water in a beaker. Mix the soil thoroughly.

4. Pour some of the soil/water mixture into the box then insert the nylon and gypsum units. Fill the box with the remaining soil that only the flexes protrude above the soil surface. Make sure the block and the unit do not touch.

5. Allow the unit to come to equilibrium over a period of a few days.

6. Measure the resistance (R) and then weigh the box and contents.

7. As the soil dries, continue to take regular readings and find the corresponding water content by weighing. Take up to 10 pairs of readings over a 7-10 day period. Beware of the clay soil cracking.

8. After the last reading, remove a sample of soil from the box and weigh it. Determine its water content by oven drying and re-weighing.

Plot log R against % soil water content for both the resistance unit and the gypsum block for both soil types.

The resistance readings can vary under normal field conditions from 80O at the saturated end of the scale to 10 MO in dry soil. It is therefore convenient to use a log scale from 1.0 t 8.0.

The mass water fraction of the soil at any reading can be calculated from

Mass of moist sample = g

Mass of oven dry sample = g

Bouyoucos, G.J. (1954). Electrical resistance methods as finally perfected for making

continuous measurement of sol moisture content under field conditions. Michigan

Quarterly Bulletin 37(1):132-134.

Fada, N.R. (1960). The use of electrical resistance units for the study of soil moisture changes

and irrigation requirements in the Sudan Gezira. The Empire Cotton Growing Review

38(2):118-134.

Wellings, S.R., Bell, J.P. & Raynor, R.J. (1985). The Use of Gypsum Resistance Blocks for

measuring Soil Water Potential in the Field. Report 92. Wallingford, Oxfordshire:

Institute of Hydrology.