Soil moisture retention curves

Soil moisture retention curves which illustrate the relationship between the percentage of soil moisture by weight and matrix potential of a soil. These curves must be developed for each soil type at the nursery and can be obtained from most soil laboratories. Soil samples should be collected from the plough layer of the nursery field.

Monitoring soil moisture status

Monitoring soil moisture status

Moisture in nursery soils is monitored by estimating soil moisture content. Several methods used for this are:

Gravimetric method

Here, soil samples are randomly collected from the plough layer with a soil auger or sampling tube. Samples are then placed in per-weighted metal containers, weighed and tied to constant weigh in an oven at 105ºC for 12 hours. Samples are re-weighed after drying, and the percentage of total soil moisture content by weight(%TSMC) is computed. The gravimetric method works well in most nursery soils because of their homogeneity, however it normally takes too long for every day use.

Neutron probe

The neutron probe can indirectly measure%TSMC(wt)if bulk density is known. High energy neutrons are emitted into the soil from a radio active source contained in the probe. The neutrons are slowed down and thermalized by elastic collisions with other nuceaui. Because the slowing down or moderating of the neutrons is caused almost entirely by hydrogen nuclei in soil water, the number of thermal neutrons detected per unit time is directly proportional to the %TSMC (Wt). Though accurate, neutron probes are expensive and cumbersome and require highly trained, licensed personnel for their operation.


Tensiometer can only provide indirect inferences about internal seedling moisture stress. A Tensiometer is basically a porous cup filled wit water which is buried in the soil and connected to a vacuum gauge. This gauge registers the pressure drop on the water in the cup which is in equilibrium with the metric potential of the soil water. Tensiometer generate well in the 0 to 0.8 bar range and they are ideal for monitoring irrigation in forest-tree nurseries. However, when metric potential drops below– 0.8 bar, air begins to enter the porous cup, and the Tensiometer becomes inoperative.

Electrical resistance

Sensors containing a pair of electrodes, usually in blocks or sandwiches made of gypsum, plaster of pairs, or fiber glass cloth are planted in the soil at specified depth to directly measure metric potential. As the water content of these blocks changes with soil moisture contents, the electrical resistance meter connected to these electrodes converts the resistance to an index of soil moisture content. Resistance blocks are sensitive over a –0.5 to –0.15 bar range of matrix potential and are therefore effective in dry soils.

The electrical resistance is calibrated against soil moisture content by the gravimetric method, and plotting a curve relating the true soil moisture content to the electrical resistance reading.

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