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Field infiltration rates

Double-ring infiltration setup

Field infiltration rates measure the rate at which water infiltrates the soil – not to be confused with hydraulic conductivity of the soil. It is important to the study of runoff, erosion, water availability to plants, irrigation, drainage, and raingarden performance. Infiltration is measured under steady state conditions. The rate is influenced by soil moisture or, more specifically, water-filled pore space, soil surface conditions, degree and type of cultivation, compaction, soil horizon changes, and the season.

Infiltrometers are used to measure field infiltration rates. An infiltrometer set consists of two rings of different diameters pressed concentrically (double ring) up to 5 cm into the soil. A constant head of water, 2–5 cm, is maintained equally in both rings, and the rate at which this water infiltrates the soil in the central ring is measured. The water in the outer ring helps prevent lateral flow in the central (measurement) ring.

We have a Hilux 4X4 and trailer unit containing all equipment and 1000 L water for measuring infiltration rates in the field. Infiltrometer ring set sizes used are: (diameters, outer & inner rings) 500 mm & 300 mm, 300 mm & 200 mm, and 200 mm & 100 mm.

A set of at least four measurements are usually made for each treatment/site, and results are given in m/s and mm/hr.

Reference:

Bouwer H 1986 Intake rate: cylinder infiltrometer In: Klute A ed. Methods of soil analysis: Part 1. Physical and mineralogical methods. 2nd edn. Madison, WI, ASA & ASSS. Pp. 835–836.

Cone penetration resistance and shear vane measurements*

Cone penetrometry is a measure of soil strength. A cone of 12 mm diameter is pressed into the soil at a rate of 2 cm/sec to a depth of 0.5 m, taking 15 readings at various intervals. A set of ten measurements is usually required to create a cone resistance profile for a treatment/site. Transects across areas of interest (tracks, cultivation) can also be made to show changes in soil strength.

Shear vane measurements are also available as a measure of soil strength. The vane is pressed into the soil of interest and turned. The maximum torque is measured until soil failure is recorded.

Oxygen diffusion rates (ODR)*

ODR measures the capability of the soil to supply oxygen to a small consumer similar in size to a plant root.

Timing of reading is important. Soil to be measured should have drained for several days from a wet state; therefore a reading should probably not be taken during summer. Several platinum electrodes (10–30) are placed in the soil at least a day before measurement. A voltage is applied to the electrodes from which the oxygen diffusion rate (µg. cm-2 min-1) can be calculated.

Plant root distribution*

Measurements of plant root distribution are made either by coring a sample of root zone and washing the soil to reveal roots or by digging a trench across a root zone and measuring the size and location of roots. The coring method is most suitable for softer roots such as maize, wheat, etc., and the trench method is more suitable for woody roots such as pine. Results can be expressed as root mass/soil mass or root numbers and sizes/location.

Sample Collection

At Landcare Research we are able to sample soil in bulk or in undisturbed core forms. Many of the tests in soil physics require careful sampling of intact, undisturbed soil to retain structural integrity. This is best done by experienced personnel and we prefer to carry out sampling for what we analyses. Sampling to 0.5-m depth is performed by hand, but deeper samples require soil pits to be mechanically dug or the use of our drilling rig, which can operate to 4 or 5 metres. We also have the capability to drill to about 20-m with a larger, truck-mounted drilling rig.

* Available only at Palmerston North
# Available only at Hamilton