soil map units where Phg Phosphatic gravelly is dominant
Soil properties
| Property | Value |
|---|---|
| Geographic distribution | Tuvalu |
| Landscape position | These soils result from the interaction of humus, guano and water with a parent material of calcareous gravel. With the exception of Niulakita, where phosphatic soils cover three-quarters of the island (and phosphate has been mined), phosphatic soils are found in relatively small patches - invariably associated with Pisonia-Hernandia woodland - and nowhere do they occupy more than 5 per cent of island area |
| Soil parent material | Coralline gravel |
| Slope | Flat (0-3°) |
| Surface outcrops | Usually the surface is littered with angular blocks, but occasionally there is a pavement of dark phosphatic rock |
| Brief soil profile description | Phosphatisation produces a thin tough crust composed of phosphate and calcium carbonate beneath a dark Ao horizon. The crust is often fractured by plant root growth and rectangular blocks of phosphatic soil cover the ground surface. Where the soil is in-situ the lower limit of phosphatisation is generally sharp and clear-cut (at a depth of 15-30cm) but very irregular with little sign of gradation into the underlying unaltered sand or gravel. |
| Soil profile texture | The phosphatic soils are typically pale to dark brown in colour with orange or reddish brown streaks, and mottled throughout with white sofler and chalky grains. Where blocky phosphate occurs gaps are often filled by a rich, dark loamy crumb structured soil which extends only a few centimetres into the substrate. |
| Minimum effective rooting depth | > 40 cm |
| Soil pH (acidity) | pH ranges from 7.3-8.1 (slightly to moderately alkaline) |
| Salinity | Low |
| Soil fertility under natural conditions | Very high total Phosphorus values and high Olsen extractable Phosphorus values. Very low levels of micronutrients, iron, manganese, copper and zinc are all present in extremely low (total) amounts. |
| Soil moisture regime | Ustic |
| Susceptibility to drought | Susceptibly to drought during dry periods but these are relatively rare, particularly in the southern islands. |
| Soil drainage | Excessively drained |
| Susceptibility to waterlogging | Not susceptible |
| Susceptibility to flooding | Not susceptible |
