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Mallee Seeps vs Dry Saline Land: Different Water Movements, Same Salinity Problem
First off – magnesia patches and dry saline land are the same thing so we’re talking about two different issues – Mallee seeps and dry saline land.
The term “magnesia patch” is colloquial term traditionally used by farmers, but in reality, dry saline land has no strong association with magnesium. While magnesia related salts exist (such as Epsom salts), it is sodium chloride that is always the dominant salt driving the degradation of these patches.
What Mallee seeps and dry saline land have in common is that the:
- Soil becomes too saline for crop growth.
- Subsoil clay is the source of salt. In seeps, saline water tables are also a source of salt.
The key difference is that good rainfall helps improve dry saline and patches but makes Mallee seeps worse.
What is it?
- Dry saline land (magnesia land) – Dry saline land is caused by transient subsoil salinity wicking to the surface through hot, dry periods. Surface impacts can change with seasonal conditions.
- Mallee seeps – A Mallee Seep is an area affected by a localised perched water table that brings water and salt to the surface soil layers. It can result in surface ponding and bare saline scalding over time.
Source of salinity
- Dry saline land (magnesia land) – Subsoil clays with high levels of transient salinity. The salt finds its way to the surface through the capillary rise of moisture, particularly after extended drought periods, to eventually become too toxic for plant growth.
- Mallee seeps – The perched water tables beneath Mallee seeps can vary from being quite fresh (from rainfall moving through sand) to highly saline (due to more historic geological saline processes). The fresher water seeps still pick up salts from subsoil clays which gradually rise and accumulate in the surface layers over time, while the rise of highly saline perched water tables can result in rapid topsoil salinisation.
Made worse during…
- Dry saline land (magnesia land) – Extended periods of hot, dry weather on bare ground.
- Mallee seeps – Excessive wet years and large summer rainfalls increase the size and impacts of perched water tables, while any dry years bring more salt to the surface layers.
What happens when it rains?
- Dry saline land (magnesia land) – Patchy areas can often decrease after periods of heavy rainfall as the rain leaches salts out or dilutes the salts in the surface layers. The patches will reappear and can lead to permanently degraded areas over time.
- Mallee seeps – Direct impacts can vary greatly between sites. Waterlogging or surface ponding will inhibit crop growth, causing bare patches that are more prone to evaporation and capillary rise, drawing salts to the surface. Rain can also increase crop growth surrounding early stage freshwater seeps as perched water table area expands.
Where do they form?
- Dry saline land (magnesia land) – Anywhere in the landscape, from the tops of hills to flat paddock areas patches, on soils with heavy clays through their profiles. Salts can wick to the surface through clayey, loamy and stony surfaces.
- Mallee seeps – Mallee seeps can form on mid-slopes through to lower lying zones, usually beneath sandier rises in areas where impervious clay layers lie within two metres of the surface.
What farming practices make it worse?
- Dry saline land (magnesia land) – Letting the area bare out, especially with grazing.
- Mallee seeps – Clearing, summer weed control, fewer deep-rooted perennials.
