The Secret to Bushcare is Soil History

The Secret to Bushcare is Soil History

How chemistry explains a stubborn patch of bush along the Lane Cove River

Background

Most of Lane Cove National Park sits on sandy, acidic soils weathered from Hawkesbury sandstone, but one area is different. Next to a bend known as Fairyland, the river was dredged for builders sand during the 1960s. To reach the sand, the crews first had to strip off the fine river silt lying above it. That silt had to go somewhere, and it was dumped in a clearly defined mound in the patch of bush (see Fig. 1).

Figure 1. Map of Fairyland area with red line around affected region

Here, the soil is compact, fine, and full of shells, mostly Sydney cockles and native mud oysters (see Fig. 2). These alter the soil chemistry even sixty years later, with visible impacts for members of the local bush regeneration effort like me.

Figure 2. Compact silty soil full of shells in the identified problem area

Findings

Currently this patch is dominated by a single plant: invasive privet forming a dense forest (see Fig. 3). Where privet is cleared, the main native that reliably takes hold on its own is kangaroo apple, a tough pioneer (see Fig. 4). Other introduced weeds do well here too, including wild tobacco and inkweed.

Figure 3. Privet forest as seen from road

Figure 4. New growth under cleared privet

The trouble occurs when we plant the species we would expect to belong in this area such as she-oaks and ferns. The seedlings are sickly and then die. Recently we noticed a common pattern between Trema tomentosa, Pittosporum, and Casuarina cunninghamiana plants in the area. The youngest leaves are yellowing, starting at the edges, while the network of veins stays green (see Fig. 5). This is a phenomenon known as interveinal chlorosis, and it is a classic distress signal indicating iron deficiency.

Figure 5. Left: Pittosporum leaves showing chlorosis encroaching from the sides of the leaf. Right: young Trema tormentosa leaves also affected by intraveinal chlorosis

So I did a deep dive in the chemistry of this soil and what’s going wrong. A pH test of the dredged soil reads a pH of 8.5, which is alkaline and very unusual for local bushland. Most Sydney bush plants are adapted to acidic ground, somewhere around pH 5.5 to 6.5, as is common for areas with eroded sandstone. This unusually high pH is due to the abundance of the shells in this area. These are made of calcium carbonate, the same mineral as limestone. As it dissolves in soil water, it buffers the soil through a chemical balance that can be written like this:

CaCO₃ + H₂O + CO₂. ⇌. Ca²⁺ + 2HCO₃⁻

Simply adding acidic material to this soil wouldn’t make a difference, as the buffering action would simply neutralise the acid.

Identified Issue

High pH is what is starving the plants, even though the soil is not short of nutrients. Plants need iron to build chlorophyll, the green pigment that runs photosynthesis. Above about pH 7, iron locks into insoluble forms that roots cannot take up, so the plant slowly runs out even while iron sits all around it in the soil. A similar lack of bioavailability due to pH may also be starving the plants of other important micronutrients (see Fig. 6).

Figure 6. Graph of plant nutrient availability according to soil pH

It is worth being clear that saltiness (due to the brackish river) and alkalinity are two separate concepts. Salt is mostly neutral, and after sixty years of rain most of the original sea salt has leached away. The alkalinity from the shells is what is now holding back plant growth.

There is a second, physical problem with the soil. Being dredged silt, the soil is very fine. Fine soil has tiny pore spaces that block easily, so water sits rather than draining, and the ground swings between waterlogged and hard-set. Roots need air as much as water, and plants used to the open, free-draining sands of the surrounding sandstone forests, or coastal plants, simply suffocate here.

Proposed Solution

So the only plants that can survive here are those that tolerate alkaline, poorly drained ground. Tidal floodplain communities are the obvious place to look, since they are built for waterlogged, salty, fine-textured soils. But these communities usually grow on acidic to near-neutral soils, so not every member will cope with our unusually high pH. There is no easy way to acidify ground with such a strong buffering capacity. What we can do instead is make sure that iron and the other locked-up nutrients become available to roots despite the high pH.

The single most powerful natural tool is mulch and organic matter, which releases naturally chelating compounds as it decomposes that bind to the vital micronutrients and hold them dissolved and available even at high pH. Over time it also opens the soil, holds moisture, and prevents weed growth.

Following this, the logical next step is to plant native plants which actually thrive on silty, waterlogged soil. The natural fit for this soil profile is the Swamp Oak Floodplain Forest, a community that is itself listed as endangered, so restoring it here is doubly worthwhile. Here is a list of the plants that belong to this community. Note that some adjustment is required considering the alkalinity of the soil, so I have included only plants likely to survive as they are not specialised to acidic soil.

Canopy:

• Casuarina glauca (Swamp Oak): requires full sun, so privet must be cleared first. It is important to plant this species, not the casuarina cunninghamiana which has currently been planted, as the former is adapted for higher pH and waterlogged soil while the latter struggles to survive

Groundlayer:

• Carex appressa (Tall Sedge): placed on compost mounds, requires more aeration than some of the other species
• Lomandra longifolia (Spiny-headed Mat-rush): placed on compost mounds, requires more aeration than some of the other species
• Tetragonia tetragonoides (Warrigal Greens)
• Juncus kraussii subsp. australiensis (Sea Rush)
• Ficinia nodosa (Knobby Club-rush)

After a couple of seasons, the soil should be improved enough for other ground covers, vines, and paperbarks.

Ultimately, soil is not a blank backdrop that plants happen to grow in, but a record of everything that has happened to a place.