Soil pH – why is it so important?

Soil pH is tricky to describe in detail and dare I say it …… a little boring! (Apologies here to chemistry buffs.) Before your mind glazes over while I discuss soil pH levels, tests, and treatments ……………….. the single most important thing you need to come away from this article understanding is that soil pH has a dramatic effect on nutrient availability.
We’ll return to this again after a pH ‘crash course’.

What is pH?

pH is the concentration of hydrogen ions in water. The pH scale ranges from 0 to 14, with 7 being neutral. Values below 7 indicate an acid soil, and above 7, alkaline. Because the pH scale is logarithmic, a pH change by 1 unit means it is 10 times more acidic or alkaline(1). The difference between a pH of 7 and a pH of 6 is 10 times the acidity, between 7 and 5 is a 100 times the acidity and so on.

The majority of plants in cultivation grow well in a soil with a pH of 6 to 7.

How do I know if there is a soil pH problem in my garden?

If your plants are growing well and your soil is full of organic matter, worms and other ‘beasties’, it is probably safe to assume all is well and you may never need to worry about soil pH. Even if in this situation, you test the pH and find it outside the range 6 to 7, it is reasonable to assume the plants are obviously coping well with the current soil chemistry and any change may indeed be detrimental.

Indicators of a pH imbalance may include:
• Plants not performing well, even with the addition of fertiliser. Fertiliser added to a soil with a pH imbalance will not be available to the plant;
• ‘Lime-induced chlorosis’ – an iron deficiency caused by high pH levels, seen as yellow-white leaves on plants growing in alkaline soils; or
• flowering hydrangeas (in your own or a neighbours garden). A soil pH of 6 or below will produce blue flowers while a soil pH of 6.8 or higher will produce pink flowers.

Testing Soil pH

The Manutec Soil pH Test Kit is available from retail nurseries and hardware stores and is the most common soil test undertaken by Australian gardeners and farmers. Developed by CSIRO, the Manutec Soil pH Test Kit is arguably not the most accurate test available, however it is economical and instant and will give you a general idea of what’s going on. It is suitable to test soils, compost and potting mixes and involves adding some water to a small soil sample together with chemical indicators. For a short video on how to test soil pH https://www.youtube.com/watch?v=S6AizqoDe5Y

Analysis in a laboratory provides the most accurate measurement of soil pH (2). It used for large scale soil management strategies such as treating acid sulphate soils.

Soil pH and nutrient availability

As already mentioned, the availability of nutrients in soil changes, as pH changes (3). From the attached diagram, it is clear that in the range pH 6 to 7.5 most nutrients are reasonably available to plants. Acid soils with a pH of less than 6 commonly have deficiencies in calcium, magnesium, phosphorus, potassium, molybdenum. Iron, manganese, zinc, copper and boron are commonly unavailable in alkaline soils with a pH of more than 7.

The effect of this nutrient availability on growing plants varies, depending on species. Acid loving plants such as Azalea, Camellia, Gardenia, Blue Hydrangeas and Blueberries prefer an acid soil pH of 5 – 6. Some plants prefer an alkaline soil with Sweet Pea, cacti, Geranium, Hebe, Hibiscus, Ivy Poinsettia and Vibernum growing well in soil pH 7 to 8.

Other effects of soil pH on plant growth

In addition to nutrient availability, the other effects soil pH may have on plant growth include:

• Amounts of nutrients held in soils. Altering soil pH can change the amounts of nutrients that can be held in a soil. As soil pH is raised, more negative charges are produced and these can hold more nutrients as cations.
• Toxicities. Plants can be poisoned in soils of excessively low or high pH. Acid soils with a pH of less than 4 commonly have toxic amounts of aluminium and manganese.
• Microorganisms. All soil microorganisms have a pH range in which they survive and grow best. The trick is to encourage those that are beneficial to plants. Decomposers of organic matter prefer a pH range of 5 to 9. Rhizobium bacteria require pH above 5 (3). This has relevance for many Australian Acacia species relying on rhizobial bacteria to fix nitrogen.

Treating soil pH

Soil pH varies with time, soil moisture and even testing methods. As such, treating soils with a view to keeping pH at exactly 6.5 is not appropriate. Aiming for a pH range or even a general desired ‘trend’ towards acid or alkaline is more realistic.

Raising pH is undertaken with the addition of liming material. Generally garden lime is made up of calcium carbonate. Although it is not pure, it does have significantly less magnesium carbonate than Dolomite. For most applications and particularly where magnesium is already present in the soil in high amounts, garden lime is the best choice. Dolomite is a mixture of calcium and magnesium carbonates and is commonly used in potting mixes where additional magnesium is welcomed.

Lowing pH is a little more tricky, with powdered sulphur the cheapest and most commonly used material.

For details on how much lime or sulphur to add, depending on soil type refer http://soilphtesting.com/?page_id=10

Another option for treating soil pH, is adding organic matter via compost, green manure crop or organic mulch. This buffers the pH, which means that it tends to bring both acid and alkaline soils closer to neutral (4).

Australian Native Plants, Bluedale and Soil pH

Australian soils have a wide variety of pH, some with quite extreme levels. Soils of the Sydney region are renowned for their low pH. On the other side of the continent, in Perth, soils close to the limestone based coastal sands are naturally alkaline and plants indigenous to this area require alkaline conditions to survive.

The majority of the plants grown by Bluedale are native to Australia. Many can cope with pH outside the preferred range of 6 to 7, however it is important to note we generally grow plants for the landscape/garden industry. As such they have been carefully selected, and are proven to grow well in the conditions prevalent in Australian garden conditions – this includes a pH of 6 to 7.
References

(1) Government of Western Australia, 2010. Department of Agriculture and Food Garden Note 174 ‘Soil pH and plant health in the home garden’.
(2) NSW Agriculture (2000) ‘Leaflet No 2. ‘Understanding Soil pH’
(3) Handreck, K.A. and Black,N.D. 1984 ‘Growing Media for Ornamental Plants and Turf’. New South Wales University Press. Printed in Singapore by Kyodo Printing.
(4) Organic Gardening Website http://www.organicgardening.com/learn-and-grow/understanding-ph

Guidelines for Successful Wetland Planting Part 3 – Reedbed Wastewater Systems

Introduction

Approximately 12% of Australian households are not connected to centralised sewerage systems and hence use on-site technologies for the management of their wastewater (Geary and Gardner, 1996 in [1]). In 2001 the failure rate for on-site sewage management systems (eg septic tanks) in the Lismore City Council area was found to be 44% [2]. In Europe, constructed wetlands (reed beds) are increasingly being adopted [1]. Given these figures and the many studies confirming the success of reed bed systems (see references list), it is likely Australia will see an increase in the use of reed bed wastewater systems.

A reed bed is essentially a channel, lined with an impermeable membrane that is filled with gravel, planted with macrophytes and used to treat wastewater2. Reed beds are a second stage wastewater treatment and are become more common in domestic applications. They are usually used in conjunction with:
• Primary collection system. This may be a septic tank, greywater tank, or aerated wastewater treatment system (AWS);
• Secondary treatment system. More than one reed bed may be used. Depending on the situation, a sand filter may augment the reed bed. Alternatively an AWS undertakes both the primary and secondary treatment roles; and
• Land application area. Treated water may be reused as subsurface irrigation or disposed of via a trench.

The ‘Treatment Train’ Process [2]

The role of reed beds in this ‘treatment train’ is to detain the wastewater for a period of time so it slowly passes the roots and stems of the macrophytes (or reeds) where it is treated via a number of physical, chemical and biological processes.

Compared to other technologies, including the commonly used AWS, reed beds have a number of advantages. They are cheap to build, require no power to operate and very little personal effort or money to maintain[3] . Reed beds can be installed on sites with soils usually not suitable for on-site wastewater disposal such as sands, clays or steep slopes[1]. Reedbeds do not use chemicals [4], relying instead on natural processes. Council inspections are usually annually – compared to quarterly inspections required for AWS [4]. In addition, a reed bed can become an aesthetically pleasing, functional part of a garden [3].

Design

Common reed bed specifications are listed below to help you get the idea. Reed beds are generally designed by specialist contractors and require Council approval. Council wastewater management codes differ may differ, so check with your local Council prior to designing your reed bed.
• Detention time – generally 5 to 7 days
• Lining – concrete or polyethylene pipes, water tanks or Duroplas moulds. Many Councils no longer recommend the use of plastic liners as they can be pierced by macrophyte rhizomes.
• Substrate – in most cases 10mm gravel is chosen. Alexandrina Council in SA [5] specify 20mm washed gravel. Larger stones (eg rail ballast 60-80mm) are placed in the first metre of the bed (inlet zone) and sometimes adjacent to the outlet structure [2] to reduce clogging.
• Water Depth – ranges of 300 – 700mm are generally specified. Reed beds tend to be designed to a depth dependent upon the rooting depth of the planted macrophytes [1]
• Slope – may be zero to 1%.

It is notable that [2] found earthworms useful in cleaning reed bed substrate. De-clogging it if you like. A management outcome of this finding could be the inoculation of reed beds with earthworms to clean the substrate and prolong their useful life [2].

Typical Reed Bed showing major components [3]

It’s all about microbes really!!!

Like all plants, macrophytes in reed bed wastewater treatment systems require nutrients for their continued growth. Simply by growing, they remove considerable amounts of nutrients, particularly prosperous and nitrogen.

This relatively high rate of nutrient uptake is, however, often insignificant compared to the loading rate to which most reed beds are subjected Cooper et al, 1986 in [1].

It seems other processes contribute to the wastewater treatment process. One of the most significant is the degradation of pollutants undertaken by the microbial populations attached to wetland substrate (gravel) and the roots, rhizomes and submerged stems of the macrophytes. Planted macrophytes not only provide the physical structure for these microbes to grow, they transfer oxygen to their root zone, providing microscopic aerobic sites for micro-organisms. These oxygenated microenvironments enhance many, if not all, pollutant removal processes within reed bed systems (IWA, 2000 in [1]).

Reed Bed by Armidale Newline Plumbing

Reed beds also provide a great buffering against fluctuating air temperature, a factor which is thought to affect the rate of many pollutant removing processes (Kadlec and Knight, 1996; Hill and Payton, 1998 in [1]).

In selecting macrophyte species for reed bed systems, the nutrient uptake capacity of the plants is not the primary selection criteria. Species should be selected based on their capacity to grow well in local conditions (which may mean survive frost) and maintain an abundant surface biomass. Stem density may also be a consideration for the reed bed designer.

Species

Until recently, Phragmites australis (the Common Reed) and Typha orientalis were the main reeds used in reed beds on the NSW North Coast [2]. However, with the understanding of reed beds growing continuously, many new plant species are being used , such as Lomandra hystrix, Baumea articulata and Schoenoplectus mucronatus [2].

The aggressive nature of the Phragmites australis rhizome system and a tendency for senescence in the top growth (leading to a rather ragged appearance) in the winter months have prompted a search for other species [3]. Some Council’s, such as Alexandrina Council in South Australia [5] specify that rigid plastic modules must not be planted with Typha orientalis, a species with considerable expansive strength, due to the risk of rupture when the module becomes packed with growth. A greywater reuse operator in WA [6] suggests Typha orientalis and Phragmites australis should not be used in domestic wastewater treatment systems in Australia because of the massive seasonal release of wind-blown seeds.

Many Council wastewater management codes recommend the use of locally indigenous species, presumably as these are guaranteed to perform well under local conditions.

Lismore City Council [2] list 9 suitable species for northern NSW (from L. Davison and T. Headley, 2003 in [2]), which includes both Typha orientalis and Phragmites australis.

Unfortunately, most lists that I have seen do not include Lomandra hystrix, a species used with great success in reed beds on the mid north coast of NSW and one suggested by [2] and used extensively by [4].

Bluedale’s recommendation for reed bed species selection is to use locally indigenous species that maintain a reasonable level of growth throughout the year. The following list is suitable for coastal areas of northern NSW.

Baumea articulata
Baumea rubiginosa
Bolboschoenus fluviatilis
Eleocharis sphacelata
Lepironia articulata
Lomandra hystrix
Schoenoplactus mucronatus
Schoenoplactus validus

For details on the macrophyte species suitable for your area, contact Bluedale on 6586 0100, sales@bluedale.com.au or www.bluedaleplantsonline.com.au

References
[1] Davison, L., Bayley, M., Kohlenberg A. and Craven, J. (2002) Performance of Reed Beds and Single Pass Sand Filters with Characterisation of Domestic Effluent: NSW North Coast. Septic Safe Research Report, NSW Department of Local Government, 112 pp. accessible at http://www.dlg.nsw.gov.au/Files/Information/R14-Lismore%20City%20Council.pdf
[2] Lismore City Council (2005) The Use of Reed Beds for the Treatment of Sewage & Wastewater from Domestic Households. Accessible at http://www.lismore.nsw.gov.au/page.asp?f=RES-PEY-54-48-10
[3] L. Davison, L., Headley, T. and Pratt, K. (2005) Aspects of design, structure, performance and operation of reed beds – eight years experience in north eastern New South Wales, Australia Water Science and Technology 51, 10, pp 129-138, 2005.
[4] Midcoast Reedbed Wastewater Systems website http://midcoastreedbeds.com.au/
[5] Alexandrina Council. Environmental Health Fact Sheet What is required with an application for a Reedbed (second-stage wastewater treatment) system? Accessible at http://www.alexandrina.sa.gov.au/webdata/resources/files/Reedbeds.pdf
[6] Greywater Reuse Systems website http://www.greywaterreuse.com.au/