What can I do if my garden has saline soil?
The problem of salinity in agricultural areas of Australia, where millions of hectares are in various stages of degradation, has become a huge environmental issue. At its worst a highly saline soil becomes toxic to biological activity and can turn into a wasteland where nothing will grow. Plants vary enormously in their sensitivity to salt and even mild cases of salinity can kill certain species. The same problems can occur in the home garden if we are not careful.
What is soil salinity?
The salt that we are most familiar with is sodium chloride or common table salt. To a scientist, however, any chemical compound that releases electrically charged particles (called ions) when it dissolves in water is a salt. Therefore all the fertilisers we use in the garden are actually salts and can cause problems if applied to excess. This applies equally to organic fertilisers as well as chemical or inorganic ones.
Where do salts in garden soils come from?
Soil naturally contains a variety of salts from the various minerals that result from the weathering of rocks. Various salts are dissolved in the water that we use, particularly town water that has chemicals added to kill biological contaminants as well as various mineral salts that may be picked up during its collection and storage. Town water that is derived from rivers can have high levels of salt such as that of Adelaide. In this case water comes from the end of the Murray River that has picked up huge amounts of salt on its journey through southeastern Australia.
Fertilisers are probably the most significant source of salts in the home garden, particularly if used at above recommended rates or too frequently. The problem will be worse if the irrigation water used is also naturally salty such as in Adelaide.
Saline agricultural soils generally have high levels of salts in the subsoil that are subsequently brought up to the surface by rising water tables, often due to artificial irrigation changing drainage patterns in the soil. These salts are generally sea salt that was deposited in ancient times when the land was below sea level. This problem can also occur in urban areas but it is not common.
How do I recognise a soil salinity problem in my garden and what can I do about it?
In severe cases a crust of salt (generally white in colour) can appear on the soil or on the surface of a pot. However, before it gets to that stage plants will usually start to show signs of water stress with wilting and yellowing, followed by browning at the margins of leaves occurring. Such symptoms can occur even when the plant is well watered although they will become more severe during dry spells because salts dissolved in the soil water will become more concentrated and cause even more damage. Usually a salinity problem develops gradually, however, a large overdose of fertiliser can cause acute problems where rapid wilting of the leaves occurs as the excessive dose of salts actually draws water out of the plant roots.
Most salinity problems in the home garden are caused by excessive fertiliser applications. Always take great care to thoroughly read the manufacturerâs recommendations and do not be tempted to apply more than is advised. Regular additions of compost to your soil will give it some capacity to absorb salts and prevent them damaging your plants. If you suspect such a problem the solution is to try and flush the salts out by running lots of water through the soil or potting mix and washing off the leaves any excess fertiliser. Keep the plant well watered for the following few weeks as it may take some time for the excess salts to be removed, particularly in a clay soil.
Measuring soil salinity in the garden
Salts dissolved in water release electrically charged ions, which in turn causes the water to conduct electricity. The more salts that are dissolved the higher the electrical conductivity that can be measured by a meter. An environmental officer at your local council or water authority will have such a meter to monitor salinity in water bodies and town water supplies and they are worth asking for help if you suspect you have a problem with your soil or water supply.
Is there anything else I can do about salinity?
In areas with saline town water a rainwater tank can be used to collect almost pure water that will be perfect for garden irrigation. Also plants that come from areas which have naturally saline soils (such as seaside locations) will be much more tolerant of salty soils. Some examples include coastal rosemary (Westringia fruticosa), coastal banksia (Banksia integrifolia), Australian succulents such as Disphyma and Carpobrotus and fairy fan flower (Scaevola aemula). You can go to the advanced search page, and either look at the ‘soil type’ box and click on ‘saline’, or go to the ‘plant environment’ box and click on ‘coastal garden’ to find a selection of salt tolerant plants.
If you know that your town water is high in salts then use a mild organic fertiliser such as well-rotted animal manure as such materials generally have lower salt concentrations that are released over a more extended period than inorganic fertilisers.