What Plants Can Tell Us about the Soil

It is quite widely accepted that the most appropriate plants will germinate and grow naturally in the soil to match the full range of conditions (soil health, mineral content & organic matter, ground cover, climate, aspect, parent body material etc.) that apply at that particular point in time.

Last week Pauline Roberts and John Polglase and from Divstrat Pty Limited were visiting Ochre Arch to help us try and locate some bore sites to drill for underground water. John is currently studying geology and commented at one point that the greatest emerging threat to soil health is acidity, driven in part to the application of some chemical (inorganic) fertilisers.

It’s curious how things can ‘connect’ closely together at times. I’m presently reading the book titled “Culture and Horticulture: A Philosophy of Gardening” by Wolf D Storl ISBN 0-938250-01-9 and came across the following text, strongly reinforcing John’s comments:

“Acidity or Alkalinity (Percentage Base Saturation, pH)), or sweetness or sourness of the soil is indicated by the pH scale ranging from 1 to 14. Soils range from very acid soils of about a pH of 4, which is about the acidity of tomatoes, beer, or grass silage, to a pH of 8, which is about as alkaline as sea water or eggs. Most plants prefer to grow in earth that has a pH of 6 or 7. Humus buffers the soil between 6 and 7. Wet soils are usually sour; they have low base saturation because the bases (Ca, Mg, Na, K, etc.) usually leach out in the rain, leaving an excess number of hydrogen ions that are the indicator of acidity. Sandy soils, peat-moss formations and the podsols of northern, wet climates furnish examples of this happening. In dry climates, as in southern California, the opposite happens – alkalinity increases and salts are deposited on the surface of the soils. Before indicator tests for pH came about, farmers could tell by looking at the weeds whether a soil was sweet or sour. The presence of sorrel, sour dock, buttercups, tussocks, hawkweeds, horsetails, knotweeds, cinquefoil and daisies indicates an acid soil; whereas alfalfa, sweet clover, burdock, coltsfoot, chamomile, and others indicate a sweet soil.

The application of chemical fertiliser tends to acidify the soils so that the addition of large quantities of lime is concomitant with their use. Humus derived from careful composting, on the other hand, has such a buffering effect that the organic gardener does not have to worry about the pH at all. Humus and microorganisms buffer the soil by letting excess H ions go when the soil is too acid, and letting Ca ions go when the soil is too base. If … the gardener wishes to increase the pH he can sweeten the soil by the addition by the addition of ground limestone or dolomite; or he can make the soil more acid by adding pine needle mulch, coffee grounds, oak leaf mulch, cotton seed meal etc.”

The book goes on a bit later to say ….

“…. plants actively work at creating for themselves the soil they need. Some plants function as accumulators and change the soil in one direction or another, e.g. daisies collect calcium in acid soils, horsetail collects silicon even in silicon-poor soils, orache collects salt etc. Upon their death, these plants will enrich the soil with these elements and change it accordingly.”

Personally reflecting on the above for a moment ….

I’ve observed on many occasions that one of the main weeds that seem to emerge underneath and around winter cereal crops sown with chemical fertilisers is Capeweed (Arctotheca calendula). These plants have “masses of yellow, daisy-like flowers with dark, almost black centres” (see http://www.dpiw.tas.gov.au/inter.nsf/WebPages/RPIO-4ZZ48E?open) and it makes me wonder whether Capeweed is related in some way to daisies and if so whether they are thus indicating acids soils.

Another observation is with Patterson’s Curse. The conditions for growth are bare and in some cases compacted soils that I’m reliably informed will also be deficient in copper. Patterson’s Curse accumulates copper, and horses which graze in paddocks that are heavily infested with these plants have been known to suffer from copper poisoning.

Just about every grazier will have observed Stinging Nettles growing in stockyards and under stock camps. This is an indicator of high nitrogen levels, associated with the breakdown of concentrated amounts of animal urine and manure.

Broadleaf plants emerge rapidly on some bare soils as nature’s way of providing protection and assisting in the build up of litter and organic matter.

Taproot generating plants such as thistles grow on compressed soils as a mechanism for ‘opening up’ the soil, allowing aerobic (air based) processes and decomposition to work effectively.

So it’s all very interesting stuff, this ‘reading’ the sign-posts of nature.

Oh, and by the way, both Pauline and John are both amazing people and I’d encourage anyone who is searching for underground water or is looking to do farm mapping to get in touch with them. There contacts are on the Divstrat Pty Limited website.