The soil supports a web of life or food chain commencing with the plants themselves, which start the whole thing off by using sunlight and chlorophyll to create simple sugars, made use of on a microbial level by bacteria, fungi and nematodes (microscopic worms). As you move up the chain, organisms become larger and more complex until you reach the climax predator(s) for the particular bioregion where you live. This is summarised by the illustration below. 
But how does all this relate to testing the soil in the veggie patch in your backyard?
Using Plants to Detect Soil Nutrient Deficiency
Symptoms and signs of disease exhibited by the plants growing in your back yard may provide an indication of the deficiency of certain soil nutrients or their abundance to the point where they cause toxicity in the plants which are growing there. I have several books in my library which contain many hundreds of photographs illustrating how nutrient deficiency and toxicity symptoms manifest in individual fruit, vegetable, grain and pasture plants. On the surface, sorting through this mass of data and decoding what these symptoms mean to you in your situation could provide a daunting task. However the following diagram (developed by the University of Minnesota Extension Service and published here) provides a wonderful summary and flow chart making the job considerably easier.
* Symptoms refer to deficiency unless otherwise stated.
** Symptoms of sulphur deficiency usually occur on upper leaves first, but a general yellowing of the entire plant may occur under prolonged deficiency conditions.
Worm Numbers in Soil
The number of worms in a given amount of soil from you veggie patch or whatever may act as an index as to the health of your soil. The test is carried out as follows –
- Ensure that the soil temperature is at least 10°C and has not recently been cultivated.
- Using a trowel and spade dig out a 30cm cube of soil and place onto a tarp, garbage bag etc.
- Gently sift the soil (if possible) through a garden sieve and then count the worms present in your sample.
- Count up the number of worms discovered.
Ten worms per test confirms that your soil is in good shape where as 9 or less may point to problems such as lack of soil organic matter (in association with other tests) or the soil is too dry, there are pH problems, there may be pesticide contamination or the soil is too compacted.
In the event of a low worm count the idea to correct the situation is not to supply more worms but to fix the underlying problem – “if you build it they will come”.
Soil Respiration Test
One advantage of the above “worm” soil test is that you don’t need much in the way of equipment to carry it out, and what equipment you do need you are likely to have hanging around anyway. The soil respiration test, while it gives you a good handle on how the life in your soil is doing, does require some specialised equipment. I need to point out at this point that while I have read about this and have used Draeger tubes for a number of different analyses when I worked in the chemical industry, I have never actually carried out a soil respiration test.
The idea behind this test is that organisms in the soil take in oxygen and emit carbon dioxide in the same way that humans do and by measuring the amount of carbon dioxide emitted from a given volume of soil over a given time will act as an indicator of the biological health of the soil. The soil respiration rate can be affected by temperature, moisture, nutrient content and level of oxygen in the soil and so the result should be interpreted in combination with the results of other soil tests.
To carry out this test you need –
- 1 x 150mm diameter metal ring about 125mm long (eg 125mm cut from the end of a 150mm diameter tube)
- lid to fit the above with rubber stoppers
- Club hammer and wood block
- soil thermometer
- two sections of plastic tubing
- 2 needles
- Draeger tubes (to read CO2 – usually 10 tubes to a box) (Carbon Dioxide 0.5%/a; Mfr No CH31401)
- 140 cc syringe
- stopwatch or timer
The test needs to be carried out when the soil is close to Field Capacity, or in other words the area being tested has been watered well and then allowed to drain for 16 to 24 hours so that it is holding as much water as it can against gravity. Soil respiration will be inhibited if the soil is too dry or saturated with water. More detail on this test is available here.
To carry out the test –
- Using the club hammer and wooden block, drive the ring into the test area to a depth of 75mm.
- Place the lid on the ring and wait exactly 30 minutes.
- Insert the soil thermometer into the soil 25mm from the ring and 25mm deep.
- Assemble the Draeger Tube apparatus by connecting a needle to one section of tubing, breaking open both ends of the Draeger tube, connecting the Draeger tube to the other end of the needle tube, using the other piece of tubing to connect the syringe to the other end of the Draeger tube. (note: ensure the arrow on the side of the Draeger tube points AWAY from the needle.)
- Insert the needle into the head space through the stopper, then insert the other needle into the other stopper just before sampling.
- Take the sample by drawing back the plunger to the 100ml mark on the syringe over 15 seconds.
- Record the soil temperature off the thermometer and read the concentration of CO2 off the Draeger tube by noting the highest point on the tube where the colour can easily be seen.
To calculate the soil respiration in pounds of CO2-C per acre per day use the following equation –
Soil Respiration = PF x TF x (%CO2 – 0.035) x 22.91 x H where:
Pressure Factor (PF) = 1
Temperature Factor (TF) = (soil temperature in °C + 273) ÷ 273
(H) = Inside Height of the Ring in cm (2 cm)
The Solvita Company also produce a colourimetric test which can give and indication of soil biological activity. Their GardenCare kit is reasonably priced at less than $50 (AUD) which buys you enough for two soil life tests (as well as 2 compost maturity tests). Draeger tubes cost a bit less than $100 a pack (inc GST) which gives you 10 tests.
