Organic mats and soil testing: What you need to know
Some farms in Gippsland have organic mats in some of their paddocks and if/when they soil test these
paddocks they unknowingly do so incorrectly. Organic mats or root mats are sometimes referred to as
‘acid’ root mats but not all organic mats are ‘acid’ mats. Organic mats can originate from bent grass
root mats (Figure 1) or other species such as fog grass on sandy soils or paspalum on heavy clay
loams.
These organic root mats build up over time due to underlying soil conditions being unfavourable for
the normal process of organic matter breakdown within the soil (roots and root hairs) and/or a buildup of organic matter (mainly dead leaves, dead plants and plant crowns) on the surface of the soil.
Organic matter build up on the surface is usually due to paddocks becoming waterlogged regularly
and for long periods or from salinity.
Acid root mats most commonly occur as a result of the soil being very acidic (low pH) and often
coincide with low soil fertility, particularly potassium, as well. This low acidity (less than pH5.1 (water)
reduces the range and activity of soil micro-organisms which would normally break down the
dying/dead organic matter into plant available nutrients and other products of plant decomposition.
This process provides essential nutrients for plant growth as well as sustaining productive healthy
soils and all the living organisms (bacteria, fungi, mites, ants, millipedes, beetles, earthworm, slugs
and snails) which live in the soil.
In the acid mat scenario, the nutrient levels and pH itself is higher in the root mat than the soil under
it. In soils, nutrients adsorb (attach) to clay minerals and organic matter and strongly so in the organic
matter. This means that much of the nutrients in the organic mat are not so available to plants and ‘tie’
up a lot of fertiliser nutrients when applied. This characteristic has implications for soil test results. In
soils with an acid mat, the critical problem being a low pH (maybe also low nutrient levels) may not
be identified or the extent of the problem not so pronounced in the soil test results.
If a 10 cm soil test core is taken from the surface in a paddock with an organic/acid mat (Figure 2),
the soil test results will indicate that the pH and nutrient levels are much higher than the actual case in
the soil. The soil test results will therefore not indicate the true soil nutrient and pH of the soil. An
example is a bent grass mat in grey sandy loam east of Inverloch tested in the mat at pH(w) 4.9 but
pH(w) 4.5 in the soil itself. The pH scale is logarithmic meaning that the lower figure is substantially
more acidic than the higher figure.
So when soil testing paddocks with organic mats, it is essential to remove the organic mat and soil
core the soil only to 10 cm depth. Carefully remove the organic mat without removing any of the soil.
Sometimes, this is not easily achieved and can be time-consuming but better than renovating and
sowing only to have the problem return very quickly, also time consuming and expensive. Qnly will
coring into the soil allow the soil test analyses to help you to identify the real problem in an acid mat
soil, usually low pH and low nutrient
Direct drilling seed into the mat itself without addressing the real problem will result in plants
germinating but then stop growing after some weeks and turning yellow from nitrogen deficiency as
very little nitrogen is released in the mat due to minimal soil micro-organism activity. Acid mats also
tend to develop allellopathic (poisonous) effects within the mat and this greatly affects plant growth,
often killing them in the older mats. Therefore accurate soil testing on soils with soil mats is crucial to
identify issues and determine remediation.
For further information contact Frank Mickan, DEPI Ellinbank, on (03) 5624 2259.
Figure 1. Bent grass acid root mat
Figure 2. 10 cm of root mat should be discarded