What is a Healthy Soil?
A healthy soil is sufficiently diverse biologically, to regulate the availability of nutrients for plant growth,
and to promote a healthy root system.
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What Makes a Soil Healthy?
Organic matter is the cornerstone of a healthy soil. Plants can grow in
the absence of organic matter, as in hydroponics, but nutrients must be
replenished continuously, and plant pathogens must be excluded
(beneficial organisms which out compete pathogens, need organic matter
for growth). Organic matter is nature’s slow-release fertiliser (Figure
1).
As organisms grow, nutrients absorbed from the soil are locked up in
their cells. Minerals in the form of waste products are released as
part of growth, and on death, their tissues are decomposed by other
organisms leading to the excretion of more mineral waste products. Like
humans, most soil organisms depend on sugars and starches (organic
carbon) to fuel growth. In the process, these compounds are transformed
into energy and carbon dioxide (a gas). Carbon dioxide represents a
nett loss of organic carbon to the system, as does the harvesting of
plant and animal produce.
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Figure 1 (click to enlarge)
Nutrient dynamics within the soil profile. Straight arrows
show inorganic processes, curved arrows show organic processes.
Inorganic nutrients in excess of plant and microbial requirements, will
be lost via leaching and volatilisation (in the case of N). Humic acids
interact with minerals fixed to clays, making them more available to
plants.
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What is Enough Organic Matter and How do I get it?
There is no standard figure for enough. As a benchmark, check the
levels of organic carbon in a relatively undisturbed patch of
vegetation (remnant vegetation or native pasture with a history of low
grazing pressure), on the same soil type. However levels of over 2%
have been measured for long-term pastures with well developed litter
layers (L -plant residues), under no tillage in Qld.
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Animal manures will improve the
availability of P and K in particular, but the organic carbon component
is relatively short-term in nature, and high application rates may lead
to P enrichment of local waterways.
Green manures also provide a
short-term organic carbon deposit, but the tougher material in sorghum
adds to the longer term pool (a mulch crop).
Co-composting animal manure
with sorghum stubble or sawdust, not only maximises the storage and
slow-release of NPK, but also adds to both the short and longer-term
organic carbon pools. Maintaining the organic matter bank in your soil
will minimise nutrient losses via leaching and erosion, improve soil
structure, and improve plant health.
Indicators of a Healthy Pasture
Under a no-till pasture management a distinct litter layer (L) will
develop, providing organic matter for soil microbes and soil animals.
Under the litter layer fine particles of partially decomposed organic
material will collect. Below this layer humic colloids (H, products of
the decomposition of organic matter) will discolour the clay particles
that they complex with. Root systems penetrate through these layers,
tapping the water and inorganic nutrients at depth. Lucerne is a
preferred pasture plant due to its nitrogen-fixing ability and vigorous
rooting system, which can extend several meters deep into soil.
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Dry heat extraction of soil animals
In order of abundance, mites (4), springtails (3), bristletails (2) and
symphylids (1 –mistaken for small centipedes)) are the most common soil
organisms feeding in the litter layer. You will need a tungsten
filament lamp (desk lamp), a plastic funnel (175 mm diameter) a glass
jar, and an empty milk carton to support the funnel. Cut two circles of
12 mm aperture chicken wire (or gutter guard) to approx. 100 mm to fit
mid-way down the funnel. Offset the mesh circles to minimise the hole
sizes through which the soil organisms will move (this will reduce the
amount of soil falling through). Half fill the far with water and place
it in the bottom of the milk carton.
Place the funnel in the carton so that the barrel is in the jar. Use a
spade to cut about a 10cm cube of litter and soil from the pasture.
Place this carefully into an ice-cream container for transport home.
Avoid breaking up any clods, as this will cause more soil to fall down
the funnel. Place the litter/soil sample on the mesh support, then
position the lamp in the funnel, as close to the soil (without touching
it), as possible. The heat and drying effect will repel soil animals.
After 24 or 48 hours, pour the contents of the jar into a white tray or
ice-cream container, to observe the numbers and diversity of soil
animals present. You may need a magnifying glass, as most of these
organisms will be less than 5mm in length!
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The soaking method for observing root systems
The best time to look at a root system is after the seedling stage, but
before the stresses of flowering and fruiting. Insert a garden fork
vertically into the soil, on each of the four sides of the plant. Lever
the soil from each side in turn, until it is possible to remove the
system as one intact clod. Ease the clod into a bucket, with as little
disturbance as possible (excessive force will break the root
connections). Immerse the clod in water, and allow it to soak for an
hour or two. Water will infiltrate the soil pores, making it easier to
wash the roots. Support the root system by putting your hands under the
water, under the soil clod.
Gently shake your hands to agitate particles away from the roots.
Discard excess soil in the bottom of the bucket, change the water, and
repeat the process. After most soil has been removed, place the root
system in 2cm of water, on a white tray (large white ice-cream
container). You should see lots of finely branched roots (6), emerging
from the main laterals and tap root. Healthy legumes may have nodules
present (5). However, only those with a blood-red pigment inside, will
be actively fixing nitrogen. Unhealthy roots will have few fine roots,
with brown or black discoloured patches, sections missing from the
outer root tissue (7), and evidence of spear-tipping (8). The ability
of the plant to compensate for and to replace damaged roots, will
determine the health status of the plant.
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