SOUTH OTAGO FARMING FOR PROFIT NEWSLETTER | AUGUST 25 2016
FARMING FOR PROFIT
AUGUST 25 2016 — TOPICS —
Soil Fertility
Soil Physical
Structure
Soil Biology
SOUTH OTAGO
Understanding your soils
The aim of this event was to provide an interesting,
interactive and credible insight into soil health,
covering the three key aspects of soil fertility, soil
physical structure and soil biology. The keynote
speaker for this event was Milton Munro, Technical
Manager of Land Production at PGG Wrightson.
Fertiliser Basics
Facilitator
Nicola Chisholm,
AgFirst Otago Ltd
Ph 027 610 2221
[email protected]
Extension Manager
Olivia Ross
Beef + Lamb New Zealand
Ph 027 801 7868
[email protected]
0800 BEEFLAMB (0800 233 352) | WWW.BEEFLAMBNZ.COM | BY FARMERS. FOR FARMERS
1
SOUTH OTAGO FARMING FOR PROFIT NEWSLETTER | AUGUST 25 2016
Soil fertility
pH
Soil tests
The pH test measures the acidity of a given soil. In chemistry
“the pH is a measurement scale for determining the acidity or
basicity of an aqueous solution”. To understand pH we need to
understand a little basic chemistry. Where there is water there
are hydrogen ions (H+) and hydroxide ions (OH-). The higher
the concentration of H+ ions the more acidic the solution is.
It is possible to measure the amount of H+ ions in a solution.
However, these numbers tend to be very small and hard to
work with. The solution is to convert them into a simple scale,
ranging from 0 (more H+ than OH- ions) to 14 (more OH- than
H+ ions) with a mid-point of 7 (where there are equal amounts
of H+ and OH- ions).
A soil test is used to give us an inventory of the
nutrient status available for the growing plant. It
can also provide us with an understanding of the
general health status of a given soil. A soil test is very
useful for identifying the base nutrient levels at the
start of the season to assist with planning nutrient
application strategies or even determining the crop
to go into the paddock.
Figure 1: pH scale showing the pH of common substances.
The pH is important because it influences the
chemical and physiological process in the
soil as well as regulating the availability of
plant nutrients (Figure 2). New Zealand soils
in their native state range in pH from 4.0-6.0.
Addition of lime is required to reduce the
availability of aluminium (which is toxic to
plant roots) and increase the availability of
phosphorus and other macronutrients such
as calcium, magnesium and potassium (which
are essential for plant growth). However,
raising the pH also reduces the availability of
some micronutrients, and so a compromise
must be reached. Consideration must also be
given to soil microorganisms which require a
pH of around 6.0 to function optimally. The
ideal pH for production is around 5.8-6.2
Figure 2: Relationship between soil pH and
the relative availability of individual nutrients
(McLaren & Cameron, 2004).
0800 BEEFLAMB (0800 233 352) | WWW.BEEFLAMBNZ.COM | BY FARMERS. FOR FARMERS
2
SOUTH OTAGO FARMING FOR PROFIT NEWSLETTER | AUGUST 25 2016
Olsen P
Base saturation
This Olsen P test is the standard phosphate (P) test for
New Zealand and is used to measure the plant available P
in the soil. This test is very well understood and has been
calibrated widely throughout NZ. The accuracy of this test is
good, but dry or very acidic conditions can cause this test to
underestimate P.
The base saturation (BS) is the proportion of the CEC
that is occupied by base cations (calcium, magnesium,
potassium and sodium). Base cations are alkaline
and therefore raise soil pH. Acid cations increase
soil acidity and therefore lower pH—these include
hydrogen and aluminium.
Soils that have been receiving reactive phosphate rock (RPR)
as their primary P fertiliser will always come back low in the
Olsen P test. It is therefore recommended to get a Resin P
test done as well as the Olsen P on these soils.
Every CEC binding site must have a cation bound
to it to maintain electroneutrality. The soil pH will be
affected by whichever cations predominate on these
exchange sites. The more base cations present, the
more alkaline the soil (the higher soil pH will be). The
more acid cations present, the more acidic the soil
(and the lower the pH).
Anion Storage Capacity (ASC)
The Anion Storage Capacity (ASC) is a physical characteristic
of the soil that measures how many anions (negatively
charged nutrients, such as phosphate and sulphate) the soil
can hold. Soils with a high ASC (70+) will require higher
inputs to raise their nutrient pools than soils with a low
ASC (>30). However, these higher ASC soils will have their
levels drop slower in the absence of fertilisers. ASC is a fixed
characteristic—it is for all intents and purposes impossible to
change.
The base saturation can be used to give us an
estimate or a double check on soil pH. A base
saturation of 70% is roughly equivalent to a pH of 6.0.
Cation Exchange Capacity (CEC)
The capacity of the soil to hold and store cations (positively
charged elements) is known as the cation exchange capacity
(CEC). The CEC is a measure of the total negative charge in
the soil, and provides a good indication of the soils nutrient
holding capacity. The CEC of a given soil depends on the
clay content and the amounts and types of organic matter
present. The CEC of New Zealand soils can range from 1-5 on
pure sand soils, up to 120+ on some peat soils.
The CEC serves to give us a good understanding on how we
should treat the soil in regard to cations. For example, a soil
with a high CEC can have aggressive rates of cation fertiliser
applied, but soils with a low CEC may need applications little
and often to maintain the nutrient pool.
Figure 3: Diagram of a soil particle showing cations
attached to negatively charged binding sites.
Note: Ca = Calcium; Mg = Magnesium; K = Potassium;
Al = Aluminium; H = Hydrogen
Figure 4: Comparison of a soil at two different pH levels,
showing the proportions of base cations (Ca, Mg, K, Na) and
acid cations (H, Al). The soil at the higher pH (6.2) has a higher
base saturation.
0800 BEEFLAMB (0800 233 352) | WWW.BEEFLAMBNZ.COM | BY FARMERS. FOR FARMERS
3
SOUTH OTAGO FARMING FOR PROFIT NEWSLETTER | AUGUST 25 2016
Cation tests
Volume weight
In soil science when we talk about cations we are usually
referring to potassium (K), calcium (Ca), magnesium (Mg)
and sodium (Na).
The volume weight is the weight of a known volume of
air-dried and ground up soil. This figure can help us to
understand the soils physical characteristics. The volume
weight is also needed to convert other test results.
There are two ways to consider cations in the soil:
•
•
In terms of the actual amounts extracted, which
gives an indication of the absolute amount available
to plants. Their concentrations are expressed in
microequivilents per 100 grams (me/100g).
As a proportion of the soil’s CEC. This is the
cation’s base saturation level, and is expressed as a
percentage.
In New Zealand we often use MAF units to express cation
levels. These units have been experimentally derived from
multiple tests and are a simple and easy to understand
value. They have become the cation measurement of
choice for most lay people.
Sulphur
There are two sulphur tests used in New Zealand:
•
•
Sulphate Sulphur test—measures the current plantavailable sulphur in the nutrient pool. This is the
sulphur that the plant has immediate access to.
Organic Sulphur test—measures the sulphur
contained within organic material. This accounts for
the major proportion of sulphur in most soils. This
form cannot be directly used by plants, but it can be
converted to sulphate by microbes in the soil.
The downside to the sulphate-sulphur test is that sulphate
is readily leachable and as such the sulphate-sulphur test
result is always low after a period of rain (especially after
winter), and high during drier periods. To overcome this
error, we can use the organic sulphur test. This measures
sulphur that is bound up in the soil organic matter and will
become available over the next 2-3 years.
Potential available N
This test provides an indication of the quantities of
nitrogen that can be readily mineralised from soil organic
matter under ideal soil conditions. The actual amounts
of nitrogen that will mineralise in the field will depend on
factors such as soil temperature and moisture, which are
impossible to emulate or predict in the laboratory. This
test must therefore be interpreted with caution, realising
that it is a measure of nitrogen mineralised under specific
laboratory conditions.
The test measures the potential of soil to provide nitrogen
to growing plants. It has been widely used for cropping
soils, but has not been widely used for pasture soils.
Pasture soils usually show high levels with this test, but
may still benefit from strategic use of nitrogen fertiliser
because of unfavourable conditions for the mineralisation
of these soil reserves at certain times of the year.
Soil test optimum levels
The soil test companies often supply information on soil
test optimums and this information is pretty good at
giving us an idea of the state of the nutrient pool. Graphs
are often displayed next to each soil test value to illustrate
the result relative to these optimum levels. These graphs
should be interpreted with caution because the optimum
levels the labs use are dependent on the way the soil was
classified. They will use different optimums for different
soil classes, types and even stock on the paddock. Be sure
to be as accurate as you can in classifying your soil tests,
and/or check the classification that is displayed on the soil
test to make sure it correctly describes your soil, pasture
or crop type and livestock system.
0800 BEEFLAMB (0800 233 352) | WWW.BEEFLAMBNZ.COM | BY FARMERS. FOR FARMERS
4
SOUTH OTAGO FARMING FOR PROFIT NEWSLETTER | AUGUST 25 2016
Soil physical structure
•
How do soils form?
Soils are formed ultimately from the rock material that
makes up the Earth’s crust. Over considerable periods
of time, as a result of various erosion and weathering
processes, the solid bedrock exposed at the surface
of the Earth is broken up and its mineral and chemical
composition altered substantially. Factors affecting soil
formation include:
•
•
Climate—most importantly, the direct effects of
rainfall and temperature. Climate also indirectly
influences soil formation via its relationship with
plant growth and therefore the amount of organic
matter in the soil.
Parent material—the underlying geological
material. Different rock-forming minerals vary in
their susceptibility to weathering processes e.g.
granite or schist take much longer to weather
and develop into soil than softer rocks such as
sandstone.
Structure—refers to the size, shape and degree of
development of the aggregation of the soil into
naturally or artificially formed structure units (peds,
clods and fragments). Structure types include platelike, prism-like, block-like or spheroidal.
Structure formation
Factors that contribute to the formation of soil structure
include:
•
Cations, such as calcium, magnesium and aluminium
which bind together clay particles
•
Climatic processes including freezing and thawing;
and wetting and drying cycles
•
Roots and worms pushing through the soil
•
Soil particles cementing together by humus, organic
glues created by fungi and bacteria decomposing
organic matter, and by polymers and sugars excreted
from roots
•
Fungal hyphae and fine roots can help to stablise
soil aggregates.
•
Organisms—determine the rate of organic matter
build up in the soil.
•
Topography—can affect soil formation in three
ways: through the influence of slope on soil depth,
by modification of the effects of climate, and by
influencing moisture relationships.
These include:
•
Compaction or pugging—by livestock and/or heavy
machinery. Soils are particularly vulnerable to
compaction and pugging when wet
Time—weathering and pedogenic processes do
not occur instantaneously, but instead occur over
substantial periods of time. New Zealand soils are
relatively young and are thereby less weathered
than that of other countries.
•
Cultivation—increases the rate at which organic
matter is mineralised by soil microbes. Loss of organic
matter makes soils more vulnerable to physical
damage
•
Removal of vegetation
•
Excessive moving or handling of the soil
•
Screening
•
Excessive sodium.
•
Soil physical analysis
In order to assess the physical condition of the soil it is
necessary to undertake a visual soil assessment.
Key characteristics to assess include:
•
Colour—this can indicate the level of organic
matter that the soil contains as well as issues with
soil waterlogging. Rusty coloured mottles are
associated with poorly drained conditions. Grey or
bluish colours can indicate a prolonged lack of soil
aeration (seasonal or permanent waterlogging).
•
Texture—this is the particle size distribution of the
solid inorganic constituents of the soil. Sandy soils
will feel gritty when rubbed between the fingers.
When moistened, silty soils will have a smooth,
soapy feel, while clay soils will feel very smooth to
sticky (and can be easily moulded into a cohesive
ball which deforms without cracking when pressed
flat).
•
Consistence—the inherent qualities of the soil that
are expressed by the way in which the soil material
holds together, deforms or ruptures when put
under pressure. This can be assessed by applying
pressure to a natural aggregate using your hands.
Consistence will vary depending on the soil water
content. Terms used to describe soil consistence
include “friable”, “loose” or “tight”.
Factors that deteriorate soil structure
Farmers need to take care to match cultivation practices
to soil type. Over-cultivation should be avoided (power
harrowing is especially damaging); but also recognise that
tight soils may require aeration before sowing.
For more information on undertaking a visual soil
assessment, and for recommendations around cultivation
and soil management, refer to the Landcare Research
Visual Soil Assessment Field Guide that is available in the
“Useful References” section.
0800 BEEFLAMB (0800 233 352) | WWW.BEEFLAMBNZ.COM | BY FARMERS. FOR FARMERS
5
SOUTH OTAGO FARMING FOR PROFIT NEWSLETTER | AUGUST 25 2016
Soil biology
Organisms that inhabit the soil profile (or the soil-litter
interface) include earthworms, nematodes, protozoa, fungi,
bacteria and various arthropods. The decomposition of
organic matter by soil organisms influences soil fertility,
plant growth, soil structure and carbon storage. The
subject of soil biology is incredibly complex and
involves interactions between the various organisms
in the soil and their interaction with the environment,
plants and animals.
The following organisms are often identified as
being integral to the maintenance of a healthy soil
environment:
•
Bacteria—these are single-cell
organisms found in huge densities
within the soil profile. Bacteria are
critical for nutrient cycling and are
responsible for:
––
Nitrogen fixation—the
conversion of atmospheric
nitrogen (N2) into nitrogencontaining organic substances.
These bacteria are found in the root
nodules of legumes and some are also free
living in the soil.
––
Nitrification—the conversion of ammonium
(NH4+) to nitrates (NO3-) in the soil.
––
Denitrification—the conversion of nitrates in
the soil into nitrogen gas or nitrous oxide.
––
Actinobacteria are critical in the
decomposition of organic matter and the
formation of humus. Their presence is
responsible for the sweet ‘earthy’ aroma
associated with a good healthy soil.
•
Fungi—important for the breakdown or
decomposition of organic matter. They spread
underground by sending out long thin threads known
as mycelium. They also produce spores which help
them to spread. Mycorrhizae are a type of fungi that
live in a symbiotic relationship with plants—they
invade root hairs and obtain carbohydrates from the
plant in return for providing the plant with nutrients.
•
Earthworms—ingest soil particles and organic
residues and enhance the availability of plant
nutrients in the material that passes through
their bodies. They aerate and stir the soil—
these processes help with the formation of
soil aggregates and aid the infiltration of
water (contributing to soil drainage).
How to enhance soil biology
The best way to enhance the biology of the soil is
to provide conditions that enable favourable soil
organisms to thrive. Almost all soil organisms require
the same things that we need to live, including:
•Food source—organic matter is the key food
source for earthworms and microbes. Growing
more dry matter will increase the food source
available for these organisms. Maintaining ground
cover and avoiding over-grazing will help to keep
up the supply of organic material into the soil.
• Optimal soil pH—this differs depending on the
organism, but generally a pH in the range of 6-7
will provide optimal conditions for soil organisms.
Liming soil will therefore enhance soil biology (in
addition to other benefits including enhancing
nutrient availability).
•
Aeration—beneficial soil organisms require oxygen.
Ensure that any drainage issues are remedied.
Minimise physical disturbance including overcultivation and compaction by machinery and
animals.
• Warmth—soil organisms operate best at
temperatures above 8°C. At low temperatures
many organisms will become inactive. Organic
matter helps to insulate the soil—so promoting
good levels of organic matter will assist soil
temperature as well.
•
Moisture—shelter-belts, ground cover and soil
organic matter all help to retain soil moisture levels.
0800 BEEFLAMB (0800 233 352) | WWW.BEEFLAMBNZ.COM | BY FARMERS. FOR FARMERS
6
SOUTH OTAGO FARMING FOR PROFIT NEWSLETTER | AUGUST 25 2016
Fertiliser basics
It is important to understand some of the basic fertiliser
types in order to ensure that you are applying the right
product for your requirements.
Superphosphate
Superphosphate is produced by dissolving rock
phosphate in sulphuric acid. This process converts the
insoluble tricalcium phosphate present in the rock into
water-soluble monocalcium phosphate. Superphosphate
contains around 8.5-10% P and 10-12% S. Superphosphate
has been used in NZ since the 1880s and is still the most
common form of fertiliser applied. Superphosphate does
not acidify the soil.
Sulphur Super
Produced by adding extra elemental sulphur to
superphosphate. Contains both elemental sulphur (slow
release) and sulphate sulphur (fast release) with 20-30%
S and 7-8% P.
Serpentine Super
Produced by adding serpentine rock (containing
magnesium) halfway through the superphosphate
manufacturing process. The end result is a pH neutral
fertiliser that can be safely sown with seed (and for this
reason it is also known as ‘drilling super’). It contains
about 7%P, 8% S and 5% Mg.
Reactive Phosphate Rock (RPR)
A soft type of phosphate rock which is applied in a finely
ground form. It is most effective on soils with pH less than
6 and rainfall of greater than 800 mm. This product is a
slow release form of phosphate. It has a liming effect on
the soil.
Urea
Urea is an inexpensive form of high analysis nitrogen
fertiliser containing 46% N. In the soil it is converted by
soil bacteria into ammonium and then into nitrate which
can be taken up by plants. Urea does affect soil pH and
lime is required to neutralise this effect (for every 100 kg
of urea applied, approximately 184 kg of lime is required
to neutralise the acidity).
SustaiN
A new urea product which contains an inhibitor (called
Agrotain) that blocks the urease enzyme that soil
bacteria use to convert urea to ammonium. This slows the
conversion of urea to ammonium, thereby preventing a
large pool of ammonium accumulating in the soil which
is then exposed to gases losses via volatilisation. This
reduces nitrogen losses to the atmosphere.
Sulphate of Ammonia
Supplies plant available forms of nitrogen and sulphur.
Commonly used in spring when sulphate levels are low
due to winter drainage. Sulphate is important for nitrogen
metabolism in plants (i.e. the conversion of nitrate into
plant proteins) and hence applying it in spring along
with nitrogen is beneficial for plant growth. Sulphate of
ammonia contains around 20% N and 24% S.
Di-ammonium Phosphate (DAP)
Supplies nitrogen and phosphorus and typically
contains around 18% N, 20% P and 1% S.
Potassium Chloride
Also known as KCl or ‘muriate of potash’, this is the most
common form of potassium fertiliser used in New Zealand.
It has a higher potassium content than other fertilisers
(containing 48% K) and is readily dissolved, making the
potassium immediately available to plants. Although it
does not affect soil pH, it can cause germination injury if
drilled with small seeds.
0800 BEEFLAMB (0800 233 352) | WWW.BEEFLAMBNZ.COM | BY FARMERS. FOR FARMERS
7
SOUTH OTAGO FARMING FOR PROFIT NEWSLETTER | AUGUST 25 2016
Useful references
McLaren, R.G. and Cameron, K.C. (2004). Soil Science: Sustainable Production and Environmental Protection.
Published by Oxford University Press. Available from the Lincoln University bookshop.
Molloy, L. (1993). Soils in the New Zealand Landscape: The Living Mantle. Published by the New Zealand
Society of Soil Science.
Useful links
Landcare Research Visual Soil Assessment Field Guide
The great kiwi earthworm survey brochure
Information on soil structure recovery (page 3, B+LNZ West Otago
Farming for Profit Newsletter August 25 2015)
Sponsors
Many thanks to PGG Wrightson for their support of this event.
For news, photos and updates about the B+LNZ South Otago Farming for Profit
Programme, visit our Facebook page:
www.facebook.com/agfirstotago
For more information about the B+LNZ South Otago Farming for Profit Programme,
or to submit ideas for future events, please contact:
Facilitator
Extension Manager
Nicola Chisholm, AgFirst Otago Ltd
Ph 027 610 2221
[email protected]
Olivia Ross
Ph 027 801 7868
[email protected]
0800 BEEFLAMB (0800 233 352) | WWW.BEEFLAMBNZ.COM | BY FARMERS. FOR FARMERS
8