So welcome everyone to the September Soils
Network of Knowledge webinar. If you are listening but not a subscriber,
here is the link to subscribe to the webinar. I’d really like to introduce Brian Murphy
who is presenting today’s webinar and the title is – Understanding soils and interpreting
soil tests – What do all the numbers mean? A bit of background on Brian, he has worked
as a soil scientist for more than thirty years, going back to the soil conservation service,
on the Agricultural Conservation and environmental aspects of soils science. He is the co-author of a book called ‘Interpreting
Soil Test Results – What do all the numbers mean?’ His experiences include production of several
soil maps for major parts of New South Wales and he has been involved in research on the
effects of land management practices and soil carbon and soil properties. Brian has also been author and editor of two
published texts on soil science and was a major contributor to the development of the
land and soil capability system in New South Wales. Throughout his career Brian worked on the
on ground application of soil science to develop and implement better land management practices. And Brian is still attached to OEH (Office
Environment Heritage) and ANU. Ok. The first thing I was going to talk about
today is the NSW Soil Knowledge Network and it sounds very much like the other network
but the NSW Soil Knowledge Network is made up of a group of 12 retired soil specialists
from around New South Wales and essentially they all used to work for soil conservation
or the organisations such as OEH Office Environment Heritage and Land and Water, which all derived
after Soil Conservation. And one of the reasons we’ve got together
as a group is that in recent years there’s been a bit of a loss of soil specialists through
retirement and redundancies , so through a citizen science project run by Sally McGuinness
Clark this Soil Knowledge Network has been set up. And there are just a few, um, it’s not for
profit so we don’t go out trying to make money or anything but it is mainly a resource
to try and provide some advice, and act potentially as mentors and trainers, to other soils people
who would like to know more about soils. And not only soils people but all land managers. Some of our projects that we’ve been involved
in – one is trying to get an effective State Soils Policy and the other one is we’ve
done an educational video series and if you look on Facebook, even if you just go to Google
and log in Soils Knowledge Network and you should be able to find those videos and the
videos are on things like the difference between sodic soils and saline soils and how to look
at soils in the paddock and a few things like that. We’ve also been involved recently in some
technical workshops with agency and community partners and the other job we’ve been working
on is with the release ad development of some Soil Landscapes mapping of the Central West. Just a couple of photos. The photo in the top left was where we had
a project, I think Roy Lawrie and Sally McGuinness Clark did some work with school children and
here are just some of the other field trips which we’ve been on, looking at different
soils. So now, I’ve told you about the Soils Knowledge
Network and certainly if you want to be involved with the Soil Knowledge Network, you know,
just contact Sally McGuinness Clark or myself. So the topic of today’s talk is about understanding
soils and interpreting soil tests. The reason for interpreting soil tests. One is for soil mapping, the other one is
for regional planning and investment plans and catchment management plans. Where you look at land capability and land
degradation risk. You also look at land suitability and the
productivity and suitability for specific uses and purposes. The other thing we use soil tests of course
is for specific paddock site information on acidity, nutrient recommendations and problems
like salinity, sodicity and soil structural problems and contamination. And at the end I might just talk very briefly
about soil sampling. Now interpreting soil tests, I think it’s
important to remember what your resources are, and one of the best resources around
is a book by Peverill, which Peverill comes from CSIRO I think, and it provides for all
the different soil tests and for different soil properties, this book provides the background
and theory behind a lot of the sampling and measurement and interpretation and it has
interpretation tables for a wide of soil properties including soil physical properties and pH,
aluminum, electrical conductivity, cations, sodicity, all the nutrients and the trace
elements. The issue with this book may be that some
people may find it a bit heavy because it does go into a lot of background and so on. The other book which is useful for interpreting
soils is one by Hazelton and myself and it also has a wide range of interpretation tables
for different soil physical chemical properties but it also includes some engineering properties
and a bit more emphasis on environmental properties in relation to erosion. But it is nowhere near as theoretical and
doesn’t go into as much detail as does the Peverill book. So in terms of soil mapping we do soil tests
and if you look up here you can see this landscape here is called ‘Mary’ and we know that
soil landscape has lots of sodic surface, sodic soils which is a problem. We know that these dark soils here in ‘Paraby’,
which is the yellow bit here, we know these dark soils here, which is the cracking clays,
they also have some problems with surface sodic soils but not as bad as ‘Mary’ and
the soil map, also by doing the soil tests, we know that this area down here called ‘Lower
Bugwar’ has lots of salinity problems but it also has lots of scalding. And if you know anything about scalding north
of Nyngan this is where all that scalding is. So the soil tests are very, very important
for helping us to interpret soil maps. The other thing is land capability. And the things we look at in land capability
are erosion problems, acidity, structural problems, salinity and we also look at things
like rock outcrop and waterlogging. So all the soil tests are helping us to understand
about the land capability. Class Two land here you can see is very good
for cropping, but I know this particular soil landscape and we know that it has some soil
acidification problems. Class Four land is good for grazing and you
can see Class Seven land is good for trees. And just for interest you can see this area
here of very good conservation farming or conservation tillage practices – double
retention and minimum disturbance at sowing so that’s very good for erosion control
and building up soil organic matter levels. And you’ll notice here on this sloping land
which has been tilled, under a rain storm there is obviously erosion. So one of the first things about soils is
to get know the general soil characterisation. Important things are texture, the colour,
the structure, and you should look, if you know they are self-mulching soils, that’s
very important because you can know that those soils are going to be good. If there is very strong lenticular structure
in the subsoil you’ll know you’ve got lots of shrink/swell problems. The other thing which is important is soil
organic matter. The other thing that is important is its response
to wetting, whether it slakes and disperses, so whether the soil structure breaks down
under wetting. And the other important thing is the depth
of the soil, the depth to the rock, the depth to the saline layers or maybe the depth to
dense sodic subsoils. And this is a texture triangle and texture
is a very useful and underrated aspect of soils and important so you should learn how
to field texture because that will tell you a lot about how to manage your soils. Sandy soils have to be managed quite differently
to loamy soils and to clay soils. And the lighter textured soils, soil organic
matter is really important for both cationic exchange capacity and for soil structure. In the clay soils it is the exchangeable cations
which control a lot about the soil structure and also the type of clay determines how much
cations exchange capacity you have. So if you have a look at some individual profiles
– this one here is a red chromosol. It has a fine loamy soil topsoil over the
red clay B horizon and you can see the Lucerne root there quite likes the B horizon. Whereas this soil here is a Sodosol and you
can see the bleached horizon which is caused by the very dense yellow sodic B horizon and
I haven’t got the ESP’s (Exchangeable Sodic Percentages) but I think they were around
about 18% from memory. So if we are going to interpret soil properties,
soil physical properties, things to look at are water holding capacity and the plant available
water, the aeration and bulk density, which is very dependent on texture, soil strength,
the potential for surface sealing which affects your germination and emergence and the infiltration
and runoff. So texture effects, this is plant available
water which is the difference between the field capacity and the permanent wilting point,
and of course clays hold a lot more water but there’s not as much water available
for the plant because it holds water so tight. And sandy loams have about 16 mm, fine sandy
loam about 20, but the two which have lots and lots are the loam/clay loams and silty
clay loams. What is also interesting about that is that
lots of the Russian Chernozems and the black soils on the plains of the US where they grow
such terrific crops tend to be in this group here with this very high water holding capacity. The limiting bulk density is different for
different textures. Sandy loam the limit is 1.8 but clay it’s
1.4 and the reason is because of the amount of air in the soil. Sandy soils are much more open and can hold,
have a lot more pores so they can have a higher bulk density without limiting plant growth. Clays have fewer large pores so their bulk
density rates are lower. Just to give you an idea of what happens as
soils dry. At the dry end of the scale soils are too
strong, and two Mega Pascals limit plant growth. At the wet end aeration has to be more than
10 percent. So as our soils go from dry to wet you get
a nice non-limiting water range in here if you’ve got a good soil structure. However, if you have a poor soil structure
which happens with sodic soils and sometimes under compaction you actually get no non-limiting
water range so roots find it very difficult to grow in soils like this because by the
time they lose enough water to increase their soils to get air available they are too strong
to allow root growth. At the same time when the strength of the
soil becomes less enough to allow roots to grow there is no air so that’s a problem. For soil chemistry the things that are important
are cation exchange capacity, pH. And I should mention here that pH less than
5.5 in water causes aluminum toxicity and might affect P availability as well, but if
you have a pH of more than 9 you’ve probably got hydrogen carbonate around and that will
only happen if your soil is dominated by sodium and you’ve got a sodic soil. Then you’ve got electrical conductivity,
soil organic matter, nutrients and toxicities. So I’ll just go through those. Exchangeable Cations. Clays have a residual negative charge because
of their structure. It is just the way they are built and the
way their atoms are made, therefore you’ve got to have exchangeable cations to balance
that residual charge. And they are made up of Sodium, Calcium, Magnesium,
Potassium and sometimes Aluminium. Sodium – if it is more than 5 to 6 % of
the exchangeable cations it causes dispersion. Calcium is very good for the soil. Soils high in exchangeable calcium provide
stability to soils. Magnesium is less effective in providing stability. It has only about 60 % the flocculating effect
of calcium. So sodium magnesium soils, are much less,
have much lower structural stability than sodium calcium soils. Potassium actually causes dispersion but it’s
not very common. Aluminium only really occurs when the pH in
water is less than 5.5 and when their aluminium ions are in solution they are toxic to root
growth and that’s one of the big problems with acid soils. So Cation Exchange Capacity. Units are Centimoles positive charges per
kilogram. You can see 6 is low, 6 to 12 is, sorry less
than six is very low, 6 to 12 is low, 12 to 25 is moderate, 25 to 40 is high and greater
than 40 is very high. Of course these would be mainly sandy soils
and they have a serious problem because it doesn’t give them much buffering capacity
against acidification and can’t hold much nutrients. These soils here greater than 40 are normally
your spec type clays and the black cracking clays of the Liverpool Plains. Just to give you an idea of how much Cation
exchange capacity there is in each of the different types. Coarse sand virtually has none, fine sand
none, silt maybe a very tiny little bit, Kaolinite clays have a little bit, Illite clays which
are really common in a lot of the red chromosols and it’s certainly in a lot, even the cracking
clays in the Macquarie Catchment and the Smectites clays are the ones that have really high Cation
exchange capacity and Smectite clays are very common from basalts. The other important component of Cation exchange
capacity is the organic matter and it can have 150 to 500 centimoles positive charge
per kilogram depending on the organic matter. Humus has the highest value so if you’ve
got good humus in your soil that can add a lot to your Cation exchange. As I mentioned in the slide, lighter textured
soils like sands and fine sandy loams organic matter can be a very important part of the
Cation exchange. Just to give you a visual these are clay plates. A clay plate here, a clay plate here. These are Cations here, Sodium, Calcium, Magnesium
and what happens when there is water around, sodium likes water and because it has only
got one charge, there are a lot of Sodium atoms and they all like water, so the water
rushes in and pushes the clay plates apart and you get dispersion. The controlling factor on that is that these
are the exchangeable Cations which are intimately linked to the clay plates. Outside you can have salt and when you measure
the electrical conductivity you measure what is effectively the free salt outside the clay
plates. If there is a lot of salt out here obviously
that is going to reduce the free energy of the water so the more salt you have outside
here the less water that will rush in and push the clay plates apart. So one of the ways of getting stability is
to add salt, but we’ll talk more about that in a minute, bit more about that. Here are some of the problems, you can see
surface sealing, problems with emergence and germination, and there’s sodic soil, huge
cloddy, you can see how cloddy that particular paddock is. Here is another problem if you get subsoils
can be extremely sodic and this farmer, I think, had the ESP of around about 30 something. That is a person just there so you can see
the severe gully erosion. That’s in the Bathurst granites. The other problem you get with sodic soils
and dispersed soils is as you can see there’s a dam, right, but it was a dam but now has
got a great big hole in the middle. So that’s caused from dispersion and sodic
soils. I might just skip that and move on because
of time. OK, managing soils and acid soils. So if the pH in calcium chloride is less than
4.7 or less than 5.5 or there’s aluminium 3 plus in solution which is toxic to plant
roots especially to sensitive plants such as Lucerne, some wheats, chickpeas, phalaris
and there’s probably quite a number of other plants which don’t like aluminium in solution. And the question is why pH in Calcium Chloride
and not in water, it is because pH in Calcium Chloride is the most reliable and consistent
measurement. pH in water is affected by the soil electrolyte
concentration and can vary seasonally by as much as one unit, or 0.5 to 1 unit. Whereas by adding the Calcium Chloride the
pH values are stabalised and that is very much what the plant sees as well because of
that so usually when you are trying to manage acid soils you’d use a pH in Calcium in
Calcium Chloride rather than pH in water and adding lime is a way that acids soils are
often managed. Here is a big pile of lime and here’s a
spreader adding lime to the soil. Electrical conductivity. There is saturated ECe and then there’s
a 1:5. We can convert one to the other because as
you see this is one part of soil to five parts of water. This one is saturated ECe, so that means the
soil is saturated. So how much water the soil holds in saturation
determines what the conversion factor is. So if you don’t have much water as is the
case with loamy sands then you have a much higher conversation factor. If you have quite a bit of water as you do
in clays then you have a lower conversion factor. The units are in decisiemens per meter, microsiemens
per centimeter and total dissolved solids and of course they all appear in different
publications and cause a bit of confusion because people quote numbers and often don’t
mention the units. As effectively microsiemens are about 1000
times decisiemens per meter and total dissolved solids depends on what the salts are because
they all have different conductivities. Like Sodium Chloride I think has the highest
conductivity whereas Carbonates tend to have the lowest. And the salinity if you see here Salinity. And salinity greater than 1.5 decisiemens
and that big Saline patch there, those soils would have an EC greater than 1.5 in a 1:5
extract and greater than 4 in a saturation extract. And Nitrogen. Ok Nitrogen. When you measure Nitrogen you measure Ammonium,
you measure total Nitrogen as soils, or you measure Ammonium and Nitrate. The issue about Nitrogen is that you can measure
total Nitrogen which is nearly all organic or you can measure the Nitrate. And just some numbers. Total Nitrogen if it is less than 0.05 then
that is low. High is greater than 0.5. Nitrate there’s some numbers. The other aspect about Nitrate is it’s that
toxic it gets into drinking water. And I might just briefly mention Phosphorus. Because Phosphorus can be in Crystalline,
surface adsorption or in organic matter and when you measure Phosphate you try to use
an extraction using Bicarbonate or lactate or fluoride and the Phosphate extract actually
takes this out, this part out and some of the surface adsorbed Phosphate. Then typical values for soil Phosphorus depend
on the buffering capacity of the soil and I should go back to the other slide. The amount of Phosphate that is held on the
surfaces, Crystalline surfaces, especially iron and aluminium determine a buffering capacity. So if you have a basalt soil you have a high
buffering capacity. If you have a non-basalt soil you tend to
have low buffering capacity. And they are typical values for what your
Phosphorus levels should be. You might notice if you’ve got vegetables
you need more Phosphorus than if you are trying to grow a dryland pasture. And soil organic matter. I think the critical thing here is that generally
from numbers from England suggest that if you’ve got more than 2% soil organic carbon,
as grams/100grams your soil is running nicely. If it’s got less than 2% then it is probably
not functioning as well as it could and interestingly if you look at some of the numbers that came
out of Australia there is a lot of Australian soils which have a lot less than 2% soil organic
carbon, especially under crops and under pastures. The other thing is that you have to make sure
that when you distinguish between soil organic carbon and soil organic matter and I guess
this last point here is about soil contamination. And soil contamination, the actual critical
levels depend on what you are going to use the land for. If you are going to use the land for say playgrounds
and playing fields then you have a different value and this one here is for home where
you might have preschools and primary schools and children picking up soil and putting it
in their mouth, your critical values are much lower than they will be for industrial. So that is why setting critical values up
is not simply a straight forward ‘Oh, that’s one set of values. We’ll just keep those’. It depends on the use you are going to put
the soil to. I just might mention that the thing about
sampling soils, when you sample a soil you actually sample a great big volume of soil
like this and then you get a mean value. When the root grows in that little bit of
soil there and only sees the soil that is sort of adjacent to it so sometimes soil tests
don’t line up that well with what you see. The next thing is about sampling a paddock
and I think some people say you should take a transect across the paddock and then use
that mean number but you can see you miss a lot of the paddock and it is probably a
good case for saying you should use a zig zag pattern to sample your soil. Even though in a zig zag pattern you might
take those samples and get the bulk sample from those it is still better to take a zig
zag pattern than a single transect. So just conclusions. Purpose for sampling and reason for obtaining
data needs to be well understood and clearly defined. Soils need to be adequately and effectively
sampled for the purpose. You need special coverage and in some purposes
you might need randomness so you can do some statistical analysis. The laboratory measurements need to reflect
and imitate the processes in the soil to provide information on the performance of the soil. Interpretation of data and tables of critical
values are usually derived from planned experiments and trials or from a long history of experience
but obviously if you are outside of the experience that people have used or outside the experience
of the trials sometimes the data mightn’t be that accurate. I guess as a final conclusion, sets of interpretation
tables of critical values for different soil properties can probably be thought of as the
language or alphabet of soil management. They can be used to put together the narratives
about the capabilities, productivity and limitations of soils. So it is just an interesting way to look at
soils. However, Albert Einstein said something that
said, “not everything that counts can be counted and not everything that can be counted
counts”. If you go into a paddock and this is what
you see I don’t think you need a soil test to know that that soil is probably sodic and
you’ve got big problems. This soil here is actually a Russian Chernozem. You look at that soil and say, I would say
that soil has probably got a lot of good things going for it and you probably don’t need
a soil test to tell you that. If you looked in a landscape like this you
can say there’s lots of good things, that’s a good soil and it is highly productive and
it might have some problems but is basically a good soil. So I think that is where I’ll leave it. Luke: Fantastic Brian! That was a big ask to try and cover all of
soil testing in 20 to 25 minutes so great effort. Julie Dart I can see has her hand raised. Julie: Ok, Brian, the question I’ve got
is – Up this way we tend to have labs that put out alternative testings like Reams and
Albrecht. Just wondering what we can do with those results
when we get them, because often they are standardised against different things and it is hard to
know what to do with them? Brian: Yeah, well all the tests I talked about
were done using the standard procedures, you know as in the, what used to be the Rayment
and Higgins book but there is a new one out now as well
I think that is always a difficult question to answer because they often use different
tests and you use different extraction methods and so on and I think one of the things I
mentioned was that if you are going to interpret soil tests and use soil tests then the way
you do the soils tests should reflect something about how the plant sees the soil or at least
reflect how you are going to use the soil test. Like if you are going to use the soil test
to predict how an earthwork performs then you try and relate that soil test to the processes
which might cause the earthwork to fail. So if you’re using it for Calcium Magnesium
ratios and so on you should think about what are the soil processes which you are trying
to talk or make interpretations about. And the other problem you’ll have is that
sometimes interpretations can come from say sets of soils which are very different and
I think I made that point that people will do a set of trials and set up some experiences
and have some interpretation values but they base that on that set of trials and their
experience. If they move into a different set of soils
then their numbers might not be so useful or so easy to interpret. And one of the things I probably should have
mentioned was that the most valuable person in trying to interpret soil tests sometimes
can be your local soil scientist or local agronomist say from Incitec or someone, because
they probably have the local knowledge to say ‘Ok I know this soil test means this
on these soils’ because they have had the experience. So I’m not sure if that answers your question
about those alternative methods. I don’t honestly know a lot about their
methods, but I know they use calcium magnesium ratio a little bit but I think there is a
calcium magnesium ratio is very important from the point of view of soil dispersion
because as I mentioned magnesium only has about 60% the flocculating power of calcium,
so sodium magnesium soil is nowhere near as stable as sodium calcium soil. I’m not sure if that answers your question? Julie: Sort of. I think the biggest difference in what I have
seen is that a lot of those tests have really high calcium percentages that are desirable
in the exchangeables and I suspect sometimes on our really acid soils that that’s not
going to be possible unless you spend a ridiculous amount of money on lime. Brian: Right, well the main thing about pH. I mean that’s what I said you have to look
at the process. If pH is regarded as your limiting factor
and the pH becomes seriously limiting or your pH is below 5.5 because that brings aluminium
into the solution. So if you are changing the pH from 6 to 6.15,
I mean that might give you a bit of a buffer against dropping the pH below 5.5 but I’m
not sure. So you have got to look at your process. What are you trying to manage? Julie: Yeah well that’s it because I think
I quite often see tests where to me the pH is adequate for what people want to do but
then they’ll say ‘but oh no my exchangeable Cations say I need more Calcium. I’m going to go spend all this money on
Calcium” and I think well do you really need to? Brian: Interestingly even if you’ve got
a sodic soil the reason people add gypsum on a sodic soil certainly to reduce the ESP
but the main effect of gypsum on a sodic soil is to increase the electrolyte concentration
and I tried to mention during the talk that if you have enough salt outside then you won’t
get dispersion and gypsum is a magical chemical because it actually absorbs just enough to
provide stability to sodic soils but doesn’t dissolve enough to start causing salinity
problems for plants so that’s why gypsum is so widely used I guess for stabilising
sodic soils. Julie: Yep, ok, thank you. Luke: Thanks Julie. Just out of interest Julie which part of New
South Wales are you talking about? Julie: North Coast. So one of our major labs up here tends to
use the Albrecht/Reams method in their interpretations and it can cause some good fun. Luke: Ok. Thanks very much Julie. I have a message from Terry Kibbler and what
he has said is he has organic matter of 10.8% is this a problem? Brian: I guess my first question would be
is that organic matter or I guess it has to be organic matter at 10.8%. To me it is going to give you some advantages. It is probably, a lot of that soil organic
matter at 10.8%, a lot of that soil organic matter is probably going to be in what they
call particulate soil organic matter so it is probably not going to be adding that much,
like you won’t get 10.8% of the organic, all that organic matter is not going to be
adding to your water holding capacity and aggregate stability and Cation exchange capacity
necessarily. A lot of it will be in the form of, more or
less, decomposing organic material and particular organic matter. However, what you can be sure of is that maybe
something like 3 to 4% of that is actually contributing to your Cation exchange capacity
and your water holding capacity and various other things so I wouldn’t see it as a problem
immediately. It depends also what texture it is. I presume it is reasonably big loam or fine
sandy loam to have that much organic matter. I don’t know, I wouldn’t see it necessarily
as a problem. Luke: OK, we’ve got a couple of comments
here from Tony Cox. Tony says ‘Hi’ and a good presentation. The research coming out of the project that
Tony is involved in ‘More profit from crop nutrition’ highlights low, medium and high
production zones in paddocks and we are looking at getting farmers to treat and sample these
areas differently. Can you comment on that in terms of chemistry
please Brian? Brian: I think I know what I guess Tony is
asking is, is there soil chemistry causing the low production zones, to medium production
zones and the high production zones. To be honest it could easily be chemistry. It may also be just a change in soil type
through the paddock or through paddocks you often get small wet zones as well, which are
caused by just a small dip in the landscape which makes them a lot wetter. And the wetter zones might have a high yield
one year and a lower yield the next, depending on the season, which makes it all very confusing
to know exactly what is happening. I would suggest that a lot of it would be
caused by soil variability, which is related to probably clay content, plant material,
just local landform and different drainage conditions. It is hard to know without, but they will
affect the soil chemistry obviously. But probably it is more, a lot more the soil
physical properties which are operating in some ways on a paddock, through the effects
of texture and waterlogging and drainage and so on. Luke: Great. So Brian, Tony has added another question
in there where he has said ‘How important so you think critical levels are in terms
of crop development?’ Brian: That’s a good question because critical
levels sometimes can be what they call a threshold or they can be a continuous variation. The one that is the typical threshold is pH
because if your pH in water drops below 5.5 or pH in calcium chloride drops below 4.7
you immediately know that you are going to start getting some aluminium 3 plus in solution
which is going to have an immediate effect on your plant production, especially if you
have a sensitive plant. On the other hand if you are talking about
perhaps Nitrate levels or Phosphate levels or even bulk density levels there is more
of a continuous variation. So I guess the more Nitrogen you have, often
the higher the yield until you’ve reached a maximum of course. Until something else is limiting the plant. The same as Phosphorus. So somethings are thresholds and somethings
continuously variable. Luke: Great. Thanks Brian. The next thing written I’ve got here is
from John Lawrie. John: Yeah, I just wanted to make a point
that Magnesium can also be toxic particularly on some soils high in iron and Magnesium can
be a problem with soils with a pH above 5.5, especially canola. That was one thing that we discovered years
ago, that you needed to have a high pH to grow canola. Also Lucerne needs higher pH so there is a
need to put lime and pH’s greater than 5.5 if you’ve got a Magnesium toxicity problem. Luke: Thanks John. So the next one is from Michael Fitzgerald
and Michael has written a question in here to Brian and says ‘How do you measure natural
background soil organic carbon over time in a soil amended with an organic amendment?’ Brian: I’ll just interrupt the question,
so the question seems to be asking ‘How do you know what is your original native level
of soil carbon compared to the soil carbon you have added since ‘. To answer that question
when biomass and carbon is added to soil, essentially that carbon will be either metabolised
and produced as CO2 by you know the microorganisms in the soil or it will be assimilated into
the soil organic carbon and the soil organic matter. So short of using something like an isotope
I’m not sure how you would try and distinguish between what was the native, original soil
carbon and the new soil carbon that you have added in. I think all the carbon atoms go together and
get all mixed up and in fact it is probably wrong to say that soils bank carbon because
the carbon goes in but that carbon has been continually processed and reprocessed by the
microorganisms and plants and so on, so it is not like the carbon just sits there it
is actually being churned around and moved from microorganisms, to soil organic matter
and back to microorganisms and then back to soil organic matter. So it all gets churned up and all becomes
one soil organic matter. So I would say the only way to do that would
be with isotypes. The only other way if you had a paddock next
door which you knew was a native, undisturbed site maybe you could just monitor what the
soil carbon level was on that paddock next door and say ok so this is what the soil carbon
level would be we hadn’t added all that soil organic matter. Luke: Great. Alright so Glen Uebergang has a written question
here. Glen: I was just wondering whether you have
a comment on micronutrients like which ones are the most significant with soils tests
and which can be accurately determined by soil tests rather than leave tissue tests. Brian: I guess you are interested in how the
micronutrients would affect the plant growth. Certainly, well Beneathalhin is certainly
an important one for legumes and so on. Glen: Do you think we can trust the results
that we get from a soil test? Are they accurate? Brian: I guess it comes back to what the experience
has been and I guess my knowledge of some of the testing of micronutrients isn’t that
strong but what I would probably recommend to you is that you go and read the book by
Paverill where he lists Beneathalhin and all the different tests and field studies for
Beneathalhin , also for voron he also lists ones for cobalt so that probably doesn’t
answer your question that well but certainly the book by Paverill has all that sort of
information in it. Glen: No worries. Thank you. Luke: Thanks Glen. Harry Kibbler, so Harry’s question is ‘I
grow native flowers that are susceptible to P, Phosphorus, What is the best way to apply
low amounts of Phosphorus over a long period?’ Do you want the question again Brian? Brian: Yeah, No, I think the obvious way is
with organic materials and the organic material, you’d probably
need to get some organic materials where you know what the carbon to phosphorus ratio is
and also the carbon to nitrogen ratio because the carbon nitrogen ratio is important because
the carbon nitrogen ratio will determine how rapidly that organic matter decomposes. So if you wanted to make it slow release you’ll
probably want to have some organic materials which perhaps have some phosphorus but the
carbon nitrogen ratio is not too high. I mean there could be some slow release type
phosphorus fertilisers available on the market as well but certainly organic materials that
don’t decompose to rapidly and that’s where you’d leave out chook manure and things
like that because they have so much nitrogen and they’ll decompose rapidly. The right compost would probably do the job
unless they’ve got some specialised fertilisers. Luke: Terrific. Phil Hurst doesn’t have a microphone and
Phil has written ‘Hi Brian, A question on CEC or Cation Exchange Capacity in acid sulfate
soil. By adding high aluminium values the results
end up quite high. Is this a good indication?’ is his question. Brian: I think determining soil properties
on acid sulfate soils is a specialised field and requires specialised tests. So measuring Cation Exchange Capacity on acid
sulfate soils probably depends an awful lot on whether it has been drained and what the
pH is and various other things. So I guess I can, if he sends me an email
I can probably pass him on to some people who would know much more about that kind of
details on that sort of soil. Acid Sulfate soils are, as you realised, have
the Sulphur, they have iron sulfides in them and when you drain them they start producing
sulfuric acid. So exactly how to determine like cation exchange
capacity and so on, I think you would want somebody who actually knows, who specialises
in acid sulfate soils to make the comment on that. Phil, if you send me an email I can put you
on to the person or pass you on to the manual as well which talks about soil tests for acid
sulfate soils. Luke: That would be good and Phil, as you
probably know, has a long background in acid sulfate soils so, yeah, it’s getting pretty
technical but thanks for the question Phil. Ok Tony Cox has made another comment. He says ‘Ensuring your lab has ASPAC Certification
is a good start to ensuring your tests are accurate. ‘And Harry Kibbler has said ‘Thanks for
your answer and with high organic matter, would increasing the soil microorganisms add
to the CEC?’ Brian: I suspect if your soil organic matter
is already high adding soil organisms probably will not add to the CEC because the CEC requires
the negative charge from the soil organic matter and it is the negative charge from
the soil organic matter which is important in providing CEC and therefore soil microorganisms
themselves are probably not going to do that, plus, soil microorganisms and the percentage
of the soil organic matter that living organisms make up is normally fairly small, possibly
around about 3 to 5% of the overall soil organic matter or maybe a bit less. So I wouldn’t, generally I wouldn’t think
adding organisms is probably not going to directly increase the Cation Exchange Capacity. Luke: Great. Thank you so much Brian for a really informative
webinar and great to have you along. It has again broken the record for attendees. So, for another fantastic webinar, thanks
again to you Brian and thank you everybody for attending. Brian: Thank you everyone. Luke: I will bring it to a close now.