Archive for the ‘soil’ Category

Our new farm

Tuesday, December 8th, 2015

After a lot of searching we finally found a new home for our calves, 173 acres in Ranceby in South Gippsland.

 

Feeding cows

Feeding out hay to the milkers in the first Winter. This is not ideal especially under wet conditions.

 

The previous owners Robin and Deb McKinnon were very helpful in showing us their production figures, explaining how the farm worked and then allowing us to move some machinery and cows early.

There are some steep paddocks but most is gently undulating. About 2/3 of the farm is accessible with a tractor. We ran production and financial models on the farm and the figures showed that it was viable.

The Strzeleckis were originally under the sea so the soil is derived from sediments. It is a gray coloured loam with poor structure. When its dry weather the soil is dusty and when it rains it turns to mud.

Our family shares the farm jobs which spreads the load and makes it manageable.

We bought the existing herd and have bought in new milkers. As at December 2015 we still have 20% of the herd yet to calve. This should bring the total cows in the vat to around 95. We were aiming for 110 cows but it seems that the existing cows are doing better than anticipated so that has made up some of the difference.

Moving yearlings back to their paddock

Moving our wandering yearlings back to their home paddock. The farm has extensive shelterbelts of Southern Blue Gums. There are also many of the now uncommon Strzelecki Gums on the property.

 

Farm soil testing – major nutrients

Tuesday, May 6th, 2014

Up to now we’ve looked at some of the main physical factors in the soil at the Gembrook horse property. Now its time to look at the other main nutrients and the soil’s fertility.

Soils have an ability to loosely hold nutrients to prevent leaching and to make them slowly available to plants. This applies mostly to the positively charged nutrients like calcium, magnesium and potassium. This is called the Cation exchange capacity and it depends on the type of clays but is higher in good structured soils like loams. Because organic matter also holds these same nutrients the amount of organic matter affects CEC.

This soil has a CEC of 17.76 meq%. This value is around mid range with sandy soils at around 1 meq% and organic soils around 30 meq%.

Nutrients are assessed in terms of absolute level, relative proportions, percent of nutrient holding capacity and levels compared to non nutrients and acidity.

Major nutrients weight for weight in the soil.

Nutrient Result Recommended range Level

ppm ppm
calcium 1859 1200 – 2000 good
magnesium 297.5 150 – 300 good
potassium 400.7 180 – 300 high
Horses at Gembrook eating hay during Autumn. Good nutrition is essential for growing or working horses and this can start with good quality pasture. A soil test can identify problems that can lead to nutritionally poor pasture and further health problems.

These horses at Gembrook are being fed hay during Autumn. Good nutrition is essential for growing or working horses and this starts with good quality pasture. A soil test can identify problems that can lead to nutritionally poor pasture.

The overall level of the major nutrients calcium, magnesium and potassium is good but a look at the makeup of the exchange capacity tells a different story.

Nutrients as proportion of (CEC) with comparison to non nutrients
Nutrient % of nutrient Recommended Level
holding capacity range
calcium 52.3 60 – 70 low
magnesium 13.8 12 – 20 ok
potassium 5.8 4 – 6 ok
Non nutrients
sodium 1 < 4 ok
exchangeable acidity 27 13 – 20 too high

The proportions (of CEC) of calcium and magnesium are low. At the same time, exchangeable acidity proportion of CEC is very high. So the soil has a too high proportion of acidity taking the place of more desirable nutrients. Its like saying, the soil has the capacity to hold more nutrients but they are not there. Instead, their place is being taken by acidity.

For similar soils, the higher the exchangeable acidity, the lower the pH.

Another way to look at this is to say that in this soil desirable nutrients (plus some low levels of sodium) occupy 73% of available capacity. This figure is called the base saturation. The base saturation represents the proportion of nutrient holding capacity CEC that is actually occupied by desirable nutrients. Around 80 – 87 % is considered desirable. The non desirable proportion is exchangeable acidity.

Sulphur was also measured in this soil. The result was 17.6 ppm. Around 10 – 40 ppm is considered desirable so the sulphur level was OK.

To change the pH of the soil some of the exchangeable acidity will need to be neutralized. This has to be done gradually, usually by applying lime. As the lime neutralizes the acidity, calcium and magnesium (if using dolomite lime) slowly occupy more of the exchange capacity and will slowly raise the pH by increasing the base saturation.

Farm soil testing – acidity and phosphorus

Thursday, April 24th, 2014

In a previous blog entry, March 11 2014 I described some of the physical factors in a pasture soil in Gembrook that I had tested. The soil was very acidic with a very low proportion of fresh organic matter.

Exchangeable acidity results from prolonged leaching of good nutrients from soils. The pool is considerable larger than that represented by pH but is in balance with pH. A high exchangeable acidity usually means low pH. In this soil exchangeable acidity is very high and is creating a low pH. A low level of exchangeable acidity is normal and is always present in soils but a high level indicates a problem. The exchangeable acidity has to be at least partly overcome to raise the pH. Therefore exchangeable acidity is a good measurement on which to base calculation of lime requirement.

Exchangeable acidity (calculated by our partner lab as Lime requirement) is 4.8 meq% which is high. The exchangeable acidity determined on the same soil by Apps Labs was 0.32 meq% which by comparison to other figures is relatively low. Around 0.5 to 1 meq% is normal and acceptable (the lower the better). It looks like our lab is including aluminium in the exchangeable acidity whereas our method specifically measures the H ions.

Gembrook pasture and soil. The photo was taken in Summer and shows exposed soil and weeds.

Gembrook pasture and soil. The photo was taken in Summer and shows exposed soil and weeds.

The M3-PSR is the Mehlich Phosphorus Saturation Ratio, an environmental and soil quality test designed to show if phosphorus is likely to be leached from the soil. Conversely it will show the tendency of the soil to fix phosphorus and to make it less available to plants. A M3-PSR < 0.062 in below the agronomic minimum and shows that P uptake by plants will be poor. The result for this soil is 0.003 which indicates a strong tendency of the soil to hold phosphorus in an unavailable form.

The red Kraznozem soils around Gembrook are highly oxidized soils and the red colour comes mainly from the oxidized iron. These are similar to many of the soils found in equatorial regions including those in Africa, Asia and south America. I already expected a problem with phosphorus lockup in this soil as phosphorus binds strongly with iron and aluminium minerals at low pH. The M3-PSR mostly confirmed this.

Soil phosphorus was extracted using Mehlich 3 extractant. Mehlich 3 extractable P has been found to correlate well with a number of other indicators for more readily ‘plant available’ or potentially available phosphorus (see my previous blog entry on phosphorus in dairy farm soil for more detail). The result for phosphorus was 6.2 ppm. The ideal range is 30 – 70 ppm.  Therefore not only will this soil tend to bind up phosphorus, the overall level of plant available phosphorus is very low.

If inorganic phosphorus fertilizers are added to this soil much could be potentially lost before being used by plants. To get around this some farmers add up to twice the calculated plant phosphorus requirement. The result is that some soils have high phosphorus levels (see my previous blog entry on phosphorus in dairy farm soil). Other solutions are to use a slowly soluble form of phosphorus like rock phosphate or to create a fertilizer made up of granules of inorganic phosphorus compounds coated in compost or organic matter.

Sustainable farming – what is it?

Wednesday, March 19th, 2014

Yesterday as a guest of Trevor and Anne-Marie Mills and the Western Port Catchment Landcare Network I attended a field day on the Mills’ dairy farm at Drouin South.

Amongst the principles of sustainable agriculture are that farming should:

– provide an amenable lifestyle for the farmer & family

– protect and enhance the productive capacity of the farm

– protect and nurture the natural environment and reduce environmental impacts

Judging by these criteria, the Mills have gone a long way to creating a sustainable farm. Much of this has been achieved by thinking ‘outside the square’ and often going against conventional thinking. For example T & A-M have fenced off and replanted many of the drainage areas and watercourses on the farm. Water is now piped to stock high up in each paddock. The result; less contamination of water, less nutrient runoff and cleaner water for the cows to drink.

The South Gippsland area was originally heavily forested and early accounts have detailed the diversity of wildlife that once existed. Now with areas on the farm returning to natural vegetation, some of the native animals are also returning. Happily these areas are often those that would be less productive and difficult to manage. The photos below taken from the same spot approx 5 years apart show the dramatic change around a natural waterway.

Before and after watercourse revegetation on the Mills Farm at Drouin South. By excluding stock from wet gullies significant improvements have been made to the quality of water flowing from the farm and as drinking water for stock. Approx 5 years between photos. Courtesy of T & A-M Mills and WPCLN.

Before and after watercourse revegetation on the Mills Farm at Drouin South. By excluding stock from wet gullies significant improvements have been made to the quality of water flowing from the farm and as drinking water for stock. Approx 5 years between photos. Courtesy of T & A-M Mills and WPCLN.

The WPCLN as part of their involvment in the property have been monitoring water quality and this has provided valuable feedback for farm planning.

On the farm management side T & A-M have adopted a rotational grazing system that takes advantage of the natural productivity of the soil and facilitates nutrient cycling whilst protecting against overgrazing and damage to pasture. The result, an increase in productivity which has meant that the herd size can be reduced whilst maintaining production.

I was especially interested to hear how Trevor had cut back on use of urea as a nitrogen fertilizer. This came about because he saw that the urea was favouring grass growth and supressing clovers. Now clovers are thriving and producing nitrogen naturally!

I think that soil testing still has a role to play on this farm. Particularly if it is done in a way that provides a better understanding of management effects on soil processes and the dynamics of nutrient movement around the property as well as off the property as natural losses and in farm products.

Judging by the attendance at the field day there is a lot of interest in sustainable farming and land management. The Mills farm is an excellent example for all to see that shows how productive farming can go hand in hand with protecting and enhancing environmental quality.

How can I improve the pasture on my farm?

Tuesday, March 11th, 2014

A complete soil test is an important first step to help restore the productivity of pastures. Most soil tests provide a breakdown of major and minor plant nutrients but many also profile the soil’s overall health, potential and fertility.

I was recently asked to visit a property in Gembrook east of Melbourne that was used to run horses. This was in February and the soil was very dry, around 3% moisture. At this level most biological activity in the soil ceases (more about this later). Now no-one expects a farm to be at its most productive during summer but its a good time to check on how well the soil and pasture is standing up to the stress of summer. Good indicators are the amount / proportion of bare ground, if there is any useable pasture left standing and the presence of weeds. Unfortunately this property showed signs of stress with little standing feed for the horses.

Horse paddock in Gembrook. A complete soil test is the first step towards re-establishing a good feed base for horses.

Horse paddock in Gembrook. A complete soil test is the first step towards re-establishing a good feed base for horses.

This is a higher rainfall area and soils are highly oxidized hence the red iron colour common in the area. On the positive side these soils have a good structure and are well aerated but they tend to be acidic and this is difficult for most plants.

Some results:

Test measured preferred comments
pH in water 5.83 6 – 7 too acidic
pH in CaCl2 5.04 5.4 – 6.4 too acidic
Bulk density 0.89 ‘light’ soil
Soil water 3.1% around 20% very dry

The soil is acidic as expected.  The bulk density value shows that the soil is lightly textured and because the soil felt soft this suggests that there may be organic materials in the soil and the soil may be well aerated. By contrast some soils in West Gippsland have a heavy consistency like butter and when dried lose their structure and become powdery.

How much organic matter was in the soil?

Test measured preferred comments
Total organic C 37184 ppm 29000 – 52000 moderate
Fresh organic C 466.5 ppm 860 – 2100 very low
Proportion fresh org C 1.23% 2.9 – 3.9% low

The soil had moderate levels of organic matter but most is ‘older’ humic type organic matter. There is not much fresh organic matter present. Overall the picture is of little recent return of plant material to the soil. Is this important? Yes, because organic matter has a significant role in making nutrients such as phosphorus and sulphur available to plants – a very important role in these types of soils. Check my previous blog entries for some results and comments for dairy farm soils.

Complete soil tests are a cost effective management tool. Tests that cover all nutrients, many of the main physical factors and organic matter levels cost around AU$150. In future blog entries I will cover more of the test results from this Gembrook soil.

Acidity and major nutrients in dairy farm soil

Tuesday, December 3rd, 2013

What is the connection between soil pH, acidity, nutrients and amount of lime required to raise the soil pH?

When we think of soil acidity most people think of pH. But pH is a measure of active acidity which can be measured with a meter, test strips or indicator solution or powder. In simple terms they measure hydrogen ions in water that’s in the soil.

But there is a ‘pool’ of acidity that is held in the soil. This is called exchangeable acidity and it creates a balance with pH in the soil solution.

An important property of soils is their ability to hold nutrients such as calcium, magnesium and potassium and make them available to plants. This is called the ‘exchange’ capacity and it is generally larger for soils with more clays and organic matter. But this capacity can be partly taken up by exchangeable acidity.

For agricultural soils generally, as pH increases (less hydrogen ions), exchangeable acidity decreases. But also with increasing pH the total exchange capacity of the soil increases and this capacity is taken up with a larger proportion of desirable nutrients. In soils with pH around 5.5 to 7 exchangeable acidity should taper off as pH rises with more of the available exchange capacity occupied by nutrients.

Soil samples were taken on a West Gippsland dairy farm at the same 3 sites described in previous entries. Exchangeable acidity was extracted with KCl salt solution. Exchangeable calcium and magnesium were extracted using Double Acid (Mehlich 1).

Results:

Exchangeable Exchangeable Exchangeable


acidity calcium magnesium
Site pH meq% meq% meq%
1 5.5 0.76 7.24 0.93
2 6 0.13 17.00 2.46
3 6 0.32 26.10 9.83

Typical figures for exchangeable acidity reported for other soils range from 0.5 to 1 meq% so Sites 2 and 3 have low exchangeable acidity.

Typical values for exchangeable calcium can range from 0.23 to 12.5 meq%. Typical values for exchangeable magnesium range from 0.25 to 4.2 meq%. Calcium levels are moderate at Site 1 to high at Sites 2 and 3. Magnesium levels are low / moderate at Site 1, moderate at Site 2 and high at Site 3.

West Gippland dairy farm Site 1. Of 3 sites tested on this farm, Site 1 has lowest pH, organic matter, phosphorus, calcium and magnesium. But exchangeable acidity is highest here.

West Gippland dairy farm Site 1. Of 3 sites tested on this farm, Site 1 has the lowest pH, organic matter, phosphorus, calcium and magnesium and it has the highest exchangeable acidity.

The unit meq% used to express acidity and nutrients is designed to allow a direct comparison between the amounts of each held on exchange sites in the soil. It also provides the mechanism for working out how much lime to apply to soil.

As lime is applied to soil it slowly reacts with the soil acidity. The active (pH) acidity is constantly replenished from the exchangeable acidity but in the process some of the calcium (and magnesium for Dolomite type lime) becomes attached to the exchange sites. The lime will displace some of the exchangeable acidity. This raises the proportion of desirable nutrients to acidity and in doing so, raises the pH.

One approach for working out how much lime to apply to raise the pH is to calculate how much would be required to neutralize the exchangeable acidity. At least this takes the guess work out of liming. Tests like the ones shown here can be used to monitor progress.

Another related approach is to estimate or measure the occupied exchange capacity then by using a diagram of pH vs exchange capacity decide the percentage change required to raise the pH a particular amount. See the reference below for more details.

So far, tests for organic matter, pH and some major nutrients have shown significant differences in fertility between paddocks on a dairy farm.

Further reading: Soil test interpretations by Apps Labs.

How much phosphorus is in dairy farm soil?

Tuesday, November 26th, 2013

How much phosphorus is there in farm soils?

I’m in the process of carrying out a quick assessment of soils on a dairy farm in West Gippsland. Phosphorus use is an increasingly important topic from a $ cost as well as environmental perspective.

In this study samples were taken from the same sites previously tested for soil organic matter. One sample was taken at each site in a core between the surface and 10 cm. One subsample was prepared after mixing the soil for each core. Phosphorus was extracted using Mehlich 3 extractant.

Mehlich 3 is a widely used extractant for several nutrients suitable for alkaline as well as neutral to acidic soils. It will extract a proportion of the inorganic forms of phosphorus. Mehlich 3 extractable P has been found to correlate well with a number of other indicators for more readily ‘plant available’ or potentially available phosphorus (Moody et al 2013).

Results:

Site Colour Organic matter pH Phosphate ppm
1 Grey low 5.5 27
2 Red-brown medium 6 36
3 Red-brown high 6 632

Phosphate measurements at other locations taken by Apps Laboratories (using Mehlich 3) have ranged from 23 ppm for Gembrook pasture through to 385 ppm for a well composted garden soil. Generally phosphate levels around 30 ppm are considered low and levels around 150 ppm high. The phosphate levels at sites 1 and 2 are low but the phosphate level at site 3 is very high. Some further tests might be useful to find if this applies to the whole paddock. Information on fertilizer history could also be helpful.

Dairy cows grazing on mixed species pasture in West Gippsland. Levels of more readily available phosphorus can vary widely between paddocks.
Dairy cows grazing on mixed species pasture in West Gippsland. Levels of more readily available phosphorus can vary widely between paddocks.

In soils, phosphorus is thought to be present in around 4 ‘pools’. Readily available P is dissolved ready for plant uptake. This amount can last between 1/2 to 3 days for average crops. Some P is temporarily attracted to and held by soil minerals (called adsorbed P). Some P is held in organic forms in the soil organic matter. Adsorbed and organic P form moderately available P and phosphorus moves slowly between these pools and soluble P. Up to 70% of the available P can be held in organic form. However some P finds its way into more permanent ‘bound up’ or ‘occluded’ pools in the soil minerals. This phosphorus is only released again very slowly so is in effect ‘lost’. The total amount of P in soils may be much larger than the amount recovered by extractants like Mehlich 3. Much of this is in the ‘occluded’ pool.

Challenge problem: A pasture contains 30 ppm Mehlich 3 extractable phosphate – a low value. This is close to 9.8 ppm phosphorus (P). This part is done – each hectare contains approximtely 9.8 Kg of P (down to 10 cm). Is this amount of P adequate for the milk produced in a year assuming that 1000 L of milk contains approx 1 Kg of P? For the non dairy farmers some approximate figures that will help are stocking rate 2 cows / ha, production 6000 L / yr / cow. What assumptions have to be made and what factors are missing in this calculation?

References:

Moody, P.W. et al. 2013. Soil phosphorus tests I: What soil phosphorus pools and processes do they measure? Crop and Pasture Science 64(5) 461-468.

Phosphorus fertility. Mississippi Agricultural and Forestry Experiment Station. Downloaded from http://msucares.com/crops/soils/phosphorus.html Nov, 2013.

Phosphorus fractions in soil diagram.

Organic matter in dairy farm pasture

Tuesday, October 22nd, 2013

The benefits of organic matter in soil are well known. Organic matter improves factors including water holding capacity, nutrient holding capacity and structure. But organic matter can be made up of more longer lasting humus through partially broken down material to fresh material from plants and animals that has recently entered the soil. This fresh reactive fraction is more likely to be a major supplier of nitrogen to a pasture as it is broken down.

How much of each is likely to be present in a pasture soil? A recent study, Culman et al, 2012, has found that permanganate oxidizable carbon in soils correlates well with widely used measurements of microbial biomass and particulate organic matter. Permanganate oxidizable carbon is also a good indicator of variation in management and environmental factors.

Dairy farm pasture in West Gippland, Site 2 of the study. The paddock is elevated and the soils has a characteristic reddish-brown colour.

Dairy farm pasture in West Gippsland, Site 2 of the study. The paddock is elevated and the soils has a characteristic reddish-brown colour.

It is relatively cheap and easy to measure the reactive fraction of soil organic matter by permanganate digestion. A simplified method is outlined in detail in the Archive for March, 2012.

In a preliminary study soil was sampled at three sites on a dairy farm in West Gippsland.

Site 1: Pasture soil mid way down a slope, known to be poorly drained. Mixed pasture species including some perennial ryegrass and poorly developed white clover. pH measured at approx 5.5. The soil has a heavy texture but becomes powdery when dry.

Site 2: Elevated pasture with mixed species. Chosen for its contrast to Site 1.  More typical West Gippsland red-brown soil. Distinct crumb structure with pH around 6. This is the site in the picture.

Site 3: Another red-brown soil in an elevated position considered to have good pasture. pH approx 6. Good crumb structure.

Partially dried samples were sieved to remove roots. Two tests were carried out: digestion with 30% hydrogen peroxide for a ‘total’ organic matter measurement and, digestion with potassium permanganate for a reactive organic fraction.

Results.

Organic matter Reactive Total % reactive Approx
Site total w/w % org C ppm org C ppm org C level *
1 5.3 865 29293 2.9 low
2 7.2 1025.5 39751 2.5 moderate
3 9.5 2085.6 52014 3.9 high

* representative values can be seen by following the SOM Method link in the Archive for March, 2012  ‘A simple test  for reactive soil organic matter’.

Across the farm, levels of total and reactive organic soil matter varied from low to high. The lowest at Site 1 and the highest at Site 3. The percentage of total organic matter weight for dry weight in the soils ranges from 5.3 to 9.5.

The percentage of reactive soil organic matter was significantly higher at Site 3 (3.9% of total). However a meta-analysis of a range of figures for total and reactive soil C from the Archive for March, 2012 shows that typically the reactive component ranges from 3.8 % to 10.6 %. Therefore overall, soils on the dairy farm in this study have low or lower than expected levels of reactive soil organic matter.

This study has provided some comparative figures for soil organic matter fractions on a dairy farm. Reliability will be improved with more tests per paddock and wider testing over the farm will be useful as part of pasture and feed management on the farm. Many of the factors that determine organic matter levels in the soil can be identified like grazing, pasture, crop and fertilizer history. This information along with tests for key nutrients can help to better understand how the current situation has developed.

References.

Culman et al, 2012, Permanganate oxidizable carbon reflects a processed fraction that is sensitive to management, 2012, Soil Science Society of America Journal.

A simple test for reactive soil organic matter.

Tuesday, March 20th, 2012

Of all the factors that can be measured in soil, fresh, reactive or labile organic matter is one of the more important. Its a tool that can be used to monitor seasonal changes that depend on strategies like cover crops, composting and residue retention. This test can be valuable for home gardens, horticulture, pastures or cropping.

A test for soil organic matter that shows the change in colour of a potassium permanganate solution is useful for quick comparisons. I’ve been working on an improved method that uses a relatively inexpensive colorimeter. Also there’s a spreadsheet calculator that handles all the necessary calculations even for soils with a wide range of organic matter content.

Soil organic matter is essential for good growth and flowering in vegetable gardens.

Soil organic matter is essential for good growth and flowering in vegetable gardens.

A description of the method can be found at SOM method. The calculations are in a Excel spreadsheet here SOM calculations.

As yet there is no kit available but if anyone wants help getting setup please let me know.

How much organic matter is in your soil?

Friday, September 2nd, 2011

Of all the factors that can be measured in soils, organic matter (OM) is probably the most useful and critical. This test measures the fresh or labile organic matter in soil. This organic matter supplies nutrients to plants because it is the food for microorganisms which in turn release nutrients like phosphorus and nitrogen.

Some tests like high temperature degradation or acid / dichromate digestion measure total or resistant organic matter. This is important for nutrient holding capacity, water holding capacity and soil structure. These tests are slightly difficult and can be slightly dangerous – been there, done that! Peroxide digestion tends to measure labile and perhaps more resistant SOM.

Potassium permanganate (KPM) is an oxidant that is safe to use. As the KPM oxidizes the OM it loses its purple / magenta colour. In the lab I use 0.2 M KPM stock solution with an added flocculant. The sample can be weighed or measured by volume (2.5 mls). The stock solution is diluted x 10 and then shaken with the soil by hand for a short time. So the method is simple and easy to carry out.

I tested 5 soils from around my property. A good compost, vegetable garden soil, soil from an old strawberry patch, soil from a paddock where very little fertilizer or compost had been added, and sub soil from an excavation. The soil in Gembrook is mostly a highly oxidized iron based soil that has a reddish colour. The results are below.

Permanganate digestion of labile organic matter in soils
Five soils from compost (left) to subsoil (right) were extracted using potassium permanganate. Soils with high labile organic matter remove most of the purple permanganate colour.

The compost sample is on the left – very high OM, then from L – R vegetable garden, unfertilized paddock, old strawberry patch, subsoil. The big surprise was that the old strawberry patch soil had relatively high labile OM, slightly more than the vegetable garden. The subsoil (on the right) had almost no OM. The unfertilized paddock (centre) had relatively low OM.

The change in colour provides a simple way to compare the OM in samples. In the lab I used a photometer to read the absorbance of the solutions at 570 nm. This method needs a calibration curve but the end result is more accurate. The amount of carbon in the OM can be related back to the amount of KPM used up (based on a simple assumption). Therefore this method can give a reading for organic carbon in gm/kg. Then that can be converted to a value for OM (organic C is about 55% of OM). Its even possible to estimate the amount of nitrogen that could be potentially released from the OM because the C : N ratio is usually , based on soil type, between 10 : 1 and 33 : 1.

The amount of fresh OM in soils is related more directly to soil fertility factors like microbial respiration and biomass and to factors that can be linked to soil management like reduced tillage, green manures etc.

The inspiration for my exploration was Weil, Ray, et al 2003, Estimating active carbon for soil quality assessment: A simplified method for laboratory and field use.

There are many good articles on soil organic matter available on the web including this one: Hoorman and Islam, 2010,  Understanding soil microbes and nutrient cycling. Ohio State University Agriculture and Natural Resources Fact Sheet SAG-16-10