Posts Tagged ‘soil organic matter test’

How to sow new pasture and forage crops

Wednesday, September 10th, 2014

In our quest to become dairy farmers we have leased a few acres in Gembrook to grow pasture and forage crops for our small herd. The land is run down pasture and I have outlined soil test results in the last few entries.

To get this pasture productive again we need to raise the pH, correct nutrient deficiencies and increase the soil health mainly through increasing organic matter.

Connor Shea disc seeder at work in Gembrook. Discs slice the soil open and the seeder drops in a trickle of fertilizer and seed. The next crop can be sown without disrupting the existing crop to get a smooth succession. .

Connor Shea disc seeder and John Deere at work in Gembrook. Discs slice the soil open and the seeder drops in a trickle of fertilizer and seed. The next crop can be sown without disrupting the existing crop to get a smooth succession.

Strategy: Make sure some legumes are included in the planting. Balance short term production and removal with longer term growth of pasture (persistance). Horse pasture, cut hay if possible but allow for some pasture suitable for horses to become established in the longer term for grazing. Perennial ryegrass, subterranean clover and cocksfoot. Hay pasture, mainly for hay cutting with some persistence into the next year. Italian ryegrass, balansa clover and cocksfoot. Forages for cows. This will be cut with a forage harvester and fed to cows. Oats, vetch and field peas. Sowing rate for the pasture mixes will be 25 kg / ha.

How much fertilizer? The major trace element deficiencies were boron and copper. We assumed that molybdenum could be deficient given the type of soil and history and because we wanted to establish legumes again we opted to include molybdenum. The final mix had 0.02% B, 0.01% Cu and 0.003% Mo.

We had CaCO3 lime added to the pastures in the previous autumn at 1 tonne / ha.

Unfertilized pasture will produce around 2 tonne / ha (as dry matter). Fertilized pasture can be expected to produce up to 10 t/ha maybe even higher for some varieties. Figures for nutrient uptake by different crops are hard to find and interpret but there are a few guideline figures available. We based calculations for fertilizer requirement on 8 t/ha. A harvested ryegrass / clover pasture (8 t/ha) will typically remove N : 104 kg/ha, P : 30 kg.ha, K : 102 kg/ha, S : 15 kg/ha, Ca : 2 kg/ha and Mg : 9.2 kg/ha. Our soil test results show that around 100 kg/h DAP should supply enough P but not all the N required. Legumes in the pasture may help fill the gap. The DAP also contains sulphur so 100 kg/ha should supply all the S required. The soil is not short of calcium and magnesium for crop growth but we have limed the soil to reduce exchangeable acidity.

Ideally we would have preferred to apply phosphorus in a organic or organically coated form because this soil has the potential to lock up P. The decision to use DAP to supply nitrogen and phosphorus was a compromise but we figured that we had to balance fast short term growth against loss to the soil. However if things go well and organic matter increases in the soil some of that locked up P will be available again (see previous entries for a discussion on P in soils).

It is an expensive business to plant pasture especially to restore a pasture. To get a return we need to concentrate on quality as well as quantity of production. That’s why we opted to resow with productive varieties and to invest in fertilizer. Also there needs to be some carry over of growth so not all the pasture needs to be resown the next year. Our strategy is to keep something growing and includes allowing some production to return to the soil. Basically that means we are preserving and enhancing our capital.

Diversity is important. That’s why we opted to include at least Cocksfoot in the mix – maybe when we better understand the potential and problems with other varieties they can be included also.

In Spring 2014 we sprayed the existing pasture with a low strength glyphosate spray. This was to weaken the weeds and reduce competition without unduly affecting existing grasses.

Direct seeding pasture. The seeder is cutting into existing pasture that has been sprayed to weaken any weeds. The slots can be instected to make sure that seed and fertilizer is being fed in at the required rate.

Direct seeding pasture. The seeder is cutting into existing pasture that has been sprayed to weaken any weeds. The cuts can be inspected to make sure that seed and fertilizer is being fed in at the required rate.

Most small seeded pasture varieties can be sown along with fertilizer with a spreader but this needs to be followed by a pass with pasture harrows and maybe a roller to help bury the seed. A direct drill seeder with either discs or tines is designed to bury the seeds along with the fertilizer. The main advantages of this are more efficient sowing where the fertilizer is placed with the more desirable species, ability to sow larger seeded varieties in the soil away from pests and less disturbance of the soil – particularly important where exposed soil can dry out. Settings on the seeder regulate the flow of seed and fertilizer but every now and again it helps to jump off the tractor to check that the seed and fertilizer is being released at a suitable rate.

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.


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.


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