Archive for November, 2013

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).


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?


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 Nov, 2013.

Phosphorus fractions in soil diagram.

Effluent management on a dairy farm

Saturday, November 9th, 2013

For the last few weeks I’ve been visiting a dairy farm in West Gippsland to learn a bit more about how dairy farms work. It’s also an opportunity to apply some ideas about soil and water management in a practical context.

Cows can deposit around 8 – 10% of manure and urine output around the milking shed and yards. Manure and urine contains significant amounts of major nutrients including nitrogen, phosphorous and potassium.

On many farms this manure is often washed directly into specially contructed waste retention dams. A typical setup is a sedimentation dam sometimes followed by an aeration dam.

Sedimentation dam for dairy waste on a farm in West Gippsland. Water is washed into the dam from the milking shed and yards without treatment.

Sedimentation dam for dairy waste on a farm in West Gippsland. Water is washed into the dam from the milking shed and yards without treatment.

The picture shows a sedimentation dam on the WG farm. There is a thick crust of manure on top which means that conditions in the dam are most likely anaerobic. At this dam I didn’t want to get too close in case I became part of the waste system! Therefore I didn’t get a sample!

In an anaerobic dam the organic matter itself provides oxygen to help drive the other processes that eventually break down most of the organic matter into methane, hydrogen, carbon dioxide and ammonia. But the disadvantage of this method is that energy in the organic matter is lost (as methane) and importantly nitrogen is lost (as ammonia).

The overflow from the sedimentation dam on the farm enters a second aeration dam. What can we expect the water quality to be in this type of dam? There shouldn’t be much nitrogen but what other nutrients will be present?

Dairy farm aeration dam in West Gippsland. Water flows into this dam from an uphill sediantation dam that takes waste directly from the dairy.

Dairy farm aeration dam in West Gippsland. Water flows into this dam from an uphill sedimentation dam that takes waste directly from the dairy.

The aeration dam is just below the sedimentation dam. The overflow pipe can be seen in the picture. The water has a brown colour and a slightly unpleasant smell. Here are some water quality tests done in the Apps Labs lab: Dissociated carbon dioxide 13.5 ppm (elevated); Turbidity (unfiltered) 57 FTU (high); Turbidity filtered (0.45 micron) 19.8 FTU (still high); pH 7.1 (very slightly on the alkaline side); UV absorbance 99.2% (very high dissolved humic materials); Conductivity 1459 microS/cm (elevated salts); redox potential (ORP) -44.7 mV (anaerobic, and that’s at the surface).

Nitrate and nitrite were checked using screening tests. Both were at low levels or absent. That’s expected anyway because nitrates usually don’t exist in low oxygen conditions and nitrites usually form from nitrates under reducing conditions. Phosphate was checked using two different test kits. One showed phosphate over 30 ppm. The other showed phosphate as 43 ppm. Both these levels are very high.

There may be significantly more phosphate present in the two dams than the amount measured as some is likely to be held in the sediments.

What about nitrogen? Ammonia – nitrogen in the aeration dam was 0.44 ppm. This is higher than normally found in natural waters but is not excessive. At pH 7 around 0.4% of this nitrogen can be expected to be in the ammonia form as opposed to the ammonium form. This is not good for water life because the ammonia form is harmful. In general as water becomes more alkaline, an increasing amount of any total ammonia nitrogen present is likely to be in the ammonia form. This same amount of ammonia nitrogen, is roughly equivalent to 2 ppm nitrogen as nitrate. This is slightly elevated for natural waters so the nitrogen in the ammonia form probably doesn’t account for all the nitrogen in the original manure entering the two dams.

What is the best way to use dairy effluent to capture maximum nutrient value?

The following web resource provides detailed figures on tests done on dairy effluent dams and suggests way to reuse the nutrients in the effluent:

DPI Victoria 2013, Using dairy effluent as a fertilizer. Downloaded from, November 2013.