Posts Tagged ‘Mehlich 3’

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.

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.