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What happens to your soil sample when it comes into a lab?

29 April 2020
Type Factsheet

Soil is a major source of nutrients needed by plants for growth. How is the soil sample sent into a lab tested for the three main nutrients NP & K. Jason Naughton, Brendan Healy and Karen Daly from Johnstown Castle give some information

The three main nutrients in soil are nitrogen(N), phosphorus (P) and potassium(K). The trio are known as NPK. Soil pH also plays a key role in soil fertility. Maintaining soil pH at an optimum level will result in better nutrient uptake.

Soil nutrients P and K

Phosphorus (P) is essential for photosynthesis. It stimulates root development and is an important energy store. Of the total P present in soil, less than 1% is available to plants. Typical values of available P in soil range from 0.1 to 30 ppm (mg/L).

Potassium (K) is essential for plant health including reproduction and growth. It is essential to regulate CO2 uptake. It is associated with movement of water, nutrients and carbohydrates in plant tissue. Typical values of available K in soil range from 20 to 500 ppm.

Forms of soluble P and K are available to a plant root for uptake.

How we extract P from soil

Morgan’s Method

Morgan’s method is a test used to determine nutrient levels in a soil sample. Soil samples are extracted using a chemical solution. The extract is filtered through a filter bed. The gathered filtrate is then analysed through the lachat instrument.

Mehlich 3 Method

Mehlich 3 method is used to determine plant available P. Soil samples are also extracted using a chemical solution. The extract is filtered through a filterbed. The gathered filtrate is then analysed through the ICP-OES instrument.

The Morgan’s extractant is a Sodium Acetate and Acetic Acid reagent with a pH of 4.8. Morgan’s reagent is designed to dissolve an amount of P and K proportional to the fraction available to the plants for uptake. Soil test are critical to determine the level of nutrients in the soil for plant uptake. Phosphorus supply in soil plays a critical role for the growth cycle in plants. In the Morgan’s method, Johnstown Castle, have calibrated Morgan’s extract to crop growth so that the Morgan’s test can indicate whether crops will respond to fertiliser, if the test indicates low plant available P.

Mehlich 3 is also used as an extractant for evaluating plant available P. Mehlich 3 reagent has a pH 2.5 and consists of several reagents including Ammonium Nitrate, Ammonium Fluoride and Nitric Acid. Mehlich 3 is more aggressive than Morgan’s, therefore it can extract more P in addition to the  plant available fraction which is why the values are higher than Morgan’s P. Mehlich extraction is used widely in the U.S. and have calibrated the test to crop growth.  In our labs at Johnstown Castle, we use Mehlich extracts for research purposes to help us understand some of the other fractions of P in Irish soils.


When soil samples are taken from a field, they are put in the oven at 40°C for a period of 24 hours. They are then sieved down to <0.2 mm. Dried and sieved (<0.2 mm) soil samples are required for analysis. The density of this soil is assumed to be 1.0 g/cm3.

For Morgan’s, a 3 ml sample is scooped into round glass flasks held in wooden racks. 15 ml of Morgan’s reagent is dispensed into each of flasks and then shaken on a 180 rpm gyrotory shaker for thirty minutes. 100 samples can be extracted at a time and these typically include standard house control samples and a sample of Morgan’s reagent for quality control purposes.

For Mehlich 3, 2 g of sieved soil sample is weighed into plastic tubes. Thereafter, the extraction procedure is similar to Morgan’s except 20 ml of Mehlich 3 reagent is dispensed into each sample tube and the tubes are shaken at 180 rpm for 5 minutes.

For both extractions, the soil solution is poured through folded filter paper on a filter bed (10 by 10 array) where a sample (filtrate) is collected (see Fig. 1).

Fig. 1: Filter bed (10x10 array) - soil solutions have been poured out after extraction process



The Lachat instrument is an autoanalyser used to measure levels of P and K in Morgan’s filtrates. A pump continuous flows samples and reagents at a set rate to different mixing areas within separate channels for P and K. For P, some of the filtrate is mixed with a number of chemical reagents to generate a P complex. The absorption of P is determined colormetrically. The amount of P present is read from a calibration curve that has measured the absorption of P standards ranging in amounts from 0 to 100 ppm.


K is measured photometrically. A portion of the filtrate is pumped through to a flame photometer where the potassium ion is converted to a vapour of potassium atoms. The emission of light from potassium atoms is detected. The amount of K present is read from a calibration curve that has measured the emission of K standards ranging in amounts from 0 to 1000 ppm.

Fig. 2: Lachat autoanalyser for Morgan’s analysis

The filtrates for Mehlich 3 are measured using an inductively coupled plasma – optical emission spectroscopy (ICP-OES) instrument. The filtrate is converted to a fine aerosol that is sent through a plasma to produce atoms of each element that are identified by the emission of characteristic UV-visible light.

Fig. 3: ICP-OES autoanalyser for Mehlich 3 analysis

Some photos of vegetables growing showing importance of soil nutrients.