Structure and Functioning of a Managed Grassland Ecosystem
Exercise E: Soil nutrient pools (plant-available Na, K, Ca and total soil N)

EQUIPMENT

Field

• tapes and pins for positioning sampling points
• random number table
• soil corer (see Exercise B for details)
• polythene bags

Laboratory

• knife
• foil trays
• balance
• 2 mm sieve (standard mesh size for soil analyses)
• drying oven
• mortar
• flame photometers
• auto-analyser
• volumetric glassware (including safety pipettes)
• ‘qualitative’ filter paper
• digestion block

Reagents

• 1M ammonium acetate solution
• 2000 mg l^-1 lanthanum chloride (CARE: POISON)
• 1000 mg l^-1 Ca, K and Na standard solutions
• H₂O₂ / H₂SO₄ digestion mixture (CARE NEEDED. See Exercise D)
• deionised water

METHOD

Field procedure

Twelve soil cores about 5 cm deep are taken randomly from each plot (mown and unmown) of the site and placed in polythene labelled bags. Three replicates are required from each plot for each of the four analyses.

Laboratory procedure

• The top 1 cm of the soil cores, containing most of the root-mat and shoots, is sliced off.
• Each core is broken up and placed on a labelled foil tray for drying at 40°C overnight or for 24 hours to constant weight (a low temperature drying regime is essential for N analyses). The dried core sample is weighed so that the samples can be related to the original core size to calculate pools m^-2.
• Each soil sample is ground separately in a mortar to pass a 2 mm sieve. Grinding should be vigorous enough to crush aggregates but not to break stones. The soil should be separated from stones but the sample should not be reduced to a fine powder, which would dilute the nutrients in the soil with rock dust. The material is sifted carefully onto a sheet of paper, without making too much dust.

Procedure for Na, K and Ca

• Exchangeable cations Ca, K, Na (but not NH₄⁺) can be extracted by shaking the soil in a solution of ammonium acetate that displaces cations that are ionically retained by clays. The procedure is as follows:
  • 5 g of sieved oven-dry soil are weighed into a 250 ml conical flask
  • 125 ml 1M ammonium acetate solution are added, the mixture is stoppered with a vented bung and placed on the side arm shaker for 30mins
  • 3 blanks (stoppered flasks containing ammonium acetate but no soil) are also run
  • the flasks are removed from the shaker and the soil is allowed to settle out for 10-15 minutes
  • the mixture is decanted into a filter funnel on a sample bottle, leaving most of the soil in the flask, to collect a volume of about 50 ml.
  • a series of standard dilutions for each element is made up to calibrate the instruments (see below).
• Setting up the flame photometers is demonstrated. Each machine is set up to analyse for Na, K or Ca. (Ca analysis can be carried out on an atomic absorption spectrophotometer, but a flame photometer is adequate and safer to use). Instruments are calibrated as in Exercise D.
• Because of the high concentrations of Ca ions (assuming the soil is not an acid one) the extracts will need to be diluted 10:1. Lanthanum chloride is added to suppress interference by phosphate and aluminium ions. To achieve this dilution and an 800 ppm concentration of lanthanum chloride in the extract:
  • to 1 ml extract add 4 ml of 2000 mg l^-1 lanthanum chloride solution and 5 ml ammonium acetate and mix (CARE: VERY POISONOUS). Correct for dilution by multiplying the result by 10.
  • to measure Ca in the blanks, take 4 ml of the 2000 mg l^-1 lanthanum solution, 6 ml of blank (ammonium acetate solution) and mix. Correct for dilution by multiplying the result by 2.5.
• Potassium and sodium concentrations are easier to determine by flame photometer because no lanthanum is required.

Making up standards for Na, K, Ca

Sodium

• A 1000 mg l^-1 Na standard solution is diluted to 100 mg l^-1
• 10 ml are pipetted into a 100 ml volumetric flask, made to volume with 1M ammonium acetate, labelled and mixed well.
• 0, 2, 4, 6, 8 ml are pipetted into volumetric flasks, made to volume with 1M ammonium acetate and mixed well.
• The standard series will then be: 0, 2, 4, 6 and 8 mg l^-1 Na. Flasks are labelled with the concentrations and marked AmAc.

Potassium

• A 1000 mg l^-1 K standard solution is diluted to 100 mg l^-1
• 10 ml are pipetted into a 100 ml volumetric flask, made to volume with 1M ammonium acetate, labelled and mixed well.
• 0, 2, 5, 10, 15 ml are pipetted into 100 ml volumetric flasks, made to volume with 1M ammonium acetate and mixed well.
• The standard series will then be: 0, 2, 5, 10 and 15 mg l^-1 K. Flasks are labelled with the concentrations and marked AmAc.

Calcium

• A 1000 mg l^-1 Ca standard solution is diluted to 200 mg l^-1
• 20 ml are pipetted into a 100 ml volumetric flask, made to volume with 1M ammonium acetate, labelled and mixed well.
• A series of standards is prepared by pipetting 0, 5, 10, 15, 20 ml of 200 mg l^-1 into 100 ml volumetric flasks.
• Using a safety pipette, to each flask add 40 ml of 2000 mg l^-1 lanthanum chloride solution, make to volume with 1M ammonium acetate and mix well.
• The standard series will be 0, 10, 20, 30, 40 mg l^-1 Ca, each one having an 800 mg l^-1 concentration of lanthanum chloride. Flasks are labelled with the concentrations and marked AmAc.

Calibration of instruments

• The standards for Ca, K and Na are run on flame photometers and calibration curves are constructed on an Excel spread sheet or on graph paper.
• As a check on the standard preparations, students are provided with quality control samples for each element. These are run on the flame photometers to find whether the results tally with their standard preparations.

Total soil nitrogen content (Kjeldahl N)

• 0.2 g (* 0.01) of ground soil from each sample is weighed and put into a digestion tube (keeping a record of the depth of each replicate sample).
• 4.4 ml H₂O₂ / H₂SO₄ digestion mixture are pipetted into each digestion tube. 40 ml 'blanks' of the mixture will also be prepared. (It is recommended that this stage is demonstrated, not attempted by students, because it involves highly corrosive oxidising agents.)
• The samples are digested at 360°C for two hours on a digestion block to a colourless solution and bleached minerals.
• They are allowed to cool, then 50 ml of de-ionised water are added to each tube, mixed well and allowed to settle.
• The clear solution is used for N determination by auto-analyser. (This technique is demonstrated to the students.)

Sample analysis

The concentrations in the extracts are read from the calibration curves (for K, Ca and Na) or obtained from the auto-analyser (for N) (see Exercise D).

Data handling and statistical techniques

The general formula used to calculate the concentration (in mg g-1 of soil) for all the elements:

sample reading (mg l-1) x volume of extractant (ml/1000) x [dilution factor]

wet weight sample (g) x dry weight correction factor

 

Means ± standard errors of the results from mown and unmown areas are calculated

Timetable

The length of the digestion procedure makes it difficult to schedule this exercise into one day. Sampling and sample preparation are usually carried out during one practical exercise, then the digestate prepared during the week in preparation for a laboratory practical.

SPECIMEN RESULTS

Exchangeable cations and Kjeldahl N

DATA INTERPRETATION

• Levels of exchangeable Ca are consistent with the pH (c. 5.5 — 6.5) of a well-buffered base rich loam.
• Na and K seem low given the large maritime inputs in precipitation.
• Total N concentrations are higher on the mown site because of the higher turnover of high quality plant material as grass mowings.

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