Agro-environmental fertilization guide

Modalities of sampling and soil analysis

This section describes the sampling method and the contents of the soil sampling report as well as (aluminium and phosphorus) analysis methods and assessment of the soil phosphorus saturation percentage referred to in the Regulation respecting the reduction of pollution from agricultural sources (RRPAS).

1. Soil Sampling Method ¹

The use of mineral fertilizers, animal waste, farm compost and various other residual fertilizing materials accounts for a large part of expenditures relating to plant production. This is because of the need to improve crop yields and quality, the increase in the cost of mineral fertilizers and the increase in the cost of managing fertilizing materials in order to minimize the risk of soil and water contamination. From this point of view, it becomes important to add to the soil only the quantities of nutritional elements required to obtain optimum yield.

Agricultural producers must be knowledgeable about the soils they are cultivating. This means they must pay close attention to the nutritional elements present in these soils. Analysis of a representative sample of the various types of soil or the different fields being used will give them useful information allowing them to use fertilizing information more rationally.

1.1 Sampling Frequency

The soil analysis used to prepare an agro-environmental fertilization plan (AEFP) or taken during regular monitoring of the phosphorus content of the soil must be representative of the fertilizing content of the soil to be fertilized. It must always be less than five years old. However, following recourse to high phosphorus content fertilization, for example, using animal waste or farm compost according to the nitrogen requirement specified in the Fertilization recommendation published by the Conseil des productions végétales du Québec inc. (CPVQ inc.) (AGDEX 540, 2nd edition, 1996), analysis must be less than one year old. This is also the case when it is expected that fertilization since the last soil analysis has altered the maximum limit for application of phosphorus to which a field is subject.

For the frequency of sampling and analysis needed to carry out the regular monitoring of soil phosphorus content as required in the RRPAS, refer to the section dealing with this aspect.

1.2 Sampling Period

Preferably, sampling should always be carried out at the same time of year, whether at the beginning of the season, in June, or after harvest. It is however necessary to avoid waiting until the day before ploughing since the analysis may not be completed in time to permit fertilization or application of lime. By obtaining analysis results early enough, you will be able to order the limestone in time, spread it when appropriate and not unduly delay work or else have your AEFP in time to carry out the first fertilization work of a crop year.

1.3 Preparation for the Taking of Samples

A person who has to carry out this task should always refer to the farm plan before going to the fields to take samples. He must identify them (on each container, give details regarding the field designation appearing in the farm plan). In this way, it will be easy to match the results with their source when the results of the analysis are available and use them when preparing the agro-environmental fertilization plan.

1.4 Nature of the Soil

When the soil is uniform in nature (same texture, same drainage, same topography) and when dealing with the same field, you can make one comprehensive sample from a minimum of 10 specimens taken from different parts of the field.

If there are variations in the nature of the soil, it may be desirable of take samples separately as you would with a different field. Such variations are sometimes marked by difference in colour or texture. It is preferable to take separate samples of sandy, silt or clay soils.

1.5 Sample Taking

Using a soil sampler or a spade and bucket, take a small amount of soil from a depth of 16.9 cm in mineral soil (soil with 30 per cent or less organic material) and at 20 cm in organic soil (soil with more than 30 per cent organic material). A minimum of 10 specimens should be collected at different points in a given field in a zigzag across it. For fields with an area greater than three hectares, the minimum number of samples should be increased to 15. Crop wastes and vegetation must be removed from the samples. The soil samples must be sufficient in number, and sampling sites so chosen as to obtain a representative value of the average phosphorus content of the arable layer of the field.

The soil collected must be well mixed with a trowel or other tool and placed in a container labelled with the number of the field. Avoid handling a sample with the hands and use only clean tools and containers.

Avoid taking samples or specimens:

• on fields fertilized less than one month before;
• on edges of fields;
• on elevated spots;
• on the banks of ditches or waterways;
• at places where animal waste, farm composts, residual fertilizing materials (paper plant sludge, etc.) or lime have accumulated;
• along roads where snow containing calcium or sodium salts has been dumped;
• along farm driveways;
• in places that are too wet (low spots).

1.6 Situations Requiring Special Attention

1.6.1 Crops That Have Received Band Fertilization

In a field where mineral fertilizer has been spread in bands, the soil must be sampled both on the bands and between them, then carefully mixed to reduce the effects of this type of fertilization.

1.6.2 Establishment of Deep-root Crops

In such cases, two samples must be taken :

• a surface sample as already described;
• a depth sample by collecting specimens at the 16.9 or 20 cm to 40 cm depth, labelling the container that it is a depth sample.

2. Field Soil Sampling Report

The sampling activity must be clearly described in a report detailing the procedure followed and, especially, the special features of the samples taken. It is important to know that field boundaries will sometimes be determined by the nature (composition) of the samples taken.

The field referred to is the one specified in section 3 of the Regulation respecting the reduction of pollution from agricultural sources (RRPAS).

Thus, after samples are taken of the soil in one or more fields, a report must be prepared containing the following information:

• name of the person who carried out the sampling;
• date of the sampling;
• number of fields sampled according to the farm plan and identification number given to each sample;
• location of the soil sampling points;
• depth of sampling;
• number of specimens making up a sample;
• crop grown;
• all particular information relating to one or more fields sampled on the same date.

3. Soil Analysis Methods

3.1 Introduction

The Mehlich III method was developed by Mehlich (1984) as a method to extract various nutritional elements present in the soil. The Mehlich III extract solution is composed of 0.2 M CH₃COOH, 0.25 M NH₄NO₃, 0.015 M NH₄F, 0.013 M HNO₃ and 0.001 M EDTA. This method is used at present to determine the phosphorus (P) available to plants for Québec's mineral and organic soils (Tran and Giroux, 1989; Tran et al., 1990).

The Mehlich III method also makes it possible to extract the following exchangeable mineral elements: potassium (K), calcium (Ca), magnesium (Mg) and sodium (Na). It is also used in the determination of the following trace elements: aluminium (Al), copper (Cu), zinc (Zn), manganese (Mn) and iron (Fe).

3.2 Equipment and Reagents

1. Rotary agitator
   
2. Erlenmeyer flasks, 125 mL
   
3. Filter funnels
   
4. Filter paper (Whatmanâ No. 42)
   
5. Disposable plastic flasks
   
6. Spectrophotometer for conventional colorimetry at 535 nm for Al and 882 nm for P or Technicon automated analyser for P or atomic absorption spectrometer or inductive plasma spectrometer
   
7. Mehlich III extract solution (0.2 M CH₃COOH + 0.25 M NH₄NO₃ + 0.015 M NH₄F + 0.013 M HNO₃ + 0.001 M EDTA)  
   
   1. Mehlich III mother solution: (1.5 M NH₄F + 0.1 M EDTA). Dissolve 55.56 g of ammonium fluoride (NH₄F) in 600 mL of distilled water. Add 29.23 g of ethylenediaminetetraacetic acid (EDTA) (p.m. 292.24) to this mixture, dissolve, top up to 1 L with distilled water, shake well and store in a plastic flask.
      
   2. In a large plastic container, place about 8 L of distilled water, add 200.1 g of ammonium nitrate (NH₄NO₃),100 mL of Mehlich III mother-solution, 115 mL of acetic acid (CH₃COOH), 82 mL of nitric acid (HNO₃) 10% v/v (10 mL of 70% concentrated HNO₃ in 100 mL of distilled water),dissolve, top up to 10 L with distilled water and shake well.
      
   3. Store in a plastic container
      
8. Solutions for the manual determination of phosphorus
   
   1. Solution A: Dissolve 12 g of ammonium molybdate [(NH₄)₆ Mo₇O₂₄ X 4 H₂O] in 250 mL of distilled water. In a 100 mL flask, dissolve 0.2908 g of antimony potassium tartrate [K(SbO)C₄H₄O₆ X ½ H₂O] in 80 mL of water. Transfer these two solutions into a 2 L volumetric flask containing 1000 mL of 2.5 M sulphuric acid (H₂SO₄) (141 mL of concentrated H₂SO₄ per litre), top up to 2 L with distilled water, shake well and store away from the light at 4°C.
      
   2. Solution B: Dissolve 1.056 g of ascorbic acid (C₆H₈O₆) in 200 mL of solution A. Prepare this solution daily.
      
   3. Standard solutions of P: Use certified solutions of P or prepare a solution of 100 mg L^-1 of P by dissolving 0.4393 g of monobasic potassium phosphate (KH₂PO₄) in 1 L of distilled water. Prepare standard solutions of 0, 2, 4, 6, 8 and 10 mg L^-1 of P in the diluted Mehlich III extract solution.
      

The phosphorus concentration in the Mehlich III extract solution may be determined by other calorimetric methods, such as Bray-1 or Bray-2.

9. Solutions for the automated determination of phosphorus using the Technicon apparatus, according to modified industrial method no 94-70W (Technicon Autoanalyzer II, 1973)
   
   1. Antimony molybdate solutions: Dissolve 30 g of ammonium molybdate [(NH₄)₆ Mo7O24 X 4 H₂O] in 600 mL of water. Add 0.15 g of antimony potassium tartrate [K(SbO)C₄H₄O₆ X½ H₂O] and complete the volume to 1 L with distilled water.
      
   2. 1 M sulphuric acid: Dilute 56 mL of sulphuric acid (concentrated H₂SO₄) in 400 mL of distilled water, cool the solution. Add 1 mL of Aerosolâ 22 agent and dilute to 1 L with distilled water. This solution must be prepared daily.
      
   3. Ascorbic acid solution: Dissolve 12 g of ascorbic acid (C₆H₈O₆) in 200 mL of distilled water, add 1 mL of wetting agent Levor IV and mix well.
      
   4. Standard solutions of phosphorus: See 8 c.
      
10. Solutions for the determination of aluminium by manual colorimetry
    
    1. Eriochrome cyanine R.

       Dissolve 0.750 g of eriochrome cyanine R in 200 mL of demineralized water. Add 25 g NaCl, 25 g NH₄NO₃ and 2 mL concentrated HNO₃. Top up to 1 litre with demineralized water.

        

2. Ammonium acetate - acetic acid buffer.

   Dissolve 320 g ammonium acetate in 500 mL demineralized water. Add 5 mL of chilled acetic acid and top up to 1 litre with demineralized water.

3. Ascorbic acid.
   2% aqueous solution
   
4. Standard solutions of aluminium.
   

   Prepare a range of standard solutions: 0, 0.80, 1.60, 2.40, 3.20 and 4.00 ppm of Al prepared in a Mehlich-III solution diluted to 1/100 with demineralized water.

11. Solutions for the determination of aluminium using atomic absorption
    
    1. Solution of ionic suppresser and interference suppresser (0.06% CsCl + 0.2% LaCl₃): dissolve 3.16 g of caesium chloride (CsCl) in 100 mL of lanthane chloride (LaCl₃ 10%).
       
    2. Standard solution: according to the instrument used and the concentrations desired, prepare a range of standard solutions for the calibration of the aluminium.

3.3 Procedure

1. Extraction

1. Weigh 3 g or measure 3 mL of 2 mm sifted soil in 125 mL Erlenmeyer flasks. For organic soils, a volume of 3 mL soil is recommended.
   
2. Add 30 mL Mehlich III extract solution (soil: solution ratio of 1: 10).
   
3. Agitate immediately for five minutes in a rotary agitator (120 revolutions min^-1).
   
4. Filter through Whatmanâ No. 42 paper filter and collect the filtrate in plastic flasks. Carry out the analyses as quickly as possible.

2. Determination of phosphorus by the manual colorimetry method 

1. Pipette 2 mL of clear filtrate into a 25 mL volumetric flask.
   
2. Add 15 mL of distilled water and 4 mL of solution B, dilute to 25 mL with distilled water and mix well.
   
3. After 10 minutes of colour development, measure absorbency at 882 nm.

3. Determination of phosphorus by the automated method (Techniconâ Autoanalyzerâ )

1. Start up the various modules of the Technicon automated analyser at least 30 minutes ahead of time.
   
2. Place each tube in its respective reagent (see diagram in Figure 1). Let it pump for 20 minutes to balance the system.
   
3. Adjust the base line using the Mehlich III extract solution and the maximum absorbency level of the recorder using the standard solution containing the highest phosphorus concentration.
   
4. Place the standard solutions and soil filtrates on the sampler and start it up. Use distilled water with a few drops of Levor IV wetting agent as washing solution.

4. Determination of aluminium by manual colorimetry.

1. Pipette 1 mL filtrate into a 100 mL graduated flask. Top up to the mark with demineralized water.
   
2. Remove 1 mL filtrate diluted to 1/100 (or to standard) in a colorimetry tube.
   
3. Add 2.4 mL demineralized water.
   
4. Add 0.5 mL eriochrome cyanine R solution
   
5. Add 1.0 mL ammonium acetate - acetic acid buffer.
   
6. Add 0.1 mL 2% ascorbic acid. The stage c to f reagents may be mixed in advance in the following manner: 48 mL demineralized water, 10 mL eriochrome cyanine R solution, 20 mL ammonium acetate-acetic acid buffer and 2 mL 2% ascorbic acid. Remove 4.0 mL of the reagent mixture and continue to step g.
   
7. Shake well to homogenize and let stand for 20 minutes.
   
8. Read spectrophotometer absorbency at 535 nm.
   
9. If the concentration is too high, dilute the filtrate to 1/100 and repeat stages b to h taking dilution into account in the final calculation.

5. Determination of aluminium by atomic absorption

1. The (Mehlich-III) soil extracts are titrated by conventional atomic absorption in the presence of an ionic suppresser and an interference suppresser combined (0.06% caesium chloride + 0.2% lanthane chloride, reagent No. 6) at a rate of 1 mL per 50 mL of diluted extract.
   
2. To avoid any simultaneous precipitation of CsCl and LaCl₃ in the presence of Mehlich-III extract, it is preferable to dilute in demineralized water. Dilution in water also decreases the concentration of dissolved salts deposited at the surface of the burner.

Example of 1/10 dilution:

• 5 mL soil extract;
• 1 mL suppresser solution;
• complete with water to 50 mL.

3. Standard solutions are also diluted in aqueous medium.

6. Determination of aluminium and phosphorus by the inductive plasma spectrometry method

The (Mehlich III) soil extracts may be titrated directly by plasma emission without adding internal standard or interference suppresser. The instrumental parameters and optical characteristics of the plasma emission spectrometer are different from one instrument to another and must be calibrated accordingly. The aluminium present in the soil extract may produce interference affecting the phosphorus channel. It is advisable that a correction factor be integrated, if necessary. In the case of soils rich in organic material (% M.O.> 5), phosphorus contents titrated by plasma emission may be higher than those titrated by colorimetry. However, this difference will not markedly affect the fertility class of the analysed soil.

References

Mehlich, A., 1984. Mehlich-3 soil test extractant: a modification of Mehlich-2 extractant. Comm. Soil Sci. Plant Anal. 15: 1409-1416.

Technicon Auto-Analyzer II, 1973. Orthophosphate in water and wastewater. Industrial method No. 94-70W.

Tran, T.S. et M. Giroux, 1989. Évaluation de la méthode Mehlich-III pour déterminer les éléments nutritifs (P, K, Ca, Mg, Na) des sols du Québec. Agrosol 2: 27-33.

Tran, T.S., M. Giroux, J. Guilbault and Audesse, 1990. Evaluation of Mehlich-III extractant to estimate the available P in Québec soils. Comm. Soil Sci. Plant Anal. 21: 1-28.

4. Soil Phosphorus Saturation Percentage

With respect to fertilization, at present there is a problem of water quality related to poor management of phosphorus from all sources. In the RRPAS, the limitation on the use of phosphate fertilizers is established on the basis of two parameters, soil phosphorus content and saturation percentage. The phosphorus content of the soil indicates the level of soil fertility. The soil phosphorus saturation percentage shows the proportion of soil fixation sites already occupied by this element. This latter parameter is related to the phosphorus fixation capacity of the soil, which is defined as the power of a soil to hold a given quantity of phosphorus. These two parameters provide an indication of the environmental risk that the presence of phosphorus represents.

In Québec, soil phosphorus content is measured using Mehlich III extract. It is one of a number of highly effective extracts that make it possible to measure the phosphorus fixation capacity of the soil. Under our soil conditions, the studies carried out by Giroux and Tran (1996) show that there is a very good relationship between the aluminium content extracted with the Mehlich III solution and soil phosphorus fixation capacity. Both of these values are regularly provided in the soil analysis reports.

Giroux and Tran (1996) also demonstrate the value of two indications of phosphorus saturation of the soil: the Mehlich III P / Mehlich III Al ratio and the Mehlich III P / phosphorus fixation capacity ratio. They obtained a strong relationship between these two indexes. Thus, the fixation capacity makes it possible to easily determine the saturation percentage and, from a practical point of view, this means that we have a reliable agro-environmental diagnostic tool based on the data provided by the soil analysis bulletin.

As we have just seen, the soil phosphorus saturation percentage is the ratio between the soil phosphorus content and aluminium content in a field, both extracted by Mehlich III solution. This value is calculated in the following manner:

[M-3³ phosphorus content (kg P/ha) / M-3 aluminium content (mg Al/kg soil) x 2.2⁴] x 100

Reference

Giroux, M. et T.S. Tran, 1996. Critères agronomiques et environnementaux liés à la disponibilité, la solubilité et la saturation en phosphore des sols agricoles du Québec. Agrosol 9 (2): 51-57.