Agro-environmental fertilization guide

Monitoring of Phosphorus Required in an Agro-Environmental Fertilization Plan

This section describes three elements that must appear in the agro-environmental fertilization plan specified in the Regulation respecting the reduction of pollution from agricultural sources (RRPAS). These are the calculation of expected changes in soil phosphorus content and saturation percentage, regular monitoring of these readings, and determination of phosphorus reception or surplus capacity in an agricultural operation.

1.1 Changes in Soil Phosphorus Content

It is known that a contribution of phosphorus beyond that contained in the harvested portion of a crop leads to an enriching of the soil content of that element. Since the nitrogen and phosphorus content of animal waste does not generally correspond to the quantities of nitrogen and phosphorus taken up by the various crops and since the quantity of phosphorus necessary to meet the crop needs is generally less than the amount of nitrogen that is needed, the quantity of manure spread to meet all or a large part of the nitrogen needs of the crops has the effect of producing a relatively large increase in the phosphorus content of the soil. This may also be true in cases where phosphated mineral fertilizers are used in addition to contributions of animal waste or beyond the crop needs. This situation will deteriorate even more with frequent repetition.

In fields whose soil is classified as "poor" or "average" in phosphorus, a controlled enrichment is desirable to achieve quality yields. However, in fields whose soil is classified "good," "rich" or "exceedingly rich," excessive enriching is harmful to the maintenance of surface waters quality, especially when the saturation level exceeds 10 per cent (Giroux and Tran, 1996).

Although, on the one hand, the lawful requirements as to the quantities of phosphorus that may be spread on a soil and a given crop become stricter over time and, on the other hand, it is not always easy to quickly make the necessary adjustments for certain agricultural operations, it is foreseeable that over a certain period (short and medium terms), operations have to fertilize beyond the desirable quantities. In such situations, the speed of soil enrichment and the consequences of reducing relatively quickly the quantities of phosphorus spread on the fields must nonetheless be assessed. This information makes it possible to adopt the fertilization procedure best adapted to the situation of the agricultural operation in question.

In consulting various research studies carried out in Québec, Giroux et al. (1996) found that it is necessary to add from two to five kg P/ha to crop samples to increase the Mehlich III phosphorus content of a soil by 1 kg P/ha. For his part, Beaudet (1996) estimated long-term changes in the phosphorus content of the soil of a silage corn crop following fertilization with liquid pig manure. He also established that it was necessary to practice fertilization exceeding the crop sample by 3.5 kg P/ha to increase the soil phosphorus content by 1 kg P/ha. In another case, Tran et al. (1996) show that each 3.6 kg P/ha contributed exceeding silage corn crop samples in the form of liquid pig manure on a Neubois silt loam produces an additional 1 kg P/ha in soil content. Similarly, they recorded an increase of 1 kg P/ha on a Le Bras silt loam for each 3.8 kg P/ha spread in the form of cattle manure exceeding the samples from a silage corn crop. Finally, Gangbazo et al. (1998) obtained on a Coaticook silt loam an increase of 1 kg P/ha for each 2.8 kg P/ha contributed exceeding the phosphorus samples from a silage corn crop as well as an increase of 1 kg P/ha for each 10 kg P/ha exceeding the samples for a meadow. These authors also noted that a contribution of 2.8 kg P/ha and 1.4 kg P/ha less than the phosphorus samples respectively for a silage corn and meadow crop led to a reduction of 1 kg P/ha of soil.

When such fertilization is expected, the fertility level may be assessed by adding to the initial soil phosphorus content (according to soil analysis), the enrichment value obtained following excess fertilization in relation to the crop sample for each season involved. This excess is determined by assessing the phosphorus contributed by the fertilizing materials used, from which the crop sample must be subtracted, and applying the conversion factor given in the studies cited above or obtained on the farm. The crop sample is calculated by multiplying the yield of this crop, obtained by the operation involved in accordance with RAAQ data, by the sample of a ton of the harvested crop (kg P/ tonne harvested according to values given in the Assessment of the Phosphorus Sample in the Harvested Portion of Crops section of this Guide).

The final phosphorus content of the soil is determined from the following data:

beginning soil content + [(contributions - sample) ¸ conversion factor] = final content.

Here is an example:

Let us assume a clay soil growing grain with a grain yield assessed according to the RAAQ as 6800 kg/ha and a moisture rate of 15 per cent. The analysis of the soil from this field carried out with the Mehlich III extract shows quantities of 132 kg P/ha, 156 kg K/ha and 1128 mg Al/kg.

According to the Fertilization Recommendations of the Conseil des Productions Végétales du Québec [Québec plant production council] Inc. (AGDEX 540, 2nd edition, 1996), this crop needs 150 kg N/ha, 40 kg P₂O₅/ha and 75 kg K₂O/ha.

The field is fertilized at seeding time with mineral fertilizers at the rate of 30 kg N/ha in the form of NH₄NO₃ and post-emergence with liquid manure from a pig breeding operation containing 3.0 kg N/m³, 2.8 kg P₂O₅/m3 and 1.5 kg K₂O/m³ sprayed and incorporated in less than one week.

Following the fertilization with 30 kg N/ha in the form of NH₄NO₃, the needs of the crop are therefore 120 kg N/ha, 40 kg P₂O₅/ha, 75 kg K₂O/ha.

Considering the fertilization modes adopted (type of manure, when spread, etc.), efficiency coefficient and loss factors that apply to this situation, the available nitrogen content of the liquid manure used is obtained as follows:

3.0 kg N/ha x 0.6 ¸ 1.4 ¸ 1.0 = 1.3 kg N _available /m³

When fertilization is carried out according to the crop’s nitrogen requirement, this represents a quantity of:

120 kg N/ha ¸ 1. 3 kg N _available/m³ = 92 m³/ ha

This quantity corresponds to a phosphorus contribution of:

92 m³/ ha x 2.8 kg P₂O₅/m³ = 258 kg P₂O₅  

This type of fertilization furnishes excess phosphorus assessed as follows:

Contribution – Sample = Excess 

Since 2.29 kg P₂O₅ corresponds to 1 kg P, this excess corresponds to:

258 kg P₂O₅/ha – (6.8 t/ha x 3 kg P/t x 2.29) = 211 kg P₂O₅/ha

For the purposes of this exercise, it was assumed that a corn crop fertilized in excess of the samples at the rate of 3.5 kg P/ha enriches the soil by 1 kg P/ha. This fertilization contributes to the phosphorus enrichment of the soil in the following manner:

[211 ¸ (2.29 x 3.5)] kg P/ha = 26 kg P/ha

Consequently, it will have the effect of increasing the fertility of the soil to the following level:

Initial level + Excess = Level attained

132 kg P/ha + 26 kg P/ha = 158 kg P/ha

To anticipate the long-term phosphorus content of the soil in this field, any assessment must take into account the fertilization used for each of the seasons preceding the sampling date. Thus, the former fertilization practice repeated for five years will lead to a phosphorus level of 262 kg P/ha, which increases environmental risk. 

1.2 Changes in Soil Phosphorus Saturation Percentage

The soil phosphorus saturation percentage provides information on the level of phosphorus availability but also the level of environmental risk this element presents to water quality. In addition to leading to an increase in the phosphorus content of the soil, over-fertilization thus also has the effect of increasing the soil phosphorus saturation percentage. So, when a soil holds 1128 mg Al/kg soil and 132 kg P/ha extracted with Mehlich III solution and is fertilized in such a way that its content reaches 262 kg P/ha, the soil saturation percentage it represents therefore increases by 5.3 to 10.6 per cent. A field may therefore change from a favourable agronomic situation to a risky environmental situation without necessarily guaranteeing better agronomic performances.

Changes in soil phosphorus saturation percentage may be calculated using the following equation:

[M-3¹ phosphorus content (kg P/ha) / M-3 aluminium content (mg Al/kg soil) x 2.2²] x 100

Here is an example:

Let us assume the same field as in the previous example whose beginning soil phosphorus content is 132 kg P/ha and aluminium content 1128 mg Al/kg soil extracted with Mehlich III solution and presenting a phosphorus content of 165 kg P/ha after fertilization with liquid manure from a pigs breeding operation.

Initially, the saturation percentage is :

[132 kg P/ha (1128 mg Al/kg soil x 2.2)] x 100 = 5.3%. 

After one year of fertilization, the saturation percentage will be:

[158 kg P/ha (1128 mg Al/kg soil x 2.2)] x 100= 6.4%.

After five years of fertilization, the saturation percentage will be:

[262 kg P/ha (128 Al/kg soil x 2.2)] x 100 = 10.6%.

1.3 Periods and Modalities

The agro-environmental fertilization plan (AEFP) must provide two former assessments for a period from the first crop year covered by the AEFP until the date the second phase of the phosphorus standard comes into effect. However, this period may not be less than five years. More specifically, values must be produced for the first crop year when the first phase and second phase standards come into effect. Once these dates are reached, the assessments must to be produced for a date ending a minimal period of five years.

Assessments must be prepared for each field of an agricultural operation.

The preparation of assessments must take into account expected fertilization and crop rotation. The conversion factor for the quantities of phosphorus introduced in excess of the samples to be used to carry out the assessments is 3.5 kg P/ha, to increase soil phosphorus content by 1 kg P/ha or any value that can be supported with data by the author of the AEFP (research work carried out in Québec or data from the agricultural operation involved).

References

Gangbazo, G., A. R. Pesant et G. M. Barnett, 1998. Effets de l’épandage des engrais minéraux et de grandes quantités de lisier de porc sur l’eau, le sol et les cultures. Ministère de l’Environnement et de la Faune du Québec. Direction des écosystèmes aquatiques. 46 pages.

Giroux, M., D. Carrier et P. Beaudet, 1996. Problématique et méthode de gestion des charges de phosphore appliquées aux sols agricoles en provenance des engrais de ferme. Agrosol 9 (1): 36-45.

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.

Tran, T. S., D. Côté et A. N’Dayegamiye, 1996. Effets des apports prolongés de fumier et de lisier sur l’évolution des teneurs du sol en éléments nutritifs majeurs et mineurs. Agrosol 9 (1): 21-30.

2. Regular Monitoring of Soil Phosphorus Content and Saturation Percentage

The only known way to monitor changes in soil phosphorus content and saturation percentage is to measure the phosphorus and aluminium content extracted using Mehlich III solution.

The objective of the regular monitoring set out in the Regulation respecting the reduction of pollution from agricultural sources is to measure real changes in soil phosphorus content and saturation percentage. This data will make it easier to plan optimum use of fertilizing materials by the agricultural operation involved or animal waste and farm compost to export or import.

Thus, if a field receives only mineral fertilizer, monitoring consists in taking a soil sample and measuring its phosphorus content at least once every three years. In other cases, it must be carried out at least once every two years.

3. Determination of Phosphorus Reception or Excess Capacity of an Agricultural Operation

Any agricultural operation fertilizing according to an agro-environmental fertilization plan (AEFP) and receiving animal waste or farm compost from one or more other operations must determine its phosphorus reception capacity according to the fertilization standards in effect. Similarly, any operation fertilizing according to an AEFP and not able to use all its waste or farm compost so as to comply with the quantities of nitrogen and phosphorus set out in the fertilization standard in effect must use a different method to determine the excess phosphorus it will have to dispose of. The present section aims at specifying the approach to be utilized in carrying out these two assessments as well as the period and modalities to achieve them.

3.1 Phosphorus Reception Capacity of an Agricultural Operation

The first stage in meeting this requirement consists in establishing, from the information appearing in the AEFP, the quantities of phosphorus that may be spread on each field of the agricultural operation according to their phosphorus content and saturation percentage, their area, the crop grown and the fertilization standard specified in the AEFP, which may not exceed the fertilization standard in effect. These quantities must then be totalled for all fields in the operation. The following stage consists in establishing the quantities of phosphorus from the operation’s animal waste and farm compost.

Finally, the operation’s phosphorus reception capacity must be determined by subtracting from the quantities which may be spread the animal waste and farm compost produced by the operation as already appraised.

3.2 Phosphorus surplus of an agricultural operation

The present assessment is based on the same parameters as the assessment of the phosphorus reception capacity of an agricultural operation. However, the last stage in this assessment consists in determining the phosphorus surplus of a farm by subtracting from the contributions from its animal waste and farm compost the quantities that may be spread as already appraised.

3.3 Period and Modalities

This requirement calls for the production of assessments at various times. Thus, in all cases, an assessment must be carried out for the first crop year covered by the AEFP. The assessment must also deal with each year when the first and second phase of the fertilization standard come into effect. The assessment must always cover a minimal period of five annual crop years.

These assessments may be prepared taking into account the fertilization and rotation set out in the AEFP, while considering the values obtained from the calculation of the foreseeable changes in the soil phosphorus content and saturation percentage that are themselves a function of the expected fertilizations and crop rotations.

1 M-3 : Mehlich III