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Soil Analysis

Maintaining Optimum Soil Fertility

Approximately 10% of all advisory soil samples analysed through Teagasc in 2015 had soil P and K levels in the optimum soil fertility range (pH >6.2 and soil P & K index 3 or 4). This means that 90% of the soils analysed had sub-optimal levels of soil pH, P and K or a combination of all 3. Soil test results indicate that soil P and K levels continue to decline, with 61 and 55% of grassland soils index 1 and 2 for P and K, respectively. There has been a very slight improvement in soil pH over the last decade but there are a large percentage of soils below the optimum pH 6.3 and 6.5 for grassland and tillage soils, respectively. Results indicate that approximately 65% of grassland and 55% tillage soil samples are below the optimum soil pH targets.

Productive soils are the foundation of any successful farming system. The increasing demand for high grass growth rates and crop yields represents an increasing demand on soil fertility levels. The ability of soils to maintain a continuous temporal supply of nutrients in the appropriate quantities for crop growth is a key determining factor of how productive a field or farm can be. Therefore, the management of soil fertility levels should be a primary objective on any farm.

Fertilisers account for a significant proportion of direct production costs on farms, but fertiliser costs represent good value for money when used correctly. However, fertiliser application rates that undersupply nutrients or oversupply nutrients, or are not in balance with other required plant nutrients will yield lower returns on investment. With fertilisers becoming more expensive, it is vital that fertiliser is managed as efficiently as possible with maximum return in farm produce.

5 Steps to soil fertility management

Managing soil fertility is about focussing on the key aspects of soil and nutrient applications, and setting targets for the farm. The following is 5 practical steps that should be followed to manage soil fertility.


1) Soil Samples

Have soil samples taken for the whole farm. Unless you know what is already in the soil, it is impossible to know how much fertiliser it needs. Therefore, by taking soil analysis and putting the results into practice, the fertiliser programme can be tailored to the needs of the soil and the crop. Repeating soil analysis over time (3 to 5 years) is also critical to monitor the effectiveness of the farm fertiliser strategy. 

Soil Analysis Step 1

2) Lime

Lime should be applied to neutralise soil acidity and raise the soil pH to the target soil pH for the crop been grown. For mineral soils, a soil pH 6.3 is recommended for grassland. The soil pH should be higher (Barley / Beet) for tillage crops and aim to maintain at pH 6.5 to 6.8. Acid soils will result in reduced release of the major nutrients (especially P) from soil, and result in a poorer response to applied fertilisers. Apply lime as a priority in line with the lime advice as per the soil test report.

3) Target Index 3

Aim to have optimum soil P and K (Index 3) fertility levels in all fields. At optimum fertility levels, nutrients being removed in products (Meat / Milk / Grain) need to be replaced. Low fertility (Index 1 & 2) soils need to be fertilised correctly to achieve soil index 3. For soils in Index 3 the fertiliser program should be designed to replace the nutrients being removed, thus maintaining the soil fertility levels. Index 1 and 2 soils have a very low to low nutrient supply, and require additional nutrients to increase the fertility levels on an annual basis. Index 4 soils have a high nutrient supply. These soils present an opportunity to save money on fertiliser inputs by harvesting the P and K soil reserves for a number of years depending on the soil test reading. For example, high P index 4's (>10-15 ppm) omit for 2/3 years and retest. For high soil K levels (>150- 200ppm) omit for 2/3 on grazing ground and one year on silage ground. Then revert back to index 3 advice until next soil tests. 

 4) Slurry and Manures

While slurry can be more difficult to manage than chemical fertiliser, it can be a very cost effective resource to increase fertility levels. Use slurry / FYM on the farm as efficiently as possible, and top up with fertiliser as required. Aim to apply slurry and manures to fields that have high P and K requirements (e.g. grass/maize silage). Apply in spring time under cool and moist weather conditions to maximise N recovery.

5) Balanced Nutrient Supply

If one nutrient is deficient, no amount of another nutrient will overcome this. For example, if a field is deficient in K, then excess N application will not be fully utilised. Make sure the selected fertiliser compound is supplying nutrients in the correct balance for the crop, the soil, and to complement any other nutrients applied in organic manures. Other nutrients such as sulphur / magnesium can play a very important role to ensure a balanced fertiliser programme and should be applied where necessary. For other crops, such as cereals/vegeatables always check minor nutrients such as boron, copper, managanese and zinc.

 

Articles and Publications

Soil Sampling Technique

The results of a soil analysis are only as good as the sample on which it is based. To give reliable advice, a soil sample must be representative of the area sampled and be taken to a uniform depth (10cm).


The principle of soil analysis is to determine the average nutrient status of an area and to give a measure of the available nutrients in the soil. A sample normally consists of 0.25 – 0.5 kg of soil and this is taken to represent the entire sampling area or field.

  1. To take a soil sample it is essential to have a suitable soil corer
  2. Ensure soil cores are taken to the correct sampling depth of 100 mm (4”)
  3. Take a soil sample every 2 to 4 ha. (5-10 acres)
  4. Take separate samples from areas that are different in soil type, previous cropping history, slope, drainage or persistent poor yields
  5. Avoid any unusual spots such as old fences, ditches, drinking troughs, dung or urine patches or where fertiliser / manures or lime has been heaped or spilled in the past.
  6. Do not sample a field until 3 to 6 months after the last application of P and K and 2 years where lime was applied.
  7. Take a minimum of 20 soil cores, mix them together, and take a representative sub-sample for analysis, making sure the soil sample box is full.
  8. Take a representative soil sample by walking in a W shaped pattern across the sampling area.
  9. Sample fields at the same time of the year to aid comparisons of soil sample results and avoid sampling under extremes of soil conditions e.g. waterlogged or very dry soils.
  10. Place the soil sample in a soil box to avoid contamination and write the field number and advisor code on the soil box with a black permanent marker.

Soil Sampling Pattern

Testing Equipment Contact Information

Soil Sampling Equipment

Tomas Fitzgerald,
Fitzagri,
Crosslow,
Co. Carlow
Phone- +35386-3915313

e-mail: fitzagri@gmail.com

  • Funnel type sampler Cost €80 plus VAT
  • Tillage Soil sampler Cost €40 plus VAT
Analysis Service NumberElements TestedTotal Charge inc. VAT €
Soil 1/S1
Basic or REPS
LR, pH, P, K €25.00
Soil 2/S2
Dairy/Beef/Sheep/Horses
LR, pH, P, K, Mg, Co and Total Mn €36.00
Soil 3/S3
Sheep - Tillage
LR, pH, P, K, Mg, Co, Cu, Zn, easily reducible Mn and Total Mn €42.00
Soil 4/S4
Tillage
LR, pH, P, K, Mg, Cu, Zn and easily reducible Mn €36.00
Soil 5/S5
Horticulture
LR, pH, P, K, B, Mg, Cu, Zn and easily reducible Mn, €43.00
Soil 6 /S6
(S1 + Mg)
Tillage / Grassland
LR, pH, P, K, Mg €27.00
Soil 7/S7

Organic Matter

Soil organic matter % €22.00
Soil 8/S8
(S6 + Organic Matter)
Tillage / Grassland / OM
LR, pH, P, K, Mg and soil organic matter % €36.00
Soil 9/S9
(S4 + Organic Matter)
Tillage / OM
LR, pH, P, K, Mg, Cu, Zn, easily reducible Mn and soil organic matter % €45.00
Soil 10/S10
(S5 + Organic Matter)
Horticulture / OM
LR, pH, P, K, Mg, B, Cu, Zn, easily reducible Mn and soil organic matter % €52.00

Advice on nutrient and trace element application rates clearly must depend on the quantity of the element in the soil that is available to the crop. Apart from N, this is determined by soil analysis. For most elements, the soil is extracted with a suitable reagent and the amount extracted is deemed to be or related to the amount available to the plant. For elements that are extracted, the analysis unit is milligrams per litre of soil (mg/l). For elements that are digested in strong reagents, the analysis units are mg/kg. These include cobalt, total manganese, sulphur and iodine.

In order to simplify advice tables, it is normal to classify soil available levels of nutrients and trace elements into classes. The class is referred to as the Soil Index. At Johnstown Castle, soil analysis levels are classified into Index 1 – 4. The exact interpretation of the Soil Index varies somewhat with the element and the crop but the definitions in Table1 apply in most circumstances.

Table 1: The Soil Index System
Soil IndexIndex DescriptionResponse to Fertilisers
1 Very Low Definite
2 Low Likely
3 Medium Unlikely/Tenuous
4 Sufficient/Excess None

Nitrogen (N)

There is, as yet, no useful Irish laboratory test for N in soils. Therefore, the nutrient N advice for grassland systems (grazing and conservation) depends mainly on land use and farming system, and particularly on the stocking rate. 

For crops requiring cultivation, the available soil N can be deduced from the previous cropping and manurial history, and the type of soil. Thus, N fertiliser advice is determined by the soil N supply status. This depends in turn on the previous cropping history. The supply status is categorised into an index system for grass establishment and tillage crops. Account is also taken of previous applications of chemical and organic manures, the requirement of the crop and the likely crop yield.

Tables 2 and 3 show how the N Index takes into account the past farm management history and reflects the likely rate of release of N from the soil.
In continuous tillage it is usually only necessary to consider the last crop grown to estimate N Index (Table 2). However, where long leys or permanent pasture occur in the rotation, it is necessary to consider the field history for longer than one year (Table 3). Previous applications of animal manures must also be taken into account.

Table 2: N Index for tillage crops that short leys or tillage. This table can also be used for grass establishment.
Previous crop
Index 1Index 2Index 3Index 4
Cereals, Maize
(no animal manures)
Sugar beet, Fodder beet, Potatoes, Mangels, Kale, Peas, Beans, Oilseed Rape    
  Swedes removed Swedes grazed in situ  
  Leys (1-4 years) grazed or cut and grazed    
  Any crop receiving dressings of organic fertiliser    
Vegetables receiving less than 200 kg N/ha Vegetables receiving more than 200 kg N/ha    
Table 3: N Index for pasture establishment or tillage crops that follow long leys (5 years or more) or permanent pasture
Previous crop
Index 1 Index 2Index 3Index 4
Any crop sown as the 5th tillage crop following long leys or permanent pasture. Any crop sown as the 3rd or 4th tillage crop following long leys or permanent pasture.
 If original long ley or permanent pasture was cut only use Index 1.
Any crop sown as the 1st or 2nd tillage crop following long leys or permanent pasture (see also Index 4). If original long ley or permanent pasture was cut only use Index 2. Any crop sown as the 1st or 2nd tillage crop following very good long leys or permanent pasture which was grazed only.

Phosphorus (P)

The P Index depends on the level of available P in soil. This is determined by measuring the amount of the element that is extracted by Morgan’s solution. The ranges are shown in Table 4. The ranges for grassland crops are different from other crops as many tillage and vegetable crops requires higher P levels for optimum production.

In the past, higher ranges were used for peat than for mineral soils for each Index level. This resulted in higher agronomic P advice rates for peats than for mineral soils. This was unsustainable from an environmental point of view, as P tends to be leached or washed out of peats each winter. This was particularly important in deep peats where the P could be lost to the drainage water instead of being trapped by the mineral layer underneath as happens in shallow peats

To minimise possible losses of nutrients to the environment, the Good Agricultural Practice for Protection of Waters Regulations 2010 requires that the fertilisation rates for soils which have more than 20% organic matter shall not exceed the amounts permitted for Index 3 soils. The effect of this is that a peat soils at P Index 1 or 2 is fertilized as if it were at soil P Index 3.

Table 4: The P index system
 Soil P Ranges (mg/I)
Soil P IndexGrassland cropsOther crops
1 0.0 - 3.0  0.0 - 3.0 
2 3.1 - 5.0 3.1 - 6.0
3 5.1 - 8.0 6.1 - 10
4 Above 8.0 Above 10.0

Potassium (K) and Magnesium (Mg)

The Index system for K and Mg are given in Tables 5 and 6.

Table 5: The K Index System
Soil K Ranges (mg/I)
Soil K IndexMineral SoilPeat *
1 0- 50 0-100
2 51- 100 101-175
3 101- 150 176-250
4 Above 150 Above 250

* To be defined as a peat, there must be no mineral soil in the upper 10cm throughout the sampled area. 

Table 6: The Mg Ranges (mg/I)
Soil Mg Inde
x
Soil Mg Ranges (mg/I)
1 0- 25
2 26- 50
3 51- 100
4 Above 100

Trace Elements Cobalt( Co), Maganese( Mn), Copper( Cu), Zinc( Zn) ,Iodine (I) and Boron( B)

The Index system for Co, Mn Cu, Zn, I and B are given in Tables 7 to 12. 

Table 7: The Co Index System 
Soil Co IndexSoil Co Ranges (mg/kg)
1 0 - 3.0
2 3.1- 5.0
3 5.1 - 10.0
4 Above 10
Table 8: The Er-Mn Index System
Soil Er-Mn IndexSoil Er-Mn Ranges(mg/l)1
11 < 90
2 90-120
3 >120
1Extrantant for Er-Mn : 0.5m EDTA-pH7.0
Table 9: The Cu Index System1
Soil Cu IndexSoil Mg Ranges (mg/I)
11 < 1.00
2 1.01- 1.50
3 1.51-3.00
4 >3.00
1 Extractant for Cu 0.5m EDTA- pH 7.0
Table 10: The Zn Index System
Soil Zn IndexSoil Mg Ranges (mg/I)
11 < 1.00
2 1.01- 1.502
3 1.51- 3.00
4 > 3.00

1 Extractant for Zn: 0.5 m EDTA - pH 7.0

2 At pH greater than 7.0, Zn deficiency may be severe when the soil contains less than 1.50 mg/I Zn

Table 11: The Iodine Index System
Soil I IndexSoil I Ranges (mg/kg)
1 0 - 5
2,3,4 Above 5

Boron (B)

The B Index (Table 12) applies only to boron-responsive crops, i.e. swedes, turnips, oil-seed rape, sugar beet, fodder beet, mangolds, all horticultural brassicae, carrots and celery. Boron is toxic to many crops at soil concentrations greater than 3.0 mg/l. Potatoes and legumes are particularly sensitive to high soil B 

Table 12: The B Index System
Soil B IndexSoil B Ranges (mg/kg)
1 <  0.5
2 0.5 - 1.0
3 1.1 - 1.5
4 1.6 - 2.0

All advisory soil samples analysed through Teagasc in 2016 show that 11% of soils had pH, P and K levels in the optimum soil fertility range (pH >6.2 and soil P & K index 3 or 4). This means that 89% of the soils analysed had sub-optimal levels of soil pH, P and K or a combination of all 3. Soil test results indicate that soil P and K levels continue to decline, with 62 and 55% of grassland soils index 1 and 2 for P and K, respectively (for further information see Soil Analysis). There has been a very slight improvement in soil pH over the last number of years (more on tillage soils in 2016).  There is still a large percentage of soils below the optimum pH 6.3 and 6.5 for grassland and tillage soils, respectively. In 2016 results indicate that approximately 63% of grassland and 47% tillage soil samples are below the optimum soil pH targets.

The information is summarised by county for all soil samples and is shown by enterprise within the county, see information below.

Soil Fertility Data 2016


Soil Fertility Data 2015


Soil Fertility Data 2014

How to read and understand your test soil results

“Soil test results are only as good as the soil sample taken.” This is one of the most important steps in attaining reliable information regarding the soil fertility on your farm. Up to date soil test results are unique for the soils on your farm and will have a large influence on the productivity of your soils over the next 4 to 5 years. This information will form the basis to formulating fertiliser / lime advice and decisions regarding fertiliser types and formulations. It is important that soil samples are taken correctly, which includes sampling to the correct sampling depth of 10cm. Now is a good time of the year to take soil samples as the majority of fertiliser P and K is applied in the spring, so taking soil samples now will ensure that you have results back in time. Allow 3 to 6 months between fertiliser P and K applications and taking fresh soil samples.
Soil test results will reveal a lot about the soils on your farm and will help explain why some fields perform better than other fields on the farm. It is also a good exercise to compare old and new soil test results for individual fields to assess the effectiveness of the fertiliser programme on your farm over the last number of years. Recent trends show a decline in national soil fertility levels, so by not soil sampling, you may be missing out on knowing your soil fertility levels.

When soil test results return from the laboratory the results can be quite confusing to interpret, given that results appear that are measured in mg/ l, and then are converted into a soil P and K index. In this article I will look at soil test results and how fertiliser advice is formulated. I will look at a standard Teagasc soil test report and explain how it is put together and what the information means. Figure 1 shows an example soil test report. Each section of the report is given a number, and is explained in turn.

Sections 1 to 3 show the client, advisor and sample information describing the sample.

1&2) Client and Advisor Details

These sections show the names and addresses of the farmer who owns the sample and the farmer’s advisor.

3) Sample Details

This section shows the date when the sample was received and analysed by the soils laboratory. Each sample has a unique code and number so it can identified. The main farm enterprise practiced on the sampled area is required to calculate the nutrient advice shown further down. Field name and the land parcel number will appear here. However, the parcel number does not have to be included in the sample information when sending in the sample. It is best to use field names that you are familiar with as the land parcel number can change over time. On the example report the farming enterprise is beef and the field name is the hill field.

Sections numbered 4 to 6 show the soil test results for the sample

4) Soil pH

The soil pH result can be confusing because 2 different results are shown. The result for pH is the true soil pH result, and this is the result that is most useful. The SMP result is a separate test, and in its own right is no value. The SMP result is used to calculate the lime requirement for the soil sample. Only use the reading for pH as a guide for the pH of the soil. The optimum pH for grassland soil is pH 6.3 and 6.5 for tillage crops.

5) Soil test results

Depending on the soil analysis requested, the lab results for P, K and other elements will be shown. In this case, Mg is also included. The soil test results are used to classify the soils into a soil Index for each nutrient

6) Soil Index

The soil is categorised into a soil Index for each of the nutrients. The basis of this classification for P, K and Mg is shown in Table 1. Soil Index 1 and 2 are considered sub-optimal while Index 4 is considered to be high. Index 3 is considered the optimal level for all nutrients. Note that there are different ranges of soil test results that determine the soil Index for each nutrient. It is also worth noting that the usage of the field as either grassland or tillage can affect the soil P index, but has no effect on the K or Mg Index.

For this example, the soil test showed that P and K were both very low (Index 1), while Mg was high (Index 4).

Table 1:- Soil nutrient Index, response to fertilisers and soil test range for P, K and Mg. (Source: Teagasc)
Soil IndexResponse to
Fertilisers
P (mg L1)
Grassland
P (mg L1)
Tillage
K
(mg L-1)
K
 (mg L-1)
1 Definite 0- 3.0 0-3.0 0-50 0-25
2 Likely 3.1- 5.0 3.1- 6.0 51-100 26-50
3 Unlikely/Tenuous 5.1- 8.0 6.1-10.0 101-150 51-100
4 None > 8.0 >10.0 >150 >100

Sections numbered 7 to 9 show the nutrient advice that is provided based on the soil test results and the information provided with the sample such as stocking rate, crop to be grown, organic manure application and levels of concentrate feeding.

7) Lime requirement

As mentioned earlier, the lime requirement shown in t/ha is the advice for lime application based on the SMP result. Sometimes the result can be shown as ‘xsl’, which means that no lime is required at all.

8) Fertiliser advice

9) Comments on the advice given

Comments are routinely printed in this section to help with interpreting the results and advice given. A comment on reducing lime advice for high molybdenum soils is often printed. This note is automatic if the sample is identified as being form an area where there has been instances of high molybdenum in herbage in the past. High molybdenum can result in copper deficiency in animals. In this example, the lime requirement is 10 t/ha (4 t/acre). Under laboratory comments it shows that this field is from a high molybdenum (Mo) area. The advice for high Mo areas is to deduct 5 t/ha from the lime advice as it is recommended to maintain a soil pH 6.2 in these areas to reduce the lock up of copper for grazing livestock. However, this advice is precautionary, and you should also consider your own experience in deciding on the final lime application rate.

For how long should you use a report?

The P and K advice on the report should be followed for the next 3-5 years, after which another sample should be taken. The lime advice relates to the total lime application to cover a number of years, while the P and K advice are annual application rates.

Cross check with nitrates limits

The advice given on the soil test report should always be cross-checked against an estimate of the total fertiliser allowance for the farm under the nitrates regulations. There are cases where the full P advice may not be permitted on the farm under the regulations, so it is important to check this before you purchase and/or apply the fertiliser.