Maintaining Optimum Soil Fertility
A review of soil sample results over 2017 and 2018 analyzed by Teagasc indicates that soil fertility levels on Irish farms may be turning a corner with some positive signs of overall improvement. Soil fertility had been in decline since the mid-noughties, linked closely with lower lime and lower compound fertilizer use, and had reached very low status between 2013 to 2015 with just 10% of soil samples showing good overall fertility in terms of pH (>6.2), P and K (≥ index 3) status. Over the last decade in particular, a worrying trend of continuous mining of the native fertility of some soils may have eroded their grass and crop production potential. Limiting their ability to maximise grass as our main fodder source and to maximize the yield potential from new cereal varieties. However, the Teagasc soils data base now indicates large improvements in soil pH levels and early signs of improvements in both soil P and K levels on farms, although the rates of these improvements are enterprise specific.
In 2017 and 2018 a total of c.90,000 soil samples were analyzed by Teagasc and these large data base of soil analysis results have been shown to generally reflect national soil fertility trends. Over this period 49% of soil samples came from dairy farms, 44% from drystock farms and 7% from tillage farms. Notably, in 2018 the number of soil samples taken on dairy farms increased (by 29%) compared to the previous 5 year average.
Across all farm enterprises the only soil fertility indicator showing significant positive signs of improvement was soil pH. Increased research and advisory emphasis on the importance and benefits of lime application to our naturally acidic soils since 2013 has helped to raise awareness amongst farmers. This is reflected by current national lime use (approx. 1 million tonnes) which had increased by on average 211,000 tonnes per year since 2013 compared to the previous 5 years. The optimum soil pH for grassland mineral soils is ≥6.3 and in 2014-16 on average 37% of soils tested were in this range whereas in 2017-18 on average 54% of soils had optimum pH levels. When soils from tillage farms were examined separately the improvements in soil pH were greater with up to 83% of samples having optimum soil pH in 207-18. This large improvement in soil pH will have significant positive effects on nutrient uptake efficiency from applied fertilizer and organic manures and also on the longevity of reseeded grassland swards and in particular the maintenance of clover.
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.
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.
- To take a soil sample it is essential to have a suitable soil corer
- Ensure soil cores are taken to the correct sampling depth of 100 mm (4”)
- Take a soil sample every 2 to 4 ha. (5-10 acres)
- Take separate samples from areas that are different in soil type, previous cropping history, slope, drainage or persistent poor yields
- 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.
- 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.
- 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.
- Take a representative soil sample by walking in a W shaped pattern across the sampling area.
- 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.
- 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
In this short clip, Mark Plunkett, Soil & Nutrition Specialist with Teagasc runs through why its so important to be testing your soil and what best way to do it.
Testing Equipment Contact Information
Soil Sampling Equipment
- Funnel type sampler Cost €80 plus VAT
- Tillage Soil sampler Cost €40 plus VAT
|Analysis Service Number||Elements Tested||Total Charge inc. VAT €|
Basic or REPS
|LR, pH, P, K||€25.00|
|LR, pH, P, K, Mg, Co and Total Mn||€36.00|
Sheep - Tillage
|LR, pH, P, K, Mg, Co, Cu, Zn, easily reducible Mn and Total Mn||€42.00|
|LR, pH, P, K, Mg, Cu, Zn and easily reducible Mn||€36.00|
|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 organic matter %||€22.00|
(S6 + Organic Matter)
Tillage / Grassland / OM
|LR, pH, P, K, Mg and soil organic matter %||€36.00|
(S4 + Organic Matter)
Tillage / OM
|LR, pH, P, K, Mg, Cu, Zn, easily reducible Mn and soil organic matter %||€45.00|
(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 Index||Index Description||Response to Fertilisers|
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.|
|Index 1||Index 2||Index 3||Index 4|
(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|
|Index 1||Index 2||Index 3||Index 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.|
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 Index||Grassland crops||Other 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 Index||Mineral Soil||Peat *|
|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|
|Soil Mg Ranges (mg/I)|
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 Index||Soil Co Ranges (mg/kg)|
|1||0 - 3.0|
|3||5.1 - 10.0|
|Table 8: The Er-Mn Index System|
|Soil Er-Mn Index||Soil Er-Mn Ranges(mg/l)1|
|1Extrantant for Er-Mn : 0.5m EDTA-pH7.0|
|Table 9: The Cu Index System1|
|Soil Cu Index||Soil Mg Ranges (mg/I)|
|1 Extractant for Cu 0.5m EDTA- pH 7.0|
|Table 10: The Zn Index System|
|Soil Zn Index||Soil Mg Ranges (mg/I)|
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 Index||Soil I Ranges (mg/kg)|
|1||0 - 5|
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 Index||Soil B Ranges (mg/kg)|
|2||0.5 - 1.0|
|3||1.1 - 1.5|
|4||1.6 - 2.0|
In 2018 Teagasc analysed a total 45,157 soil samples comprising of dairy, drystock and tillage enterprises. 42,067 soil samples were from grassland farms with 21,676 from dairy farms and 19,801 soil samples from dairy farms. The following is a summary of the main changes for soil pH, phosphorus (P) and potassium (K) in 2018.
- 59% of soils with a soil pH >6.2
- 56%of soils at P index 1 & 2 (4% Decrease)
- 48% of soils at K Index 1 & 2 (11% Decrease)
- 43% of soils with a soil pH >6.2
- 63%of soils at P index 1 & 2 (4% Decrease)
- 52% of soils at K Index 1 & 2 (10% Decrease)
Soil pH levels improvements have remained relatively stable in 2018 (see figure 1) with 54% of soils in the optimum range for soil pH on grassland soils.
Figure 1. Proportion of soils analysed which fall into each soil pH range across all farm types. Based on total of 223,000 soil samples analyzed by Teagasc between 2014 and 2018.
Soil P levels show small improvements on both dairy and drystock farms with a 4% decline in soil testing at soil P index 1 & 2. Soil K levels show significant improvements of between 10 and 11% in 2018. This maybe a reflection of the targeted application of organic fertilisers plus the application of straight potassium as muriate of potash (50% K).
Overall all farm enterprises have made progress in relation to soil fertility over the last couple of years. Soil fertility on dairy farms showed that approximately 18% of soil samples with the optimum mix of pH, P and K for grass production. Drystock farms showed that 11% of samples with good overall soil fertility. These poorer results on drystock farms may be influenced by a number of factors such as lack of profitability, lower fertilizer use, lower feed demand and need to maximise grass production especially where stocking rates are low. While the majority of these overall soil fertility improvement has resulted from positive changes in soil pH across all farming enterprises there are also indications that soil K, and possibly P, are also improving.
Soil Fertility Data 2018
County Specific Data:
Carlow 2018- Cavan 2018- Clare 2018- Cork 2018- Donegal 2018- Dublin 2018- Galway 2018- Kerry 2018 - Kildare 2018- Kilkenny 2018- Laois 2018- Leitrim 2018 - Limerick 2018- Longford 2018 - Louth 2018- Mayo 2018-Meath 2018- Monaghan 2018- Roscommon 2018- Sligo 2018-Tipperary 2018- Waterford 2018-Westmeath 2018- Wexford 2018
Soil Fertility Data 2017
County Specific Data:
Carlow 2017 - Cavan 2017 - Clare 2017 - Cork 2017 - Donegal 2017 - Dublin 2017 - Galway 2017- Kerry 2017 - Kildare 2017 - Kilkenny 2017 - Laois 2017- Leitrim 2017 - Limerick 2017 - Longford 2017- Louth 2017- Mayo 2017 - Meath 2017- Monaghan 2017 - Roscommon 2017 - Sligo 2017- Tipperary 2017 - Waterford 2017 - Westmeath 2017- Wexford 2017 -
Soil Fertility Data 2016
County Specific Data:
Carlow 2016 - Cavan 2016 - Clare 2016 - Cork 2016 - Donegal 2016 - Dublin 2016 - Galway 2016 - Kerry 2016 - Kildare 2016 - Kilkenny 2016 - Laois 2016 - Leitrim 2016 - Limerick 2016 - Longford 2016 - Louth 2016 - Mayo 2016 - Meath 2016 - Monaghan 2016 - Offaly 2016 - Wicklow 2016
Soil Fertility Data 2015
County Specific Data:
Carlow2015 - Cavan2015 - Clare2015 - Cork2015 - Donegal2015 - Dublin2015 - Galway2015 - Kerry2015 - Kildare2015 - Kilkenny2015 - Laois2015 - Leitrim2015 - Limerick2015 - Longford2015 - Louth2015 - Mayo2015 - Meath2015 - Monaghan2015 - Offaly2015 - Roscommon2015 - Sligo2015 - Tipperary2015 - Waterford2015 - Westmeath2015 - Wicklow2015 - Wexford2015
Soil Fertility Data 2014
County Specific Data:
Carlow - Cavan - Clare - Cork - Donegal - Dublin - Galway - Kerry - Kildare - Kilkenny - Laois - Leitrim - Limerick - Longford - Louth - Mayo - Meath - Monaghan - Offaly - Roscommon - Sligo - Tipperary - Waterford - Westmeath - Wicklow - Wexford
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 Index||Response to |
|P (mg L1)|
|P (mg L1)|
|2||Likely||3.1- 5.0||3.1- 6.0||51-100||26-50|
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.
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.
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.