pH & pH SMP Buffer
Soil pH is a key variable and should be considered before addressing other soil nutrients . Soil pH is a measure of the acidity or alkalinity of a soil. A soil pH test contain s two tests a water pH test which determines the pH level of the soil and the SMP pH which is required to calculate lime advice.
Crops differ in their sensitivity to soil pH. The optimum use of fertilisers containing N, P and K
is obtained when soil pH is between 6.2 and 7. The pH scale ranges from 0 14. A pH of 7 is neutral which is neither acidic or alkaline. Water pH is the measure of acidity of the soil at present. It measures the acidity of the soil water. A pH of 5.5 is ten times more acidic than a pH of 6.5.
In Ireland the majority of soils naturally have a requirement for lime. Lime is a soil conditioner which corrects soil acidity by neutralising the acids in soils so that micro organisms can thrive, break down plant material and animal residues and frees the elements necessary for plant growth The SMP pH is a measure of the extractable acidity from the acid reserve in the soil and is an indicator of the lime requirement to achieve optimum pH for soil fertility.
See below table outlining t he optimum pH for grassland and tillage.
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Morgans Extractable P,K & Mg
Advice on major nutrient application rates depends on the quantity of that nutrient in the soil and available to the crop. The Morgan’s Extractable is the standard test used for providing nutrient management advice in Ireland for P, K and Mg.
The soil test result can be used to identify the index of your soil for P, K and Mg. Soils are classified into Index levels 1 4.
The indices are computed as follows:
To be defined as a peat, there must be no mineral soil in the upper 10 cm throughout the sampled area.
The exact interpretation depends on the element and the crop. The ranges also differ depending on whether the soil is mineral or peat. To minimise possible losses of nutrients to the environment, the regulations require peat soils to be capped at Index 3. A peat s oil contains more than 20% organic matter.
Total C, Total N & Total Organic Carbon (TOC)
Total C is the total amount of carbon in the soil . Total organic carbon is the total amount of organic carbon in the soil i.e. organic carbon sequestered in the soil . The usually small difference between Total C and TOC is the Carbon from non organic sources Both are measured as a percentage . A value below 2% for Total C indicates a loss of soil function and vulnerability to degradation in the context of climate change. Total N is also measured as a percentage and is used in calculating the C:N
Ratio.
Cropland soils generally store less organic carbon than grassland because cropland has greater disturbance from cultivation, a lack of organic manure being returned to the system, has a winter fallow period and has less plant root and shoot material returned to the soil.
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Total Organic matter can be used to calculate the % Soil Organic Matter TOC % X 1. 724 (Bemmelen Factor)
Soil organic matter (SOM) forms due to the decomposition of plant and animal residues that enter the soil system. These inputs include leaf and straw residues, root material, soil biota and any applied animal manure. Soil organic carbon (SOC) is the main constituent of SOM, accounting for 50% of the total, and sequestration of carbon (C) in soils plays a vital role in removing C02 from the atmosphere.
The role of SOM in enhancing plant productivity can be classified into three broad categories: biological, physical, and chemical. SOM contributes soil nutrient retention by increasing the cation exchange capacity (CEC), which determines a soil’s ability to retain positively charged plant nutrients. Thus, SOM can act like a slow release fertiliser . 3.4% is t he soil organic matter threshold value. Soils above 3.4% have adequate levels of SOM, soils below 3.4% require some remedial action to be taken. Contact a Farm Advisory System (FAS) advisor for guidance.
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C:N Ratio
The C:N ratio is a quick way to evaluate the balance between two elements present in the soil that are both essential for crop growth and microbial health. The average C:N ratios for Grassland is 9 12 :1 and for Tillage is 10 14 :1 . Higher than average ratios
indicates the stability of organic matter for long term carbon storage.
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MEHLICH III (B, Al, Fe, Mn, Cu, Zn, Mo, P, S Se, Co)
The Mehlich III is an alternative test to Morgans which give a more complete picture of the soil fertility status across a broad range on macro and micronutrients and will play a greater role in major and minor nutrient advice in the future.
The nutrients which can be analysed under the Mehlich extraction are:
- Boron (B)
- Aluminium (Al)
- Manganese (Mn)
- Copper (Cu)
- Zinc (Zn)
- Molybdenum (Mo)
- Phosphorus (P)
- Sulphur (S)
- Selenium (Se)
- Cobal t (Co)
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The test provides an indicator of the amount of nutrient available to plants and also potential antagonisms between nutrients. In particular, the Aluminium:Phosphorous ( Al:P ) ratio provided by this test gives an indication of the potential lockup of available P. Where the ratio of Al:P exceeds 11.7 there is a likelihood that applied P will quickly become unavailable for plant uptake.
In time, Mehlich is a test which can provide valuable information on major and micronutrients which play a key role in animal and crop production. At present t he Mehlich values are not comparable with the national guidance index system (see Green B ook, SI) and further research is required to fully utilise the potential of this test.
Total Cation Exchange Capacity (CEC)
The Cation Exchange Capacity (CEC) indicates the capacity of the soil to hold positively charged ions such as K, Mg, C a, N a, in an exchangeable form. It indicates the capacity of a soil to store and resupply nutrients to plants over the growing season It is a key soil property which influences soil structure, nutrient availability, soil pH and the soils reactions to fertilisers. It also reflects the capacity of the soil to retain and not leach nutrients.
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CEC refers to the exchange capacity for the four major cations: Magnesium (Mg), Potassium (K), Calcium (Ca) and Sodium (Na
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CEC is an inherent and stable property of soil and is hard to alter
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Sandy soils have a low CEC
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Soils with higher clay content will tend to have a higher value
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Organic Matter has a very high CEC
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What does the result tell me about my soil?
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A low CEC (0-10 meq/100g) would suggest that the soil has a poor binding capacity. Typically, sandy soils with little clay or organic matter. This soil would be prone to leaching.
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A high CEC (30-40 meq/100g) would suggest that the soil has a strong binding capacity, typically seen in heavy soils with high clay or organic matter content.
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Leaching may be observed with soils of a CEC up to and including 15/16 meq/100g. CEC value higher than 16 meq/100g should have considerate clay content.
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An ideal soil has a CEC between 18-27 meq/100g.​
What can I do to improve my soil’s Cation Exchange Capacity?
There are two main ways to improve your soil’s CEC. Indeed, both solutions will improve the overall health of your soil.
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Adding lime to low pH soil can help.
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Adding organic matter to the soil. Organic matter has a substantially greater CEC compared to clays.
Therefore, adding organic matter to soil will increase your CEC and improve the structure, improving all aspects of soil health.
Simply adding nutrients to optimize the base saturation can be futile if the soil is prone to leaching. In cases where the soil has an extremely low or high Cation Exchange Capacity, i.e. 0 or 40 meq/100g, little can be done to optimise growth without completely changing the structure of the soil.
Base Cations Saturation and Ratios
Base cation saturation is the % of the total CEC occupied by base cations (Ca++, Mg++, K+, Na+) and can be calculated from the test results provided.
Base saturation % is equal to (Sum of base cations divided by the CEC) x 100.
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The data collected from the CEC can be manipulated to determine a ‘base saturation’. Base saturation expresses the percentage of potential CEC occupied by the cations Ca2+, Mg2+, K+ or Na+.
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The optimum base saturation % is:
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Na <10%
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K 2-7%
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Mg 15-20%
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Ca 65-75%
The ratio of Calcium to magnesium is critical for plant nutrient uptake. As a rule of thumb, the Ca to Mg ratio should be 2:1.
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Extended-Spectrum Beta-Lactamase producing Escherichia coli (ESBL – E.coli)
Microbes adapt and change over time and develop resistance to medicines. This makes the successful treatment of diseases and infections harder over time. This is known as antimicrobial resistance (AMR) and poses a significant threat to human and animal health.
ESBL-E.coli is a type of bacteria which is used to monitor resistance across the One Health sectors (human, animal, plant and environment sectors). ESBL samples taken under the national Soil Sampling and Analysis Programme facilitates better monitoring of AMR throughout Ireland.
The ESBL sample taken for your farm is a composite sample, including soil from all plots tested and does not come from an individual LPIS parcel. If your farm shows a positive ESBL reading and you wish to contact DAFM, please phone 01 505 8600 and ask for the Antimicrobial Resistance Section who will assist you.
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For further information on Antimicrobial Resistance and Ireland’s action plan please visit: www.gov.ie/amr
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Disclaimer.
When using the data sup plied to make nutrient management decisions please consult with a professional advisor.
For a comprehensive guide to crop nutrient management
https://www.teagasc.ie/news--events/news/2020/revisedteagascgreenbook.php
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