1. Leaving Certificate Agricultural Science
Chemical Properties & Fertility of Soil
H. Jones, St. Columba’s College

2. Learning Outcomes 1
Define cation exchange and describe its significance.
Define cation exchange capacity.
Test a sample of soil for pH.
Describe how liming can raise the pH of soil.
Describe the requirements for ground limestone in Ireland.
Describe how the pH of soil affects the availability fp nutrients in a soil.
Distinguish between macro and micro nutrients.

3. Cation Exchange Capacity (CEC)
Cations are positive ions in soil e.g. H+, Ca+, Mg+, Al+
These cations are held in the soil (adsorbed) by soil colloids – the smallest clay or humus particles.
Soil colloids are said to have high cation exchance capacity.
Cation exchange is the ability of soil particles to attract, retain and release cations.
Cation exchange capacity is a measurement of the quantity of cations a soil can adsorb.
Negative charges on colloids comes from chemical weathering.
Important feature of CEC is that it is greatest on smallest particles and it holds onto soil nutrients.

5. Soil pH
Soil pH is the negative log of the hydrogen ion concentration in the soil solution. In other words the more H+ the lower the pH.
The pH of Irish soils generally range from 5.0 to 7.5 The optimum soil pH is between 6 and 7.
Al3+ and H+ cause acid soils and these occur where the soil is weathered from granite or sandstone or where there is acid in the soil.
Ca++ and Mg++ cause neutral or basic soils and these occur where the soil is weathered from limestone soils or where the farmer spreads lime.

7. Soil pH & Mineral Nutrients
The pH of the soil influences the availability of elements (both major and minor) in the soil.
Most elements are at maximum availability at pH 6 – 7 except for some minor elements.
This is the ideal pH and therefore should be the aim for good soil management.
Liming is an essential farming operation due to acid leaching.

9. Soil pH and Liming - 2
Liming is carried out every 5 – 10 years, depending on the soil and rainfall and evaporation amounts.
Liming raises the pH by replacing H+ ions (acid ions) with Ca++ ions.
The H+ ions are then leached.
It takes up to two years for the liming to take full effect.
Ground limestone is the main source of lime in IRL

10. Ground Limestone Requirements
Ground limestone must meet the following guidelines:
Should have a TNV (Total Neutralising Value) of not less than 90%
All particles must be less than 3.35mm
At least 35% must pass through a 0.15mm sieve.
Must have more than 3.0% water.

12. Soil Fertility
All plants require certain elements and minerals to carry out functions in the plant.
These minerals allow for proper growth of the plant.
A lack of the minerals results in a deficiency disease or stunted growth.
An essential element is defined as “a chemical element required for the normal growth of the plant”.

13. Learning Outcomes 2
Distinguish between macro and micro nutrients, giving examples of each.
Briefly describe the metabolic use of macronutrients in crops.
Describe the nitrogen and carbon cycles.
Describe how a soil is sampled and analysed.
Describe investigations to test for the presence of N,P and K in a soil sample.

14. Plant Nutrients
There are 17 essential elements, but Carbon, Hydrogen and Oxygen making up most of plant tissue (95%)
These elements however are taken from the air and are rarely responsible for limiting crop growth.
The other 14 are taken up from the soil and are more likely to restrict growth if lacking.
Six of these elements are required in large amounts and they are called macro elements.
The other eight are called micro elements and are only required in small amounts.

15. Macro Elements (Soil)
Macro Elements (Air)
Micro Elements (Soil)
Nitrogen
Carbon
Iron
Phosphorous
Hydrogen
Manganese
Potassium
Oxygen
Boron
Calcium
Molybdenum
Magnesium
Copper
Sulphur
Zinc
Chlorine
Cobalt

16. The Major Elements
The major elements occur in the soil in two major forms:
Complex, insoluble forms and
Simple, soluble forms.
The conversion of each element from its complex form into the simple form is very important.
The complex and simple forms of each elements are on the next slide:

17. Major Element Complex Form Simpler Forms
Nitrogen (N)
Organic: (Proteins, amino acids etc)
Ammonium Salts (NH4+)
Nitrite Forms (NO2-)
Nitrate Forms (NO3-)
Phosphorous (P)
Inorganic: Rock minerals, and phosphates
Organic: Nucleic Acids
Phosphates of Calcium, magnesium and potassium
(HPO42- or H2PO4-)
Potassium (K)
Inorganic: Rock minerals e.g. feldspar, micas, silicates
Potassium ions on soil colloids (K+)
Calcium (Ca)
Inorganic: Rock minerals e.g. feldspar, calcites etc
Calcium ions on soil colloids (Ca2+)
Magnesium (Mg)
Inorganic: Rock Minerals as above and clays
Magnesium ions (Mg2+)
Sulphur (S)
Inorganic: Rock minerals
Organic: Proteins and amino acids
Sulphites and sulphates of calcium and magnesium
(SO32- and SO4-)

18. Nitrogen
Nitrogen is an important component of plant proteins, chlorophyll and nucleic acids.
Therefore it is vital for growth, photosynthesis and cell reproduction.
It is the most important and widely used fertiliser element.
A deficiency in N causes sharp reductions in crop yields.

19. Nitrogen 2
When N is abundant in soil, the crop grows quickly, is dark green in colour and fruits and seeds have high protein levels.
When N is short, growth is restricted and the plant goes yellow.
The ways in which N is supplied, used and reused is outlined in the Nitrogen Cycle:

21. Important Agricultural Stages of the Nitrogen Cycle
Fertiliser Application
Organic and Inorganic fertilisers contain N in Urea, Ammonium and Nitrate forms.
The Urea is generally converted into Nitrates in good soils.
Some of the fertiliser materials are converted in proteins and are immobilised, not available to plants.

22. Important Agricultural Stages of the Nitrogen Cycle - 2
Nitrogen Fixation
Nitrogen fixation is the process of changing atmospheric nitrogen into other, simpler forms (e.g. NH4+) usually by bacteria
The principle bacteria are called Rhizobium.
These bacteria are also found on root nodules on legumes (e.g. Clover)
The bacteria have a symbiotic relationship with the plants, meaning each get what they want.
The Rhizobium provide N for the plant, while the plant provide food.
This is why clover is of huge importance in pasture.

23. Important Agricultural Stages of the Nitrogen Cycle - 3
Organic Matter Breakdown
When organic matter or manure is applied to soil, the complex forms of N in the material are converted in NH4+.
Nitrification
Nitrification is the conversion of NH4+ (Ammonium Salts) into Nitrates using O2.
The result of Nitrification is that NH4+ ions (which cannot be absorbed by plants) is made available as NO3-, which can be.

24. Important Agricultural Stages of the Nitrogen Cycle - 4
Plant Uptake
The uptake of NO3- by plants completes the cycle – N begins at plants and end at plants.
De-nitrification and Leaching
All parts of the Nitrogen cycle so far are favoured by good, moist, high pH, warm soils with the presence of O2.
But De – Nitrification occurs in wet, anaerobic or waterlogged soils.
It results in a loss of N from the soil and its conversion to N2O or Nitrous Oxide and N2 gas.
Leaching also results in the loss of NO3-, so fertiliser application should be done in spring or summer.

25. Phosphorus
Phosphorus is involved in plant growth and in cell division.
It is the second most critical plant nutrient.
A lack of P results in stunted growth and a bluish colour on the leaves.
Phosphorus in soil exists in a range of complex and simply forms.
Phosphorus ions are released by chemical and biological degradation processes.

26. Phosphorus 2
If the soil has a pH of 5.5 or less, the P reacts with Iron, Manganese and Aluminium compounds and are made almost unavailable to plants.
Above pH 7.5, a similar problem occurs with Calcium, making P unavailable.
This is called phosphorus fixation or immobilisation.
Therefore, to improve P availability, the soil should always be kept between a ph of 6 and 7.

27. Potassium
Potassium performs a number of functions in plants, including disease resistance and carbohydrate translocation.
A lack of K results in withering of the vegetation and a loss of yield.
The availability of K+ is affected by clay mineral composition.
Some clays can trap K+ ions into the inside of the clay colloid, making the ions unavailable for the plant.

28. Potassium - 2
Again, this is known as potassium fixation or immobilisation.
The areas where these soils are found are recorded, and the farmers on this land must apply more K fertiliser.
Potassium is found in most artificial fertilisers.

29. Calcium, Magnesium and Sulphur
These minerals, although major elements, are not applied in fertilisers for two reasons:
The are relatively abundant in soils
They are not subject to immobilisation like N, P and K.
Lime and inorganic fertilisers contain these elements also as part of the active parts.

30. Trace Elements
Trace elements like Fe, Mn, B, Cl etc are involved in enzyme systems of the plant and are therefore important for normal plant function.
Deficiency of the major elements is rare but often occurs with trace elements.
This occurs for two reasons:
Lack of the element in the parent rock
Over liming or high pH
These deficiencies can be rectified by soil applications or foliar sprays.

31. Soil Analysis
Soil must be analysed to measure its fertility and the fertiliser requirements
It involves three steps:
Soil Sampling
Soil must be sampled in various areas of the field considering different coloured areas & sloped areas.
Consider the past history of the field
Take a minimum of 25 samples, usually in a W shape throughout the field are taken at root level.
A composite sample is made by mixing all of these samples.

32. Soil Analysis - 2
Soil analysis generally means determining lime requirements, available P and available K.
Available N is hard to measure so is not done generally
Most testing is carried out by Teagasc, in their soil testing lab at Johnstown Castle, Co Wexford.
The samples are dried and sieved and treated with Morgan’s extracting solution and another solution for P & K.
The amount of P and K is measured in ppm (parts per million)

33. Soil Analysis - 3
Lime requirements are expressed in tonnes per hectare.
Morgan’s extracting solution mimics a growing plant and takes out P & K like a plant would.
The results of the soil analysis are compared to previous reference tests carried out by Teagasc.
This allows the technician to give guidelines for the farmer on P and K application and liming requirements.

34. Finding N, P and K in Soil
Certain chemicals are used to analyse soils and determine the presence of N, P and K
Test for
Reagent
Positive result
Nitrates (NO3-)
Diphenylamine
Blue colour
Phosphates (HPO42-)
Ammonium Molybdate
Yellow precipitate
Sulphates (SO42-)
Barium Chloride
White cloudiness
Chlorides (Cl-)
Silver Nitrate
White precipitate