RHS Level 2CertificateYear 2 Week 9 – Review of soil waterand of nutrients and pH.
Learning objectives To state the mechanism of CEC To define pH correctly To identify the effect of pH on nutrient availability for specific nutrients To state TWO mechanisms by which pH may adversely affect nutrient availability To describe TWO mechanisms by which excess of certain nutrients may make others unavailable To apply this knowledge to the diagnosis of plant problems
Plant nutrient review - macronutrients Nitrogen – used by plants in two forms. Ammonium (cation) and nitrogen (anion). Vital for green, leafy growth – deficiency symptom yellowing of older leaves first. Phosphorous – vital for root development and germination. Deficiency symptoms – stunted growth and bluish/purple tinged leaves. Potassium – vital for fruiting and flowering and hardiness. Deficiency symptom – brown, often curling edges to leaves, poor flowering and fruiting.
Plant nutrient review – key micronutrients Iron – vital component of chlorophyll. Deficiency symptoms – interveinal chlorosis, younger leaves. Calcium – vital component of cell walls. Deficiency symptom – ‘bitter pit’ in apples, ‘blossom end rot’ in tomatoes, black spots on leaves.
Review Question answers1. Because nutrient presence in the soil may not mean it is available to the plant or that the plant can take it up. A leaf test may also be required.2. Nitrogen – older leaves first because the plant can move nitrogen from plant tissues in the event of deficiency.3. To the amounts needed by the plant not to the importance of the nutrients.
CEC - Terms used Ion – an atom (single elemental particle) or molecule (combination of more than one elemental particles) in which the number of protons and electrons are not equal – therefore has an electrical charge. Cation – an ion with a positive charge. Anion – an ion with a negative charge. Remember – opposite charges attract. Like charges repel.
Cation Exchange Capacity Clay and humus particles have a negative surface charge. Certain nutrient (and other) ions found in soil water have a positive charge (cations). Negative charges attract positive charges, so holding these ions in the soil. Which cations are held depends on their relative concentrations in the soil water.
What is cation exchange? The ability of one cation to displace another from the negative charge site on the clay or humus particle. This means that the nutrient cations can become available to the plant as they are displaced. Exchange sites therefore act as a reserve for cations of nutrients in the soil.
Soil pH A measure of how acid or alkaline a soil is. Technically a measure of concentration of hydrogen ions. Scale runs 1 – 14 where 1 is very acid, 14 very alkaline Plants grow best (with some exceptions) in the range 6.5 -7.5
Soil pH scale Logarithmic scale – each point is 10 times greater than the one before. So pH 7 is 0 (balanced), pH 6 is ten times more acidic, pH 5 is 100 times more acidic (10 x 10), pH 4 is 1000 times more acidic (10 x 100) etc. Alkaline soils are less acidic so pH 8 is 10 times less acidic than pH7 etc.
Soil pH – link to cation exchange mechanism Cations fall into two types – acidic and basic (or alkaline). H+1 (hydrogen) and Al+3 (aluminium) are acidic; Ca+2 (Calcium), Mg+2 (Magnesium) and K+1 (Potassium) are alkaline or ‘basic’ (meaning that they have acid neutralising capacity). So the more acidic the soil the more H+1 ions will be either free in the soil water or attached to the negative charge points on clay or humus soil particles. This displaces the nutrient cations that can then leach from the soil.
Plant nutrients and pH So high soil acidity can lead to soils being unable to hold alkaline (basic) nutrient cations. This means that these are subject to leaching and may become deficient. Soil pH can also affect the way that plant nutrients form chemical compounds with other elements. If these compounds are less soluble in soil water then the nutrients may become unavailable to the plants. This is called Immobilisation
Plant nutrient insolubility For example: Phosphorous forms an insoluble compound with Iron at low pH and with Calcium at high pH. Excessive application of calcium will lead to an insoluble compound being formed with potassium. Another possible cause of deficiency in the plant is ion antagonism – that is high concentration of one ion (like calcium) may block the root surface membrane and prevent it from taking in another (such as magnesium).
pH Worksheet answers1. The concentration of hydrogen ions in soil solution2. More alkaline (the higher the number above 7 the more alkaline it is)3. Positively charged (cations)4. 1000 (7=0, 6=10 times, 5= 100 times, 4 = 1000 times)5. Hydrogen ions will be exchanged for calcium ions on the surface of clay or humus particles. The calcium is displaced into the soil water .
pH worksheet answers6. Iron7. As the level of calcium (lime) in the soil itself is normal it is unlikely to be low pH. Either potassium (forms insoluble compound) or magnesium (ion antagonism) is in excess . Nutrient tests should diagnose8. Nitrogen deficiency. Either the soil lacks nitrogen because no N fertilizer or organic matter has been applied for some time or the soil pH is below about 5 or above 8.5. To tell which it is carry out a soil nutrient test for N and a pH test.
Learning Outcomes To state the mechanism of CEC To define pH correctly To identify the effect of pH on nutrient availability for specific nutrients To state TWO mechanisms by which pH may adversely affect nutrient availability To describe TWO mechanisms by which excess of certain nutrients may make others unavailable To apply this knowledge to the diagnosis of plant problems