Concentration (Potential of Hydrogen ions) pH Concentration (Potential of Hydrogen ions)

Learning outcomes What makes soil acid or alkaline Explain the pH scale Impact of nutrient availability on plants

Acid and alkaline soils Soils are classified as acid, neutral or alkaline. This factor has an impact on what plants can be grown

What makes a soil acid or alkaline It is the number of hydrogen ions in the soil that determines its classification. The greater number of hydrogen ions the more acid is in the soil.

pH scale This measures the acidity or alkalinity on a scale of 1 to 14. A measurement of 7 is considered neutral: below 7 is acid and above alkaline. The neutral value of 7 is based on a measurement of hydrogen ions in pure water.

Determining the pH value There are a number of soil testing kits that will provide a pH value. The simplest entails a small sample of soil, placing it in a test tube with some water and a tablet that comes with the kit. The water will turn a different colour depending on the ph and this colour can be matched against a chart to give the pH value. There are also meters available that come with a probe that is pushed in the soil but these have not proved to be accurate. Soils in the UK range from a pH of around 4 up to 8.

Equipment

Impact on ph on nutrient availibilty Nutrients restricted in acid soils: Calcium Phosphorus Molybedenum Nutrients restricted in alkaline soils: Iron Manganese Boron

Impact of pH on plants Most plants will grow best in a pH of 6-7, although many will tolerate a wider band. Plants prefer that prefer an acid soil are called calcifuges and plants that prefer an alkaline soil are called calcicoles. In your group using the Learner Resource Centre- list as many as you can of calcicole and calcifuge plants (10mins )

Calcifuges Rhododendron Camelia Erica Magnoloia

Calcicoles Clematis Dainthus Scabious Viburnum It is often possible to gauge the ph of the soil by looking at the plants that are thriving in it

Student activity Look at the pictures and discuss in your group what makes the flowers on the Hydrangea a different colour.

It is much easier to change a hydrangea from pink to blue than it is from blue to pink. Changing a hydrangea from pink to blue entails adding aluminum to the soil. Changing from blue to pink means subtracting aluminum from the soil or taking it out of reach of the hydrangea. That is why it is easier if the plants are planted into containers rather than in the ground.

Changing the pH value Over a period of time soil tends to become more acid because: Alkaline nutrients like calcium are leached from the soil Alkaline nutrients like calcium are used from the plant and removed with it Micro-organism activity releases acids Rainfall tends to be slightly acid Some fertilisers are acid

Cont.... Lime (calcium carbonate/ground limestone) is used to raise the pH of the soil Sulphur is used to reduce the pH of the soil

Conclusion In your group why do you think it is so important to know what type of soil the plants are growing in.