LO 4.1 - pH and its effects on nutrient availability and soil borne diseases

Question 1

Question

What does pH stand for?

pH

Answer 1

pH (power of Hydrogen or potential of Hydrogen)

It is a measure of the acidity or alkalinity of a solution.

pH

Question 2

Question

What does the pH scale from and to?

pH

Answer 2

The pH scale ranges from 0 (most acidic) to 14 (least acidic or most alkaline).

pH

Question 3

Question

What is neutral on the pH scale?

pH

Answer 3

7 on the pH scale is neutral – neither acidic nor alkaline.

pH

Question 4

Question

The pH scale is a negative logarithmic scale.

What does that mean?

pH

Answer 4

• The more Hydrogen ions there are in a solution, the pH is lower.
• Each point on the scale is 10 times more acidic than the point above it.

pH

Question 5

Question

Why is pH important in soils?

pH

Answer 5

• Nutrient availability
• Functioning of soil organisms

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Question 6

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How does pH relate to nutrient availability in soil?

pH

Answer 6

Different plant nutrients become more or less available to plants at different pH levels.

pH

Question 7

Question

Different plant nutrients become more or less available
to plants at different pH levels. Give examples

pH

Answer 7

• Soils with a low pH may contain soluble aluminium and manganese at levels that are toxic to many plants.
• Calcium, magnesium, phosphorus and molybdenum become unavailable at low pH leading to deficiencies in plants.
• Acid loving (low pH) plants (eg Rhododendrons, Azaleas) have a high requirement for iron, so these plants do better in acid soils in which iron is more available.

pH

Question 8

Question

What are some problems caused by high pH?

pH

Answer 8

• Soils with a high pH may create problems for plants as elements such as boron, iron and manganese become less available.
• These problems can arise if too much lime is added to agricultural land.

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Question 9

Question

What is the ideal pH for most plants?

pH

Answer 9

• The ideal pH for most plants is somewhere between 5.5 and 7.5, with the majority preferring the middle part of this range.

pH

Question 10

Question

What pH range are most nutrients available for plant growth?

pH

Answer 10

• In mineral soils, most nutrients required for plant growth are available in the pH range 6.5 to 6.8

pH

Question 11

Question

What pH range is optimum in soil-less soil like peat or coir?

pH

Answer 11

• In soil-less composts or peat soils, the optimum pH is lower: around 5.5 to 5.

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Question 12

Question

Many soil fungi and bacteria do not function well at pH levels below about 5.5. Why is this?

pH

Answer 12

• The restriction in their functioning slows down breakdown of OM and release of nutrients (particularly nitrogen, phosphorus and sulphur) back into soil. It also leads to a build-up of undecomposed organic matter in the soil.
• Nitrogen-fixing bacteria do not function well at low pH, so most plants that can fix nitrogen prefer a neutral to alkaline soil.

pH

Question 13

Question

What pH level do earthworms prefer?

pH

Answer 13

• Earthworms prefer neutral or near-neutral conditions, so their activity is limited in acid soils.

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Question 14

Question

The activities of harmful organisms are also affected by pH. What are some examples?

Give two, one high and one low.

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Answer 14

• The soils in coniferous nurseries are often kept below 5.5 to discourage fungal rots and damping-off.
• Clubroot of brassicas** prefers acid conditions**, so these soils are limed to keep the pH high.

pH