Green manures are crops grown and then dug into the soil to improve soil health. Examples include mustard and rye. Benefits are preventing erosion, adding organic matter and nutrients, and weed suppression. Limitations include tying up land and potentially becoming weeds. Green manures are used in soil management programs. Soil pH indicates acidity or alkalinity on a scale of 1-14. Plants generally grow best between 6.5-7.5. Soil pH affects nutrient availability and plant selection. Lime can raise pH while sulfur and acidic compost can lower it. pH impacts whether plants receive enough nutrients like calcium and iron.

1. RHS Level 2 Certificate Year 2 Week 6 – Green Manures and Soil pH

2. Learning objectives 5.10 Describe what is meant by the term ‘green manure’ and name TWO plant species that can be grown for this purpose 5.11 State TWO benefits and TWO limitations of using green manures 5.12 Describe the use of green manures in a soil management programme 6. Understand the importance of soil pH. 6.1 Describe the pH scale and state the pH range that normally supports healthy plant growth. 6.2 Explain the effect that soil pH has on plant selection 6.3 Describe what is meant by ‘lime-induced chlorosis’ 6.4 Name TWO plants that require an acid soil. 6.5 State TWO methods by which soil pH can be raised, including the application of garden lime, use of composted green waste and wood ash. 6.6 State TWO methods by which soil pH can be lowered, including the application of Sulphur and use of an acidic form of organic matter. 6.7 Describe the effects of pH on nutrient availability.

3. Green Manures A crop that is grown in order to be dug into the soil before it has reached maturity. Prevents soil from being left bare. Bare soil is at risk of capping and erosion. Rain fall will leach nutrients from bare soil more quickly. Bare soil will also become colonised by weeds.

4. Green manures - benefits Prevents soil capping and erosion by wind and water run off. Boosts organic matter Can provide nutrients Suppresses weeds Improves soil structure Encourages beneficial soil organisms and may suppress some pests.

5. Green manures - limitations Soil is not available for crops Not really suited to ornamental gardens – generally used for vegetable plots Can suppress seed germination after digging in Need to be planned into crop rotation If allowed to seed they can become problem weeds

6. 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 Logarithmic scale – each point is 10 times greater than the one before.

7. pH Testing You could use a test kit like this one – it takes a bit of practice. Or use an electronic meter – but the cheap ones are less reliable than the kit.

8. pH influence on plant choice Calcifuges – cannot grow on alkaline soils. Calcicoles – cannot grow on acidic soils. Some plants do better on soils one side or the other of neutral but will grow on either. Knowing the soil pH and the plant preferences helps to avoid costly mistakes

9. Adjusting soil pH – reducing acidity Acidic soil can be made more alkaline by applying garden lime. Required regularly on intensively cultivated soil as this becomes more acidic over time. Another way to do this is to apply composted green waste or wood ash. However the effect is less predictable than for garden lime.

10. Adjusting soil pH – raising acidity Much harder to do – will not be possible if the soil contains free calcium carbonate. Adding sulphur will change pH over time – but may take a number of years. Alternatively – use composted bracken or recycled peat.

11. Effect of pH on plant nutrients Plant nutrients are chemicals that behave differently depending on pH. This can mean that they form compounds that are not available to plants. Acid soils may have a lack of available calcium and phosphorous. Alkaline soils limit available iron and manganese. These are components of chlorophyll – so affected plants have yellowing leaves.

12. Learning outcomes 5.10 Describe what is meant by the term ‘green manure’ and name TWO plant species that can be grown for this purpose 5.11 State TWO benefits and TWO limitations of using green manures 5.12 Describe the use of green manures in a soil management programme 6. Understand the importance of soil pH. 6.1 Describe the pH scale and state the pH range that normally supports healthy plant growth. 6.2 Explain the effect that soil pH has on plant selection 6.3 Describe what is meant by ‘lime-induced chlorosis’ 6.4 Name TWO plants that require an acid soil. 6.5 State TWO methods by which soil pH can be raised, including the application of garden lime, use of composted green waste and wood ash. 6.6 State TWO methods by which soil pH can be lowered, including the application of Sulphur and use of an acidic form of organic matter. 6.7 Describe the effects of pH on nutrient availability.