Soil Modification for Growers

Acidifying Soil

1. Acidifying soil is often more complex and slower than raising soil pH through liming. Here are some methods to lower soil pH:

2. Sphagnum Peat: Once a common practice, the use of sphagnum peat is now discouraged due to environmental concerns. Alternative organic materials may also be used to acidify soil.

3. Sulphur: This is a cost-effective method, though it works slowly.

4. Aluminium Sulphate: This is effective but costly.

5. Iron Sulphate: In addition to acidifying the soil, it helps correct iron deficiencies.

6. Nitrogenous Fertilisers: For example, ammonium sulphate can also contribute to soil acidification.

pH Ranges for Selected Trees and Shrubs:

• 4.5-5.5: Rhododendron, Calluna, Erica, Picea, Sciadopitys

• 5.5-6.5: Acer, Quercus, Betula

• 6.5-7.0: Syringa vulgaris, Sorbus, Salix

Forms of lime

1. Calcium Carbonate (CaCO3): Also known as carbonate of lime, it is available in several forms:

2. Ground Limestone: Produced by crushing, drying, and grinding limestone. It should pass through a 100-mesh sieve and is suitable for all soil types, especially those rich in organic matter.

3. Limestone Dust: An impure form of calcium carbonate with both lumps and fine powder. It dissolves quickly and provides a long-term effect.

4. Chalk: Requires drying before grinding. It is more expensive than ground limestone but acts faster. It is suitable for sandy soils and light loams and is often used in seed and potting compost.

5. Magnesium Limestone (CaMg(CO3)2): Provides both calcium and magnesium. Commonly used in nursery stock composts.

6. Hydrated Lime (Ca(OH)2): Obtained from slaked calcium oxide, it is a finely divided powder with a very high pH of 12. More expensive but highly effective.

Signs of Lime Deficiency

1. Presence of weeds such as Sheep's Sorrel, Corn Spurry, Woodrushes, and Corn Marigold.

2. Occurrence of club root in Brassica crops.

3. Absence of earthworms and beneficial bacteria.

4. Calcium deficiency in crops.

Signs of Over-Liming

1. Chlorosis in certain plants, such as Hydrangea, due to high pH making iron and other nutrients unavailable, a condition known as "lime-induced chlorosis."

2. Increased prevalence of diseases such as scab in potatoes.

Cation exchange and soil pH

Cation exchange capacity (CEC) is a key indicator of soil fertility and its ability to absorb and retain cations, including biocides and radioisotopes. CEC is measured in milliequivalents, representing the amount of matter that can replace or combine with 1 mg of hydrogen per 100 g of dry soil. Most exchangeable cations are absorbed on the negatively charged surfaces of clay particles, with varying CECs among different clay minerals. For example, Kaolinite has a low CEC of 3-15 me/100g, while montmorillonite has a high CEC of 70-100 me/100g. Humus also plays a significant role in cation support, with very high CEC values ranging from 150-400 me/100g.

• Soil pH is crucial for horticulture and agriculture for several reasons:

• Many plants and soil organisms have specific pH preferences, affecting which crops or plants can grow without pH adjustments.

• Diseases often thrive in soils with particular pH ranges.

• Soil pH influences nutrient availability.

Common pH preferences for various plants

1. Acid Soil (pH 4.5-5.0): Blueberry, Heather, Cranberry, Orchid, Azalea, and blue Hydrangea (less acidic for pink).

2. Acid Soil (pH 5.0-5.5): Parsley, Potato, Conifers, Pine, Sweet Potato, Maize, Millet, Oats, Radish, Ferns, Iris, Rhododendron, and Camellia.

3. Moderately Acid Soil (pH 5.5-6.0): Bean, Brussels Sprouts, Carrot, Peanuts, Rhubarb, Soybean, Crimson Clover, Aster, Begonia, Canna, Daffodil, Larkspur, Petunia, Primrose, Violet, and most bulbs.

4. Slightly Acid Soil (pH 6.0-6.5): Broccoli, Cabbage, Cauliflower, Cucumber, Pea, Sweet Corn, Pumpkin, Squash, Tomato, Turnip, Red Clover, Sweet Clover, White Clover, Candytuft, Pansy, Rose, Snapdragon, Viola, Wallflower, and Strawberry.

5. Acid to Neutral Soil (pH 6.5-7.0): Asparagus, Beet, Celery, Lettuce, Melons, Onion, Parsnip, Spinach, Lucerne, Dianthus, Chrysanthemum, Dahlia, Stock, Sweet Pea, and Tulip.

6. Neutral to Alkaline Soil (pH 7.1-8.0): Clematis and Lilacs.