Soil pH

Different types of plants require varying degrees of soil acidity. In fact, some plants are very sensitive to soil pH.

Rhododendrons and heathers will not tolerate lime in the soil. On the other hand Clematis prefers an alkaline soil.

What does soil pH mean?

The acidity or alkalinity of the soil is measured by pH (potential Hydrogen ions).

Basically it is a measure of the amount of lime (calcium) contained in your soil, and the type of soil that you have. Generally, soils in moist climates tend to be acid and those in dry climates are alkaline. A soil with a pH lower than 7.0 is an acid soil and one with a pH higher than 7.0 is alkaline.

The soil must be adjusted to suit the plant which will occupy that area if it is not already within that plants requirement range.

Testing your soil pH

Most good garden centers will even gladly pH test a soil sample for you, or you can buy an inexpensive pH test kit at most nurseries, or hardware stores.

These test kits generally consist of a test tube, some testing solution and a color chart. You put a sample of your soil in the tube, add a few drops of test solution, shake it up and leave it for an hour or so to settle.

The solution in the tube changes color according to the pH of your soil. Compare the color of the sample with the color chart that came with the kit. Matching colors will tell you the pH of your sample. The better kits will also advisory booklets about how to interpret your result.

Adjusting your soil pH

Once you have determined the pH you can amend the soil, if needed to accommodate the plants in your garden using materials commonly available at your local garden center.

Raising the soil pH to make it more alkaline

Generally speaking, it is easier to make soils more alkaline than it is to make them more acid. Because different soil types react in different ways to the application of lime you will have to add more lime to clay soils and peaty soils than you will in sandy soils to achieve the same result.

To increase your pH by 1.0 point and make your soil more alkaline:

Add 4 ounces of hydrated lime per square yard in sandy soils

Add 8 ounces of hydrated lime per square yard in loamy soils

Add 12 ounces of hydrated lime per square yard in clay soils

Add 25 ounces of hydrated lime per square yard in peaty soils 

Correction of an overly acid soil should be considered a long term project, rather than trying to accomplish it in one year. It is better to test your soil each year and make your adjustments gradually. The addition of hardwood ash, bone meal, crushed marble, or crushed oyster shells will also help to raise the soil pH.

Lowering the soil pH to make it more acidic

If your soil needs to be more acidic, sulfur may be used to lower the pH if it is available. To reduce the soil pH by 1.0 point, mix in 1.2 oz of ground rock sulfur per square yard if the soil is sandy, or 3.6 oz per square yard for all other soils. The sulfur should be thoroughly mixed into the soil before planting. Sawdust, composted leaves, wood chips, cottonseed meal, leaf mold and especially peat moss, will lower the soil pH.

Popular - Cottonwood

Latin plant name Populus Common plant name Poplar or Cottonwood (chopo) Plant type Decidious tree Size (meters) < 15-50 m Distinguishing features The bark on young trees is smooth, white to greenish or dark grey, often with conspicuous lenticels; on old trees it remains smooth in some species, but becomes rough and deeply fissured in others. The flowers are mostly dioecious and appear in early spring before the leaves. They are borne in long, drooping, sessile or pedunculate catkins produced from buds formed in the axils of the leaves of the previous year. Cottonwood are ofter in wetlands. They have the advantage of growing very big, very fast. Almost all poplars take root readily from cuttings or where broken branches lie on the ground. Trees with fastigiate (erect, columnar) branching are particularly popular, and are widely grown across Europe and southwest Asia. However, like willows, poplars have very vigorous and invasive root systems stretching up to 40 m from the trees; planting close to houses or ceramic water pipes may result in damaged foundations and cracked walls and pipes due to their search for moisture.

Betula - Birch

Latin plant name Betula Common plant name Birch (abedul) Plant type Decidious tree Size (meters) <10m Distinguishing features They are typically rather short-lived pioneer species widespread in the Northern Hemisphere particularly in northern temperate and boreal climates. Birch species are generally small to medium-sized trees or shrubs, mostly of temperate climates. The simple leaves are alternate, singly or doubly serrate, feather-veined, petiolate and stipulate. They often appear in pairs, but these pairs are really borne on spur-like, two-leaved, lateral branchlets. The fruit is a small samara, although the wings may be obscure in some species. They differ from the alders in that the female catkins are not woody and disintegrate at maturity, falling apart to release the seeds, unlike the woody, cone-like female alder catkins. Birches often form even-aged stands on light, well-drained, particularly acidic soils. They are regarded as pioneer species, rapidly colonising open ground.

Comfrey

Comfrey (also comphrey) is an important herb in organic gardening. It is used as a fertilizer and as an herbal medicine. 

Comfrey is a particularly valuable source of fertility to the organic gardener. It is very deep rooted and acts as a dynamic accumulator, mining a host of nutrients from the soil. These are then made available through its fast-growing leaves (up to 4-5 pounds per plant per cut) which, lacking fibre, quickly break down to a thick black liquid. There is also no risk of nitrogen robbery when comfrey is dug into the soil as the C:N ratio of the leaves is lower than that of well-rotted compost. Comfrey is an excellent source of potassium, an essential plant nutrient needed for flower, seed and fruit production. Its leaves contain 2-3 times more potassium than farmyard manure, mined from deep in the subsoil, tapping into reserves that would not normally be available to plants.

There are various ways in which comfrey can be used as a fertilier. These include:

• Comfrey as a compost activator - include comfrey in the compost heap to add nitrogen and help to heat the heap. Comfrey should not be added in quantity as it will quickly break down into a dark sludgy liquid that needs to be balanced with more fibrous, carbon-rich material.
• Comfrey liquid fertilizer - can be produced by either rotting leaves down in rainwater for 4–5 weeks to produce a ready-to-use 'comfrey tea', or by stacking dry leaves under a weight in a container with a hole in the base. When the leaves decompose a thick black comfrey concentrate is collected. This must be diluted at 15:1 before use.
• Comfrey as a mulch or side dressing - a two-inch layer of comfrey leaves placed around a crop will slowly break down and release plant nutrients; it is especially useful for crops that need extra potassium, such as fruit bearers but also reported to do well for potatoes. Comfrey can be slightly wilted before application optionally but either way, avoid using flowering stems as these can root.
• Comfrey potting mixture - originally devised to utilize peat, now environmental awareness has led to a leaf mold-based alternative being adopted instead; two year old, well decayed leaf mold should be used, this will absorb the nutrient-rich liquid released by the decaying comfrey. In a black plastic sack alternate 3-4 inch layers of leaf mold and chopped comfrey leaves. Add a littledolomitic limestone to slightly raise pH. Leave for between 2–5 months depending on the season, checking that it does not dry out or become too wet. The mixture is ready when the comfrey leaves have rotted and are no longer visible. Use as a general potting compost, although it is too strong for seedlings.

In the school garden we prepared liquid comfrey fertilizer. We cut the leaves of the comfrey and stuffed them into a 'tea bag' that we left inside a container for two weeks. After those two weeks we took our the 'tea bag' and let the comfrey liquid in the container for future use.