Acid and alkaline soils FAQs

What is the meaning of pH?

This is the logarithmic scale which defines the acidity or alkilinity of a olution. The pH scale ranges from I to 14. A pH of 7.0 is neutral; bove 7.0 is increasingly alkaline nd below 7.0 is increasingly acid. (pH of 5.0 is ten times more acid nan pH 6.0; pH 9.0 is ten times lore alkaline than pH 8.0. Different groups of plants require different soil pH: rhododendrons nd most heathers, for example, ke acid conditions, and so cannot irive in alkaline (chalky or limy) oils: cabbages, on the other hand, ke a neutral or alkaline soil.

Why do brassicas not grow well in my sandy soil?

Sandy soil is often rather loose and, as we have seen the cabbage family needs firm soil. mother reason is that brassicas refer soil with lime present. Unless me has been applied within the last year or two, your sandy soil will probably be too acid because lime can readily be leached out (washed away) from such soils.

How can I lower the pH of my soil?

Avoid using lime. Use acid-reacting fertilisers like sulphate of ammonia and sulphate of potash. Sulphur can also be used to reduce alkalinity- a suitable rate would be 135 g/m (4 oz per sq yd) while acid sphagnum peat can be employed as a bulky organic material.

I have read that peat is acid, and yet a sample I tested was clearly alkaline, with apH 7.5. Can you explain this?

Not all peat is acid. Sedge peat, for example, is likely to be alkaline if it is dug from areas which are fed by rivers flowing through chalky (alkaline) soil.

How can I raise the pH of my soil?

The soil is made more alkaline by adding ground limestone, chalk, hydrated lime, or marl (clay with a high level of calcium). Dolomitic limestone can also be used; it adds magnesium as well as lime.

Why is it important to apply lime to the soil?

Lime neutralises acidity, and this enables bacteria to become more active. The majority of plants require a soil which is either neutral or very slightly acid (a pH of 6.8 to 6.4). There is a natural tendency, however, for the soil to become increasingly acid with time. Lime corrects this tendency and also makes available essential plant foods. Clay soil is often improved by an application of lime, which flocculates the particles to form crumbs.

I am told that beech leaves should not be used to mulch lime-hating plants. Why is this?

The leaves of many trees can make a valuable contribution to your compost heap. Beech leaves, however, contain a considerable amount of lime in the form of calcium, so they should not be used as mulch or compost in association with rhododendrons, azaleas, ericas, and other lime-hating plants. Continual use of such leaves will raise the alkalinity of the soil.

When should I apply lime?

Lime should be applied to the soil only if a pH test indicates that it is necessary. If too much lime is present in the soil, certain plant foods such as magnesium, manganese, and iron become locked up in the soil and so cannol be absorbed by plants. When liming is necessary, it should be applied to unplanted soil during autumn, so that winter rains wash the powder into the ground and it becomes fully incorporated into th« soil in time for spring sowing and planting.

Why do some of the plants in my chalky soil have unhealthy yellow leaves?

Although a certain amount of lime in the soil is required by many plants, an excess will make certain plant foods unobtainable to their root systems. Iron deficiency causes young leaves to go yellow (a condition known as chlorosis), while magnesium deficiency cause older leaves to go yellow, with the veins usually remaining green. Iror sequestrene can be used to correcl the first deficiency, while Epsom salts is used to control magnesium deficiency. The remedies should b applied according to label instructions for the type of plant in question.

Why should lime be added to the compost heap?

The vegetable material added to the compost heap is fermented and converted by the microscopic bacteria present in the heap. Bacterial activity, however, causes an acid reaction, and unless lime is added, satisfactory composting will not occur. Proprietary activators recommended for use on the compost heap usually contain lime. Otherwise a dusting of lime should be applied and watered in every other 150 mm (6 in) depth of material as the heap is built up.

Can I apply lime and fertiliser it the same time?

It is better not to do so unless it is already formulated together in a compost activator. Lime and some fertilisers (for example, those containing sulphate of ammonia) react together and the nitrogen content is lost to the atmosphere. This occurs in the soil application as well as in the compost heap.

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