pH Levels of Garden Soils
The question of acidity and alkalinity has cropped up from time to time whilst looking at the different types ofso it would be sensible to look at it in a bit more detail. Acidity, neutrality and alkalinity are expressed as the pH value of the soil. For gardening purposes, the scale runs from about pH5 to around pH8, with neutral being from pH6.5 to pH7. The lower the figure, the more acid the soil.
Most of the soils that we garden on are in the range pH5 to 8. Although the optimum pH for different plants varies, 6 to 6.5 is generally the area to aim at as this encompasses the greatest number of plants. It is slightly acid but not enough to cause any problems. All the normal sorts of vegetables that we’re likely to grow in the gardens are quite happy with this.
The pH value of a soil is really quite easy to alter; all that has to be done is to apply a material with the opposite reading. Therefore, if you garden on a strongly acid soil the answer is to apply chalk or lime, whereas gardening on chalk or limestone may necessitate heavy dressings of bulky , such as garden or farmyard manure or, in extreme cases, even powdered sulphur. These cases are extremely rare, though, and I can’t recall seeing a garden which couldn’t because it was too strongly acid or alkaline.
Where vegetables are concerned, very acid soils have more problems than chalky, alkaline ones. Just as a chalky soil will ‘lock up’ certain elements, so will an acidic soil, though not the same elements. Acidic soil conditions also reduce the population of, and therefore the beneficial effects of, many of the soil’s micro-organisms. Because of this, the plants are not able to make full use of the naturally occurring plant nutrients as they are taking longer to break down into an available form.
The simplest way to reduce the acidity (or raise the pH) of a vegetable plot is to treat it with ground chalk, limestone or agricultural lime. In most cases chalk and limestone are better than agricultural lime because they are far kinder to any plants with which they come in contact. For this reason, agricultural lime is best applied only after winter digging, when it is going to be weathered and washed in during the winter and be gone well before anything is planted. Lime, does, however, have about double the corrective value of chalk or limestone.
Fortunately, there are fairly accurate guidelines as to how much chalk/limestone should be applied to a soil to have a given effect. As a rule, to raise the pH of a soil by 1.0, 500gm per sq m (1 lb per sq yd) needs to be applied.
The texture of a soil and the effect that this will have on its nature and how well plants will grow in it has already been discussed (see The Different Characteristics of garden Soils). Now it is time to look at the meaning of another word — structure. This is often confused with texture and one is frequently used when the other is meant. Whereas the texture of a soil refers to the size of its individual soil particles and its true character, the structure is exactly what it implies; the way in which the soil particles are put together.
To illustrate this, imagine that the individual soil particles are represented by marbles. These are of different sizes but in heavy soils they would be predominantly small whilst in light soils they would be large. We now put the marbles into polythene bags and pack these into a huge crate. If the ‘bags’ are very large and the marbles are mainly small, as in the case of clay soils, there will be very little room for air in the bags or crate. There will also be hardly any spaces for water to drain through if it rains so it will tend to lie on the top layer of bags.
At the opposite end of the scale, we would find a sample of predominantly large marbles representing a sandy soil. These would be packed in small bags. When placed in the crate, there would be a large amount of air between the bags, as well as between the individual marbles. Air can flow through the crate quite easily, as can water, and in this soil structure roots can grow more or less unhindered. This is called a ‘loose’ structure and, as you can imagine, it has its problems just as much as a tight (clay) structure.
When we come to the medium textured soil, we find that not only are the marbles just about an equal mix of large, medium and small, but so also are the bags into which they are packed. This gives a soil that is truly between the two extremes with, one hopes, the virtues of both.
The texture of a soil is very difficult to alter, as it would mean adding marbles of different sizes and hoping that they would get into the bags and alter their size and shape. However, it is perfectly possible to improve the structure of the soil. Nature’s way of doing it is to return dead vegetation to the ground and encourage it to be integrated not only into the polythene bags but also around and between them. This dead vegetation has the remarkable ability to bind together loose-structured sandy soil and break up clay and silt. In the former case it enables the soil to hold more water and plant foods; in the latter, it allows water to drain away easier and so improve a soil that hitherto lay wet. Thus, the structure of all soils is improved, whether they be heavy, medium or light. However, the dead vegetation, or organic matter, does not last for ever in a soil. The more there is, the greater the population of micro-organisms and, hence, the faster the organic matter is decomposed.
The end result of this process of decomposition is a substance that is known to, but not always understood by, all gardeners humus. Humus isn’t just another word for organic matter. It is a very mysterious substance that has a mainly chemical role in the soil rather than the largely physical effect of organic matter it originates from.
All soils contain both organic matter and humus and the level of both must be maintained if plants are to grow properly. From the gardener’s point of view, there is little we can do to add pure humus to the soil, but a great deal can be done to increase the organic matter content, which will ultimately break down into humus. The effect that organic matter has on the soil, as has already been touched upon, is that it opens up heavy soils, binds together sandy soils and generally improves the structure and fertility of all soils.