Gardening

The pH of the soil, substrates and water

What is pH? The pH is a very important characteristic that all soils have, substrates for pots, planters, etc. and irrigation water. The definition, if you do not have knowledge of chemistry, will not tell you anything. The pH is expressed with a number and can be between 1 and 14, but in 99% of cases it will be between 3 and 9 .

  • ACID soil has a pH less than 7.
  • NEUTRAL soil has a pH equal to 7.
  • BASIC or ALKALINE soil : pH greater than 7.

Therefore, if we say: “This soil has a pH 6”; means it is acidic. Or: “This soil has a pH of 8.2”; means it is basic or alkaline. Neutral pH, although 7 is indicated as a theoretical value, is normally considered neutral if it is between 6.5 and 7.

The best pH for most plants is between 6.5 and 7, that is, neutral. Some, called acidophilic, prefer it lower than 6, and others (calculus), are happy with a pH higher than 7.

How is the pH determined?

I have mentioned three pH’s before:

  • Soil pH
  • The pH of the substrate (those used for pots, planters, etc.)
  • Irrigation water pH

The determination in the case of a land or substrate can be done in the following ways (for waters, you have it a little further down):

1.You can take the soil to be analyzed in a soil laboratory . It is the best, but it is not usually done at the private garden level.

2. Use some inexpensive kits that bring some reagents to throw a sample of soil and comparing color to find out the pH. The pH can be acidic, neutral or alkaline. If it is alkaline, it most likely contains a lot of lime (limestone soil).

3. Take some soil, pour it into a glass with distilled water, stir well and after a while introduce a strip of paper or cardboard pH indicator (sold in pharmacies, for example) into the liquid . Depending on the color it takes, you will have if it is acidic, neutral or alkaline.

4. Pour vinegar, ‘Sulfatán’ (cleaning product) or diluted hydrochloric acid on a soil sample. This provides an acceptable estimate for the reaction it produces:

  • If the effervescence that occurs is strong, the pH is said to be greater than 7.5 and the soil is alkaline or limestone.
  • If the effervescence is small (some bubbles), the pH will be around 7.
  • If it does not produce effervescence (no bubbles come out) it is a soil with a neutral pH (pH 6.5-7) or acid (pH less than 6.5).

To know the pH of the irrigation water you can:

– Take a sample to a laboratory that analyzes water.

– Use litmus paper.


– Use instruments called ph-meters, although they must be well calibrated to be reliable.

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Why is it interesting to know the pH of a soil, substrate or irrigation water?

The pH influences the soil or substrate in several aspects, but the most significant and the one we are going to study here is the availability of nutrients . That is, the influence of pH on the greater or lesser amount of nutrients (Phosphorus, Potassium, Iron, Copper, Boron … up to 13) that are in a soil so that the roots of the plants can take it.

For example, in a soil there may be a lot of phosphorus, but if it is not soluble, the plant is useless since it cannot take it. As the pH influences the solubility of Phosphorus and other minerals and, following the example, in alkaline soils, there is a large part of insolubilized Phosphorus and in these soils there is a greater risk of deficiencies of this element than one that is acidic or neutral.

I repeat: extreme pH’s can (not necessarily) cause a shortage of some or other nutrients and the plants will accuse it by yellowing the leaves, flowering less, bearing less fruit, slowing growth, etc., etc. The problem is aggravated if they are Stronger pH’s, both very acidic (pH = 5 or lower) and very alkaline (pH = 8 or higher).

Let’s look at each of the three situations:

a) Acidic soil (pH <7)

b) Neutral soil (pH = 7)


c) Alkaline soil (ph> 7)

a) If your soil is acidic (pH <7)

In Spain, Galicia or Extremadura, for example, are regions that due to their geological nature abound with acid soils. These soils are ideal for acidophilic plants such as Azalea, Rhododendron, Hydrangea, Camellia, Heather, Gardenia, etc.

An acid soil has the problem that the following nutrients may be lacking:

  • Match
  • Calcium
  • Magnesium
  • Molybdenum
  • Boron

Knowing whether the plants are suffering from deficiencies in any of these elements is not easy, you have to know the specific symptoms, but based on the fact that the soil is acidic, it will be an important clue and, in your case, it would be necessary to:

  1. Provide the nutrients that are lacking through fertilizers.
  2. Also, raise the pH by adding ground limestone .

On the contrary, in acid soils Iron , Manganese, Zinc and Aluminum are abundant , and they can even produce excess toxicity.

If it were very acidic, that is, with a pH <5.5, ONLY acidophilic plants could be planted , and it would be more than convenient to raise that pH value if we do not want to limit ourselves to a handful of species, the acidophilic ones. It is raised by incorporating ground limestone , which is called making a ‘liming’.

b) If your soil is neutral (pH = 7)

Most plant species will do well. The aciphophiles if you lower the pH something will be much better.

Regarding nutrients, there is an optimal availability of all those that plants normally need, without ruling out any specific deficiency independent of pH.

c) If your soil is basic or alkaline (pH> 7)

Here there are usually quite a few problems because it is a very frequent situation.

The acidophilic plants mentioned above will almost certainly not do well; the leaves will turn yellow and bear few flowers. This is due to the fact that in alkaline soils several soluble elements essential for all plants are scarce:

  • Iron
  • Manganese
  • Zinc
  • Copper
  • Boron

But not only acidophilic plants (Hortensia, Heather, Gardenia, etc.) can show these deficiencies of Iron, Manganese and others, but many other plants. For example, an orange tree, a lemon tree, a hibiscus, a rose bush, etc., etc.

The symptoms of nutrient deficiencies in plants are varied and as I said before with acid soils, it is not easy to know exactly what specific element or elements it is. Iron, for example, usually manifests initially with a yellowing of the leaf, the nerves remaining green. Later the leaf turns completely yellow. It is seen in the young leaves, not in the old ones, at least in a first phase, in advanced chlorosis all the leaves, new and old, remain yellow.

Therefore, if the leaves of your trees and shrubs turn yellow, one of the possible causes is due to a lack of Iron and / or other microelements (Manganese, Zinc, Copper and Boron) caused by being planted in alkaline soil (ph <7) that insolubilizes them (not absorbable by the roots). It is a possibility, it is necessary to study well if it is a lack of micronutrients or is another reason, for example, an excess of irrigation.

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What to do if this situation occurs?

Well, in the first place it could have been avoided to know the pH before choosing the species to plant, and discarding those that were not suitable. For example, if you really like a Rhododendron and your soil has a pH = 8, forget about planting it in that alkaline soil and do it better in a good pot with acidic substrate. However, there are people who want, despite everything, to grow certain plants even if their natural soil is not suitable for them. In this case they will have to do the following below.

The common problem is that once the symptoms appear (sometimes several years, for example, in fruit trees), and since the plantation is not going to be uprooted, a remedy must be found that consists of these 3 actions:

  1. Provide fertilizers that contain the nutrients that are scarce : Iron, Zinc, Potassium, Manganese, etc.
  2. Lower the pH of the soil.
  3. Lower the pH of the irrigation water , if you are also watering with one that is alkaline (contains a lot of lime).

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