Complete Guide to Measuring and Adjusting Soil pH: Methods, Interpretation, and Solutions

Measuring the pH of the soil It is a vital process for anyone interested in growing, gardening, or farming, whether professionally or as a hobby. Knowing the degree of soil acidity or alkalinity allows for more efficient agronomic management, helps optimize nutrient absorption by plants and helps them develop to their full potential. Furthermore, understanding and controlling pH helps prevent problems such as salt toxicity, nutritional deficiencies, or difficulties with plant germination and growth.

What is soil pH and why is it relevant?

El soil pH reflects the concentration of hydrogen ions present in the liquid fraction of the soil, which determines its acidity or alkalinity. pH scale It ranges from 0 to 14, with 7 being the neutral value. Values less than 7 indicate soils acids, while values greater than 7 correspond to soils basic or alkaline.

• Acid pH: until 6.5
• neutral pH: 7
• Alkaline pH: from 7.5

This indicator is essential, as it influences all biological and chemical processes in the soil. A well-balanced soil pH ensures an optimal environment for plants. beneficial microorganisms, improves soil structure and regulates the nutrient availability essential for plant growth.

Factors that affect soil pH

Soil pH is not a fixed value and can change over time for different reasons. Among the main factors that alter it are:

• Parental material: The parent rock or original material of the soil largely determines its natural tendency toward an acidic or alkaline pH.
• Weather conditions: The amount of rainfall, irrigation, temperature and sunlight can modify the leaching of salts and minerals, affecting the pH.
• Agronomic practices: The use of fertilizers, amendments, organic fertilizers and the type of water used for irrigation (hard or soft water) directly affect pH.
• Mineralization of organic matter: The decomposition process of plant waste releases acids or bases that can alter the pH.
• Presence of certain salts: As a result of natural processes or the addition of fertilizers and amendments, some salts can raise or lower the pH.

Additionally, pH can vary throughout the year, especially in cultivated soils, so it is advisable to take periodic measurements to monitor its evolution.

Impact of pH on crops and nutrient availability

The degree of acidity or alkalinity of the soil determines which plants can develop normally and which cannotEach plant species has an optimal pH range; outside of these values, the absorption of nutrients is compromised. For example, the micronutrients such as iron, manganese, zinc and copper tend to be less available in alkaline soils, while aluminum and manganese toxicity may appear in acidic soils.

The following table describes the ideal pH ranges for different groups of plants:

• Acidic pH (less than 6): Ideal for blueberries, azaleas, rhododendrons, camellias, heathers, hydrangeas, and fuchsias. Many of these plants are acidophilic.
• pH close to neutral (6.5 – 7): Suitable for cereals, vegetables and most fruit trees.
• Alkaline pH (above 7): Some crops such as alfalfa and spinach tolerate it better.

Failure to adjust the pH to the plant's requirements can lead to nutrient deficiencies, chlorosis (yellow leaves), poor growth, or even plant death. Therefore, many growers and gardeners pay special attention to this parameter when selecting species or designing fertilization plans. For more details on how to change the pH of the soil, you can visit our section on specific techniques.

Methods for measuring soil pH

There are various methods To measure soil pH, from affordable home techniques to highly accurate laboratory procedures. Below, we explain the most commonly used methods and their key considerations.

Homemade method with vinegar and baking soda

• White vinegar: When adding vinegar to a soil sample, if you observe bubbling, it indicates alkaline pH. For a more precise measurement, see our article on use of pH meters.
• Sodium bicarbonate: If effervescence appears when adding it (after moistening the soil with distilled water), the soil is acidic.
• Interpretation: If neither test produces a reaction, the soil is probably neutral or near neutral.

This method is indicative and useful for a first approximation, although its precision is limited.

Method with pH strips or pH kits

• Mix one part soil with one part distilled water and stir well.
• Let the mixture rest for a few minutes.
• Immerse a pH indicator strip and compare the color obtained with the scale provided by the kit.

This method is more accurate than the previous one and highly recommended for checking pH quickly and economically.

Professional and laboratory methods

There are standardized procedures, such as those included in technical regulations for the pH determination (UNE 77305):

1. Take representative soil samples from different areas of the plot.
2. Weigh 20 g of sample and mix with 50 ml of distilled water (ratio 1:2,5).
3. Shake vigorously for 2 minutes and let stand for 15 minutes.
4. Measure the pH in the supernatant with a pH meter, indicator strip or pH kit.

For professional crops, it is also common to use , some with special electrodes that allow direct measurement in moist soil, without the need for prior sample preparation.

Recommendations for sampling

• Take samples from several points in the orchard or garden (10-15 cm deep for crops grown in soil, 5-10 cm in pots).
• Avoid areas near paths, walls, or recent fertilizers to avoid distortions.
• Mix the samples well to obtain a composite and representative sample.
• Always use distilled water for testing, as tap water can affect the results.

Home and professional procedures for analyzing soil pH

Homemade step-by-step technique

1. Collect samples: Use a shovel to take several samples, collecting approximately 10 to 15 cm of soil depth. It's advisable to take samples from several distant locations to get a realistic picture of the average pH.
2. Prepare the soil: Remove any root debris, stones, and plant matter. Gently dry the sample if it's very moist.
3. Mix with distilled water: Place the sample in a clean container, add enough distilled water to cover the soil, and stir the mixture well.
4. Let stand: Wait a few minutes for the solids to settle.
5. Try vinegar and baking soda: Perform a first test by dividing the sample in two, adding vinegar to one part and baking soda to the other, and observing the bubbling. You can then obtain a more precise measurement with pH strips, dipping them into the supernatant.

Professional Analysis

• Preparation according to technical standards: Use the ratio indicated by the technique (for example, 1:2.5 soil-water) and mix in a beaker or sterile container.
• Using digital meters: Insert the electrode into the decanted solution and wait for the reading to stabilize. Some devices allow direct measurement in the soil, provided it is moist, making the procedure easier and providing almost instant results.

Interpretation of results and optimal ranges according to crops

After measuring the soil's pH, it is essential to correctly interpret the results to decide on possible corrections.

• pH less than 6,5: Acidic soil. Suitable for acidophilic crops, but can cause deficiencies in conventional crops.
• pH between 6,5 and 7: Neutral soil. Ideal for most crops.
• pH greater than 7: Alkaline soil. Suitable for basophilic plants; there is a risk of micronutrient deficiencies.

For a correct choice of crops and soils, you may be interested in consulting this page about importance of soil pH in plants.

How to change soil pH: techniques to lower or raise pH

How to lower the pH of alkaline soils?

If your soil is too alkaline and you need to make it more acidic, there are several strategies:

• Sulfur powder or granules: It is one of the most effective and long-lasting methods for acidifying soils. Its effect is gradual and long-lasting, although it may take 4 to 8 months to be fully noticed. To learn more, visit our page on how to correct soil pH.
• Acidic organic matter: Adding manure, blond peat, or pine bark compost can gradually acidify the soil and improve its structure.
• Iron sulphate: Useful for acidophilic plants, especially if iron chlorosis is detected.
• Aluminum sulfate: Fast, but should be used with caution and only in soils with sufficient organic matter.

When watering frequently with hard or limestone water, consider acidifying the irrigation water with citric acid or vinegar and adding chelated trace elements to compensate for any deficiencies. For soil care information, check out our recommendations.

How to raise the pH of acidic soils?

In very acidic soils (pH below 6), some crops may not grow well. Here's how to raise the pH:

• ground limestone: Sprinkle and mix the recommended amount into the soil, depending on the acidity level. This is the most common method for increasing pH. If you want to know how to do it, check out how to change soil pH.
• Wood ashes: They provide potassium and increase alkalinity, although their effect may be irregular.
• Crushed eggshell: It provides calcium carbonate, although its action is very slow.
• Hard irrigation water: Continued watering with hard water often increases the pH of the substrate over time.

It's important to make these changes gradually and with periodic measurements to avoid imbalances that could affect your plants.

Common mistakes, frequently asked questions, and tips for success

• Not taking enough samples: Remember that pH can vary greatly from one area of land to another, even in small spaces.
• Use tap water for measurement: Always use distilled water to avoid distortions.
• Omission of organic matter: Regular addition of organic matter not only improves soil structure, but also helps stabilize pH over the long term.
• Expect immediate results: Especially in pH-modifying treatments, the process is often slow and requires patience and monitoring.

If there are any doubts about the interpretation of the analyses or the most appropriate correction plan, consult with a agricultural technician o specialized advice to receive personalized recommendations based on the specific characteristics of your land and crops.

Having a good knowledge of soil pH It allows for better selection of species to grow, optimizes the use of fertilizers and amendments, and prevents problems with nutrient deficiencies or toxicity. Regular measurement and adjustment contributes to agricultural and gardening success, whether in orchards, urban gardens, or field crops, by promoting healthy plant development and increasing productivity. Taking the time to monitor pH and adjust the soil is the first step toward healthier and more productive crops.