Publié sur TECA (http://teca.fao.org)
How to measure soil pH value
RÉSUMÉ:
Most soils in Ghana are too acidic, limiting the nutrient uptake of plants. The following video presents one
easy way to measure a soil?s pH value using pH indicator strips. Knowing your soil's pH value helps to take
action to improve your soil quality.
MOTS-CLÉS:
Fertilisation [1]
Soil testing [2]
Acidity [3]
Soil improvement [4]
Soil pH [5]
Soil physical properties [6]
CATÉGORIE:
Production végétale [7]
PAYS:
Ghana
DESCRIPTION :
The following video describes why it is important to know if a soil is acid or alkaline and shows a cheap
way to measure a soil?s pH value. This video is also available in the Ghanaian language Twi (click here [8]
).
Many farmers spend a lot of money on fertilizers, but good soil conditions must be in place for plants to be
able to take up nutrients such as nitrogen, phosphorus and potassium in an optimal way.
Most of soils in West Africa are too acidic. Under these conditions, plants roots are not able to take up the
nutrients optimally, even though a sufficient amount of nutrients is present in the soil. In these cases, buying
and applying fertilizers won?t bring much benefit.
Most crops grow well in soils with pH values between 5.5 and 6.5. However, most soils used for agriculture
in southern Ghana typically have a pH value ranging from 4.5 to 5. They are usually too acidic and limit the
nutrient uptake of plants.
Measuring a soil?s pH value allows to see if a soil is too acidic or alkaline. Knowing this, actions can be
taken to increase or decrease a soil?s pH value.
A pH indicator kit with a pH scale is needed to measure the pH value of a soil. Water, a container and a
wooden stick to stir are also needed. Measuring a soils pH value is done in the following procedure.
Taking soil samples
1. Identify several spots on the field from where you would like to take soil samples.
2. Use a spade to remove the first few centimeters of the soil.
3. Dig a hole to take a soil sample. The depth of the hole depends on the type of crop you are going to
plant. For shallow crops like chili peppers, tomatoes or lettuce, the depth of the hole should be 0-15
cm. For perennial crops like mangos or oranges, the depth of the soil should be 0-30 cm or more.
Collect the samples in a container.
4. Mix the different samples together and take your sample for the analysis.
Measuring the pH-value
1.
2.
3.
4.
5.
Take the soil sample and add water. The ratio of earth and water should be 1/2.
Stir the mixture for 30 min.
Allow the soil mixture to settle for 2 hours.
Pick one stick from the pH indicator kit and dip the tip in the mixture for a few seconds.
Compare the color on your stick with the pH scale. The color will indicate the pH value of the soil
(see picture below).
[9]
Improving a soils pH value
If a soil is too acidic it can be improved by adding compost, lime or wooden ashes. About 10 bags of lime
per acre are needed to increase the pH value by 1 unit. This means, for example, about 5 kg of lime are
needed per mango tree. The lime can be applied on a 1 m wide band around the tree (see picture below).
If a soil is too alkaline, compost or sulfur can be added to lower the pH value.
[10]
SOURCE:
GIZ - Market-Oriented Agriculture Program (MOAP) [11]
Country:
Ghana
URL source: http://teca.fao.org/fr/node/8241
Liens:
[1] http://teca.fao.org/fr/keywords/fertilisation
[2] http://teca.fao.org/fr/taxonomy/term/16464
[3] http://teca.fao.org/fr/taxonomy/term/12418
[4] http://teca.fao.org/fr/keywords/soil-improvement
[5] http://teca.fao.org/fr/keywords/soil-ph
[6] http://teca.fao.org/fr/taxonomy/term/16465
[7] http://teca.fao.org/fr/technology-categories/crop-production
[8] http://www.youtube.com/watch?v=GZ2giqm2hEg
[9] http://teca.fao.org/sites/default/files/pH1_0.png
[10] http://teca.fao.org/sites/default/files/pH2_0.png
[11] http://teca.fao.org/fr/node/7916