Soil pH – An Important Factor In Crop Production

Soil pH is a measure of the acidity or alkalinity of soil, which is determined by the concentration of hydrogen ions (H+) in the soil. The pH scale ranges from 0 to 14, with pH 7 being neutral. A pH value less than 7 indicates acidic soil, while a pH value greater than 7 indicates alkaline or basic soil. Soil pH value is considered as one of the most vital factors which may determine the yield of the crops. Soil pH can regulate and control many chemical and biochemical reactions within the soil. If the soil pH is too high or too low, it can obstruct nutrient absorption, reduce plant growth, and even lead to plant death. Thus, it is important to maintain an optimal pH level in soil for healthy plant growth.  

Different ranges of soil pH:

Level of acidity and alkalinity	Range of pH
Strongly acidic	pH below 5.5
Moderately acidic	pH between 5.5 and 6.5
Slightly acidic	pH between 6.5 and 7.0
Neutral	pH of 7.0
Slightly alkaline	pH between 7.0 and 7.5
Moderately alkaline:	pH between 7.5 and 8.5
Strongly alkaline	pH above 8.5

Importance of soil pH:

• Soil pH influences the availability of nutrients to plants and maintaining the optimal pH range for a given crop.
• Adjusting soil pH to the optimal range can improve fertilizer use efficiency by ensuring that nutrients are available in a form that plants can use.
• It can affect the susceptibility of plants to certain diseases and pests. For example, Clubroot (Plasmodiophora brassicae) is a soil borne disease that affects brassica crops. This disease causing pathogen thrives in acidic soils and cause stunted growth and  root deformation. Similarly, potato scab is a common soil-borne disease caused by bacteria Streptomyces scabies in alkaline soil. These bacteria can cause the formation of rough, scabby patches on the potato tubers. Lowering the soil pH may decrease the severity of the disease.
• Soil pH can also affect the decomposition of organic matter in the soil.
• It influences the types and activity of soil microorganisms, which play an important role in nutrient cycling, disease suppression and soil health.

How to measure soil pH?

Soil pH can be measured using pH meter or by sending the soil samples to a soil testing laboratory for analysis, which typically involves chemical testing using specialized equipment. 

Acidic Soils (pH<7)

Acidic soil is soil that has a pH value of less than 7. Acidic soil can have low nutrient availability, especially for essential plant nutrients such as calcium, magnesium, and phosphorus. This is because these nutrients can become less soluble and available for plant uptake at low pH values. As a result, plants grown in acidic soil may exhibit stunted growth, yellowing of leaves, and reduced yield. Acidic soil may contain high levels of aluminum, iron and manganese which can be toxic to plant roots and inhibit plant growth. This can also result in reduced water and nutrient uptake. Acidic soil have reduced microbial activity.  

Management of Acidic soil:

• The most common method for reclaiming acidic soil is to apply lime, which is an alkaline material that neutralizes soil acidity. Lime can be applied in various forms, such as powdered or pelletized, and the amount of lime needed depends on the soil pH and the crop being grown. They can be broadcasted in the field and incorporated well during tillage.
• Alternate amendments for lime include dolomite, press mud and basic slag.
• Fertilizers that contain alkaline cations, such as calcium and magnesium can help increase the soil pH over time.
• Use only optimum dosage of nitrogen and sulphur fertilizers.
• Use alternative nitrogen fertilizer like Calcium nitrate that have non-acidifying effect.
• Crop rotation can be done with legumes which can fix nitrogen in the soil, thus reducing the requirement of synthetic nitrogen fertilizers that can accelerate acidification.
• Selecting crops that are adapted to acidic soil, such as beans, groundnut, maize, rice or potatoes.
• Proper management of soil and water by regularly checking on leeching of bases.

Alkaline soil (pH > 7)

Alkaline or basic soil is a type of soil with a pH level above neutrality (pH > 7). This alkalinity is often caused by the presence of minerals such as calcium, magnesium and potassium, which are commonly found in alkaline soils. Despite the high mineral content, alkaline soil often lacks nutrients like iron, manganese, zinc, copper due to the high pH level. The high pH level of alkaline soil can cause certain nutrients to become chemically bound in the soil, making them less available to plants. This can lead to poor plant growth and development. In addition, the lack of organic matter in alkaline soil can lead to poor soil structure and decreased water holding capacity. 

Management of Alkaline soil:

• Applying soil amendments such as sulfur, acidifying fertilizers such as ammonium sulphate or acidifying organic materials like peat/sphagnum peat moss can help to lower the pH of alkaline soil.
• Application of gypsum can also be done.
• Adding organic matter such as compost, green manures or FYM to the soil can improve soil structure and fertility. It increases the water holding capacity of the soil.
• Mulching with crop residues can help to increase soil organic carbon.
• Ensuring proper irrigation management with drainage channels.
• Deep tillage can increase infiltration rate in the soil.
• Planting tolerant crops such as barley, sugar beet can help to reclaim alkaline soil.

Effects of Abnormal Soil pH:

• Poor root establishment
• Poor plant vigour
• Poor nodulation in legumes
• Abnormal leaf colours
• Increased disease incidence
• Decrease in beneficial microorganisms and increase in plant pathogenic organisms
• Stunted crop growth
• Plant showing with nutrient deficiencies
• Poor flower and fruit setting

Optimum pH range for crops:

Crop	Soil pH Range	Crop	Soil pH Range
Rice	5.5 – 7.0	Peas	6.0 – 7.5
Wheat	6.0 – 7.0	Okra	6.0 – 6.8
Maize	6.0 – 7.5	Brinjal (Eggplant)	6.0 – 6.8
Sugarcane	5.5 – 8.0	Cucumber	6.5 – 7.5
Cotton	5.0 – 7.5	Watermelon	6.5 – 7.5
Groundnut	6.0 – 6.5	Mango	5.5 – 7.5
Soybean	6.5 – 7.5	Banana	5.5 – 7.5
Mustard	6.5 – 7.5	Pineapple	5.0 – 6.0
Potato	4.8 – 5.4	Guava	4.5 – 8.2
Tomato	6.5 – 7.5	Citrus (Orange/Lemon)	5.5 – 7.5
Onion	5.5 – 6.5	Grape	6.5 – 7.5
Carrot	6.0 – 7.0	Cauliflower	5.5 – 6.5
Cabbage	5.5 – 6.5	Chilli	6.5 – 7.5
Papaya	6.5 – 7.0	Barley	7.0 – 8.0
Barley	7.0 – 8.0	Garlic	6.0 – 7.0
Beans	5.5 – 6.0	Radish	5.5 – 7.0

Above are the indicated optimum pH ranges for the crops. Soil test analysis during crop planning will help in deciding the crop to go in particular soil. Soils with extreme pH may be corrected to a certain extent with the above-mentioned amendment agents.  

NOTE: 

Majority of the plant nutrients are most available to plants within the pH range 6 – 7.5. Thus, it is important to maintain this soil pH to ensure healthy plant growth and development.