pH is a measure of the concentration of hydrogen ions in a substrate which operates on a logarithmic scale. Each incremental change in pH represents a 10x difference in the concentration of hydrogen ions. This means that pH serves as a critical determinant of the chemical environment within a substrate. It influences various biological and chemical processes essential for plant growth and development in greenhouse agriculture1.
Impact of pH on Plant Growth
Each plant species has a preferred pH and is very sensitive to pH changes as the acidity of the soil influences the solubility of nutrients. Soil pH controls the solubility, mobility, and bioavailability which determine their translocation in plants. When the pH is too low, micronutrients become more mobile, leading to potential toxic growth environments. Conversely, when the pH is too high, micronutrients are less mobile, causing deficiencies1,2. For example, phosphorus, calcium, and magnesium are more soluble in acidic soils, while nitrogen, potassium, and sulphur are more soluble in alkaline soils. Fig1. shows the effect of soil pH on nutrient availability3.
Additionally, it drives differences in the microbial community structure in the rhizosphere and has an impact on nutrient cycling and disease prevention. Maintaining proper pH levels also influences soil structure including water infiltration, drainage, and aeration. This only stresses the importance of custom pH management strategies in greenhouse cultivation. In essence, effective pH management sets the foundation for healthy crops, optimised yields, and sustainable production systems in greenhouse agriculture4,5. Previous studies have shown that soil pH influences many biogeochemical processes and determines the fate of substances in the soil. Unfortunately, soil pH is often measured casually as a norm without careful consideration for its role in soil5.
Nutrient availability at different pH levels
Iron: Iron is involved in chlorophyll synthesis, electron transport, and enzyme activation. An iron deficiency causes chlorosis, or yellowing of leaves, especially in young tissue. Iron is more easily available in acidic soils.
Manganese: Manganese is involved in photosynthesis, respiration, and nitrogen metabolism. A manganese deficiency causes interveinal chlorosis, or yellowing of leaves between the veins, and reduces plant growth. Manganese is also more available in acidic soils.
Molybdenum: Molybdenum is involved in nitrogen fixation and nitrate reduction. A molybdenum deficiency causes stunting, leaf curling, and chlorosis, especially in legumes. Molybdenum is more available in alkaline soils.
Effects of pH imbalances
Acidic Soil: Acidic soil can cause nutrient deficiencies, especially of calcium, magnesium, and molybdenum. It can also cause nutrient toxicities of aluminium, manganese, and iron. These can result in reduced root growth, impaired nutrient uptake, chlorosis, necrosis, wilting, and reduced yield. Acidic soil also inhibit the activity of beneficial microorganisms, f.e. nitrogen-fixing bacteria and mycorrhizal fungi. They favour the growth of plant pathogens, such as fungi and nematodes.
Alkaline Soil: Alkaline soil can cause nutrient deficiencies, especially of iron, manganese, zinc, copper, and boron. It can also cause nutrient immobilisation of phosphorus, nitrogen, and sulphur. These can result in reduced shoot growth, impaired nutrient translocation, chlorosis, stunted development, and reduced yield. Alkaline soil can also inhibit the activity of beneficial microorganisms, f.e. decomposers and nitrifiers. They favour the growth of plant pathogens, such as bacteria and viruses6, 7.
Methods for measuring pH
Methods include soil testing, nutrient solution analysis, and electronic pH meters.
Soil testing provides accurate and comprehensive information on pH and other parameters, such as electrical conductivity, nutrient levels, and soluble salts. However, this method is time-consuming and costly, and may not reflect the current pH of your substrate.
A nutrient solution analysis is a method that involves collecting the drainage water from the substrate and testing it with a pH indicator or a colorimeter. This method provides quick and simple information on pH and other parameters, such as alkalinity and hardness. However, this method may not be accurate or consistent, and may not represent the actual pH of the substrate.
Electronic pH meters are devices that measure the pH of a solution by using a probe and a display. This method provides instant and precise information on pH and other parameters, such as temperature and moisture. However, this method requires regular calibration and maintenance, and may not work well with some substrates, such as peat or bark1,8.
Greenhouse pH Management
The pH of the growing substrate depends on several factors, such as water quality, substrate composition, and fertiliser application. All these factors can cause the pH to increase or decrease over time, depending on the crop and environmental conditions. Because of this there are different strategies for adjusting and maintaining pH levels in greenhouse systems. They are liming, acidification, and the use of pH-buffering agents.
Liming is useful for correcting acidic conditions, improving nutrient availability, and enhancing microbial activity. However, this strategy requires careful calculation and application, and may not be effective for some substrates, such as coir or perlite.
Acidification is useful for correcting alkaline conditions, preventing nutrient deficiencies, and reducing disease incidence. However, this strategy requires proper safety and handling, and may not be suitable for some crops, such as geraniums or petunias.
The use of pH-buffering agents is beneficial for stabilising pH fluctuations, optimising nutrient uptake, and supporting plant growth. However, this strategy may not be sufficient or compatible with some factors, such as high alkalinity or hard water9, 10.
Conclusion
pH management is a critical aspect of greenhouse crop production, and it requires regular monitoring and adjustment based on crop type, growth stage, and environmental conditions. By maintaining the optimal pH level for each crop, growers can ensure that the plants can access and absorb the essential nutrients needed for growth and development. Keeping a proper pH can improve quality & quantity of yield, protect plants from diseases, reduce the need for pesticides, improve stress tolerance, and so much more.
