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The availability of plant nutrients is significantly affected by media pH.

Understanding pH is essential to maximize the potential of your crops. Inadequate pH – when the media pH is too low or too high for crop needs – is often associated with crop damage. Until the plant begins to show symptoms, it may be too late to resolve the problem.

pH and plant nutrients

Media pH affects the mobility of the nutrients to the plant. Nutrients may become more available for plant uptake or less available depending on pH . If the pH value is too low, the nutrients become more mobile and are absorbed more than necessary by the plants, which often leads to toxicity. For example, plant absorbs high levels of iron and manganese , which leads to burned leaves. Symptoms of too low pH usually appear on the lower leaves and roots. For example, speckling and marginal necrosis. Poor root development can also occur due to calcium deficiency in cases where the pH is not high enough. If the pH value is too high, the nutrients are less mobile and the plants are unable to absorb them, which accordingly causes multiple deficiencies  – iron, manganese, zinc, copper, boron. Symptoms first appear on the upper leaves, can be observed interveinal chlorosis, twisted new leaves, root rot. Not all crops have the same symptoms, and the symptoms of plants of the same crop do not always appear at the same time. What and how many plants show symptoms can also be affected by watering systems and weather conditions. Stunted growth, loss of roots and plant death may be observed. As a result, this can have a negative effect on yields.

Plants cry for help!!!

The images below show changes in plant appearance due to inappropriate pH.

Micronutrient toxicity due to low pH. Stunted growth, brown leaf edges and speckling appearance for Petunias. Photo: Premier Tech Horticulture.

 

 
Pepper grows unevenly due to high pH, chlorosis is observed. Photo: Premier Tech Horticulture.

 

Your goal as a grower is to ensure a stable pH over the entire plant’s life cycle. It’s not that easy, because pH of growing media is not a static condition. There are several factors that can affect the ph of your growing media. pH may increase or decrease during the crop cycle. You should not expect symptoms of toxicity or deficiency to appear in your plants, otherwise it is a risk to your plant’s health and profits.

What is pH?

pH is a measure of how acid or alkaline a solution is. pH below 7 is more acidic, pH above 7 is less acidic (basic). pH indicates the amount of hydrogen ions in the solution. The scale used to measure pH is logarithmic. Each increase in pH contains 10 times more hydrogen ions than the previous increase. Thus, a media with a pH of 5.0 contains a hundred times more hydrogen ions than a substrate with a pH of 7.0. pH is the balance between hydrogen and hydroxyl ions, the lower the pH – the higher the concentration of hydroxyl ions relative to hydrogen ions.

The ideal range for most plants is  5.0-6.5. This range provides the plant with the maximum availability of the most nutrients. But some plants like Blueberries,  Rhododendron,  Azalea, Cammelia, Pieris , conifers  prefer more acidic soils and others like Peony, Lilac and Salvia prefer more alkaline soils. Plants also tolerate slight deviations from the desired pH, but, for example, acid-loving plants in alkaline soil will have neither the expected beauty nor yield.

This pH scale represents the acidity ranges preferred by various crops

CEC (Cation Exchange Capacity) and pH buffer

The CEC is used to describe the buffering capacity of the growing media. Buffering means resistance to changes in pH or nutrient concentration in the soil solution. Thus, growing media with a high CEC help growers maintain a stable pH or nutrient concentration over time. Growing media components, such as peat moss have negatively charged “exchange sites” that allow the particles to loosely hold onto positively-charged cations. Cations include acid (hydrogen H +), fertilizer cations (ammonium NH4 +, calcium Ca2 +, magnesium Mg2 +, potassium K +), and other cations such as sodium Na +. The larger the capacity to adsorb cations, the larger the CEC. Growing media components vary in their ability to attract and hold ions. Peat as organic material is capable of adsorbing ions to a much higher degree compared for example to sand or perlite, which have almost zero CEC. If a solution has a low buffering capacity, the pH of the solution will change immediately in response to the pH of the added chemical.  It is important to maintain the pH in a limited range. For example, a substrate with a large buffering capacity will resist pH changes when fertilizers and other chemicals are added.

Factors affecting the pH of Growing Media

Irrigation – Water Source Quality

As growers, it is very important for you to know the quality and characteristics of your water source. The complex interaction of water, growing media and fertilizer determines the availability of various nutrients that are essential for normal plant growth. Water quality can vary considerably depending on factors such as geographical location, season and water source. For example, rainwater and most surface water supplies are pure because they are relatively low in minerals. In contrast, water from deep aquifers can contain a lot of minerals.

Liming

Several substrate components have natural acidity. For example, the natural acidity of peat products is 3.5-4.5. As the ideal pH range for most crops is 5.0-6.5, addition of lime is necessary. To adjust the pH of the substrate to the desired level, carbonates (CO32-) are added, which neutralize the acid (H+).  Calcitic limestone (CaCO3) dissolves quite poorly and therefore slower, while dolomitic limestone CaMg(CO3)  dissolves better and faster. The particle size of the liming material is also important. Finer lime will dissolve faster than lime with a larger particle size. The frequency of watering will also affect the activation of lime. The more often watered, the faster the lime is activated. Lime is usually activated during the first two weeks of watering.

Fertilizing

Fertilization during cultivation can cause an acidic or basic effect on the pH of the substrate. It depends on what type of fertilizer you use during the culture. In general, the higher the percentage of ammoniacal nitrogen (NH4) in the fertilizer used, the more acidic the reaction and the faster the pH of the media will decrease. In turn, fertilizers with more nitrate nitrogen (NO3) and calcium (Ca) tend to increase the pH of the media. It also depends on what water you use and what type of substrate you add fertilizer to. As mentioned above, the larger the substrate’s pH buffer, the more stable the pH during the culture.

Crop

Plants have an effect on the pH of root environment. When plants receive fertilizer elements through their roots, all of these elements have either a positive or negative charge. The plant needs to maintain its internal electrical balance, so to get a positive charged element such as calcium, magnesium, potassium, the plant will release hydrogen into the growing medium, causing a slight drop in pH near the root of the plant. Likewise, if the roots of a plant absorb negatively charged elements such as phosphorus, nitrates and most trace elements, hydroxide ions will be released, which will slightly increase the pH level. Depending on the crop need for these individual elements, some use a higher ratio of positively charged fertilizer elements, thus more effectively acidifying the growth medium ph. Other plants use a higher ratio of negatively charged fertilizer elements, thus more effectively raising the pH of the growing medium.

 

 

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