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Managing Low pH in Growing Media for Reliable Crop Performance

Managing Low pH in Growing Media for Reliable Crop Performance Featured Image

Low pH can cause serious production problems for growers. Learn steps to determining low pH in growing media, how to properly sample for accurate testing and the best options for raising growing media pH. This valuable knowledge will help growers maintain the right growing media pH for their crops to ensure reliable production results.

Three Steps for Determining Low pH

Know Optimal pH for the Crop

Before deciding whether growing media pH is too low, the grower must determine what constitutes low pH for a given crop. For example, if geraniums (Pelargonium spp.) are the main crop, the desirable pH during production should be in the 5.8 – 6.5 range.  Or, if the crop is calibrachoas, petunias, or pansies, the optimal pH during production should be more in the 5.2 – 5.8 range.  Growing media with low pH is generally an issue for plants that are sensitive to excess minor nutrients, which are more available at lower pH ranges.

Know Growing Media pH

Once the desired pH range for a crop is determined, growers must look at the pH of their growing media. Media pH values can vary widely due to a number of factors. The main factors influencing growing media pH are age, ingredients, moisture levels, and storage temperatures. The grower must have a general idea of what the growing media pH will be during production. Media may have an out-of-the-bag pH that is lower than the desired optimal range, but once in production, it should rise within the desired range in 7 to 14 days, assuming water quality and fertilizer selection have been taken into account.

 Know Irrigation Water and Fertilizer pH

Water quality and fertilizer also impact media pH. The pH of irrigation water can vary widely. Highly alkaline water leads to a pH increase. Fertilizer ingredients also impact pH. High nitrate and calcium fertilizers are more alkaline and tend to increase pH over time. High ammonium fertilizers are more acidic and will immediately cause a decrease in pH, followed by additional pH lowering over time.

Testing for Low pH

So how can one determine if the pH is too low? The grower can run tests over time on the unused mix to determine if the pH rises after initial wetting. Generally, growing media that has been wetted down with irrigation water will show a pH increase of 0.3 to 0.5 or more after 24 to 48 hours. If testing shows this expected pH increase, the media should eventually get to the desired range. Media pH can continue to rise slowly over the next 7 to 14 days, but generally if we see the expected increase after 24 – 48 hours it is safe to assume that media pH will be adequate for crop production.  If out-of-the-bag growing media pH is in the 5.4-6.0 range, it should be adequate without further testing.

Sampling Methods for Testing

Collecting proper samples is essential in helping growers determine the pH of growing media. Packaging formats determine the sampling method. For small formats, such as 2.8 cubic foot bags or 3.8 cubic foot bales, the best samples should represent a composite from a few unused bags or bales.  For a small-format composite sample, gather equal samples from 2 or 3 bags, being sure to collect from the middle of the bag or bale, and then combine them. For large formats, such as 55 cubic foot bales or 110 or 120 cubic foot towers, collect composite samples of equal size from a few areas of the bales or towers.

Growing media samples may also be taken from pots during production. To test used mix in production, take equal samples from the middle of numerous pots to create a composite sample for testing.  Avoid sampling from the very top or bottom of the pots.

Different volumes of media should be taken, depending on whether you are testing once or over a period of time. For one-time testing, a quart-sized composite sample is adequate. For testing over time, a gallon-sized composite sample is better.

Label samples to maintain correct testing information and match testing results. Labels should contain the following information: date, type of mix, production code from bag or bale labels, and any special notes regarding the mix. Take notes! All of the information on the label should tie back to the total information of the sample.

If samples are to be sent to a lab, send them overnight or via 2-day air to make sure you get results in a reasonable amount of time. Sample at the beginning of the week to avoid having samples sit over a weekend.

Growing Media pH Testing Methods

There are two main options for testing media pH: on-site testing or lab testing. On-site testing provides immediate results and allows the grower to control the testing process. Lab testing reduces labor input and provides a more complete analysis, including data on all major nutrients. Both methods are recommended. Test on-site, but also send some samples to a lab to support on-site results and to get a more complete analysis of the growing medium.

When testing on-site, a simple and repeatable testing method is achieved by creating a 2:1 dilution. This means adding 2 parts distilled or deionized water to 1 part growing medium. It is easier than the saturated media extract method, as it eliminates the subjective determination of when a mix is “saturated”. After mixing the samples, allow them to sit for 15-20 minutes at room temperature, then test the pH and electrical conductivity (EC) if you have an EC meter.  Stay consistent in the time you allow the sample to sit.

Continue to test the pH of your growing media. Test media before using it in production, and test it again 7 -10 days after initial watering. Then continue to test on a weekly or bi-weekly basis during crop production. Track your results and review them from year to year.  Retest immediately if your results change dramatically to make sure you are not experiencing a test error.

Options for Raising Growing Media pH

Table 1. Options for raising growing media pH.

Table 1. Options for raising growing media pH

There are a number of options when dealing with low pH in growing media. (The different possibilities are listed in Table 1, along with the pros and cons of each method.) The decision on which method (or methods) to use will depend on available labor, greenhouse conditions, and grower preference.

The two easiest options are to stop acidifying water or use potentially basic fertilizers, but they may not provide a quick enough pH change, especially for crops requiring high pH. The use of potentially basic fertilizers should be part of any of the other choices offered in the table.

If a low pH issue is observed prior to potting, lime can be added to the growing media on the filling line, if possible. This will have the best overall impact. Test various rates, but incorporating 2-3 pounds of lime into one cubic yard of mix should bring the mix pH up about 0.5-0.75, depending on the type and coarseness of the lime.  Calcitic lime will increase the pH faster, but it will not last as long as dolomitic lime. Topdressing with lime is another possibility, if the growing media is already in the pots or the crop is already in production, though the impact on pH may not be as great.

The addition of potassium bicarbonate when the mix is already in production is relatively easy, but it offers only a temporary solution. It may be a good choice if a grower feels that the fertilizer regimen will eventually bring the pH up, but they need a short-term quick fix. Otherwise, the addition of potassium bicarbonate will need to be repeated every 10-14 days to keep the pH up. Rate for potassium bicarbonate is 1 to 2 pounds per 100 gallons of final solution (1 to 2 pounds per gallon for a 1:100 injector). Some growers have reported issues with potassium bicarbonate in the stock tank when water pH is above 7. Do not combine potassium bicarbonate in the same stock tank with the regular fertilizer!

The final option would be to use a flowable or liquid lime addition. This can be done when the crop is already in production. Examples of flowable lime are CalOx® and Cal-Flo®. They are generally direct-injected from the container at a ratio of 1:100. This should provide around a 0.5 pH unit increase. CalOx® can be injected at 1:5 ratio, providing a greater pH increase. If a higher pH increase is preferred, multiple applications can be made 7-10 days apart, or the product can be diluted in a large tank and applied with a water pump. These products can cause issues with emitters, so they are commonly applied by hand with a hose and regular breaker.

Sun Gro Media Testing:

Sun Gro’s grower technical specialists exist nationwide. Find your state’s Sun Gro contact by visiting To read a full list of our testing options and sampling methods visit

Air Porosity and Water-Holding Ability of Media Components

Air Porosity and Water-holding Ability of Media Components Featured Image

Most soilless growing media are composed of multiple components with distinct physical properties that fit together to form pore spaces of various sizes. Coarse particles create larger pores (macropores) that maintain air spaces and facilitate drainage, while fine particles create smaller pores (micropores) that hold available (and unavailable) water. Each component functions to meets a series of essential requirements. The media formulation is equally important because individual components will not retain their original porosity when combined. The combination of media components amounts added, and their distribution determines the character and porosity of a medium.

Why should growers be concerned about growing medium components, formulations, and essential requirements? Ensuring plant roots have sufficient air, water, and the right balance of available nutrients safeguards crop success. Stressed plants are more susceptible to pests and diseases as well as environmental stresses, which increases maintenance costs and decreases revenue.

Media Purpose and Properties

The purpose of growing medium is to support plants and their roots. A well-formulated medium creates a stable substrate that provides the right characteristics for a crop. Essential requirements must be met for the medium to offer the right amount of available oxygen and water for plant-root uptake while also providing a reservoir of available nutrients.

The essential requirements of a growing medium can be broken into two categories—physical and chemical requirements. The physical requirements are impacted more by component selection. The chemical requirements are impacted more by water and fertilizer selection and include pH and soluble salts. These requirements must be met to facilitate healthy root systems for strong growth.

The physical properties of a medium are generally dictated by the coarseness and fineness of its component particles. Key physical properties include bulk density, texture, porosity, available/unavailable water, and decomposition time. The cultural requirements of a crop determine the physical properties of a medium. Getting the balance of physical properties right for a given crop will aid in disease resistance, root respiration, and root development.

The chemical properties of a medium are equally important. Cation exchange capacity (CEC), electrical conductivity (EC), and pH are the three key properties that aid in retaining and releasing nutrients available for plant uptake. A good mix will maintain the right pH for a given crop while holding and providing needed nutrients for uptake.

Air and Water Pore Space

Particle sizes and the spaces they create are fundamental because particle size and distribution dictates the air-holding and water-holding capacity of a medium. Total porosity indicates a growing medium’s volume percentage of pore space. The number of macropores and micropores characterize media pore space.

There are general porosity and air space percentage ranges for soilless media in commercial ornamental production. On average, media has a “normal range” with a total porosity of 75-95%, air space of 10-30%, and container capacity of 65-80%. The recommended air-space range varies by crop: propagation mix should have 10-15% air space, general greenhouse production media should have 15-25% air space, and perennial mix should have no more than 25% air space.

Pores are the spaces between the solid components of the medium. Most soilless growing media contains 60% to 80% total pore space. Pore size determines the rate of drainage and gas exchange. The size and distribution of pores are the most critical factors in developing a growing medium with optimum physical characteristics. The amount of air and water pore space is determined by particle size. Larger or coarser particles have a greater percentage of air pore space, while smaller or finer particles have a greater percentage of water pore space. Adequate distribution of large and small pores is essential.

A mix containing coarse particles has fewer but larger pores. Large pores permit air to reenter the medium following irrigation. On average, most mixes contain 10% to 30% air following irrigation. Despite losing more water more quickly, larger pore spaces offer plants more available water at watering time.

A mix containing fine particles has more but smaller pores and a greater percentage of water pore space. The larger the quantity of water held by a medium, the lower the percentage of pore air space it maintains. The closer a water molecule is to a solid, the more tightly it is held through the forces of adhesion and cohesion. Therefore, a fine mix may hold more water than a coarse mix, but less water may be available to the plant. In general, the amount of unavailable water is relatively high in soilless growing medium.

Water Holding Capacity Stages and Media Components

Three stages are applied to water holding capacity: saturation, container or field capacity, and wilting or permanent wilting point. At saturation, most pores are full of water during irrigation. Gravitational water drains from the macropores due to gravity, and these pores refill with air. At container or field capacity, gravitational water has drained, and the medium contains available water for plant growth. Capillary action allows the micropores to retain water. At wilting point or permanent wilting point, no more water is available to plants and most plants wilt and fail to recover turgor when irrigated. All water between container or field capacity and wilting point is available water.

Different media components contribute to water holding capacity at various levels. Consider the following ingredients: coir, rock wool, peat, pine bark, vermiculite, perlite, and sand. Each lends unique characteristics with respect to saturation, container capacity, and the amount of water held. For example, components with smaller, finer particles and pores, such as Canadian Sphagnum peat moss, processed coconut coir, and rock wool, hold more water. In contrast, those with larger, coarser particles, such as pine bark, perlite, and sand hold little water. The percentage of each component making up the mix will determine the duration of container or field capacity.

Let’s consider an example of high- and low-bark media and how bark percentage impacts total porosity, container capacity, air space, and bulk density. A high-bark medium (50% peat, 10% perlite, 15% vermiculite, and 25% bark) maintains the following physical characteristics: total porosity of 87%, container capacity 71%, air space 16%, and bulk density 13-16 pounds/cubic foot. A low-bark medium (55% peat, 15% perlite, 15% vermiculite, 15% bark) maintains the following physical characteristics: total porosity of 90%, container capacity 78%, air space 12%, and bulk density 10-12 pounds/cubic foot. The higher bark medium is more heavyweight and maintains a lower container capacity, but it has more air space. As particle size increases, so does porosity.

 Container Height and Media Water

Container height also impacts the relative amount of water versus air in a medium. The taller the container, the less evenly water is distributed throughout the container, regardless of the media. Here is how it works.

Water is pulled down through the pot and drainage holes via gravity. Gravity in the container is constant but the gravitational potential at the container’s top is higher than it is at the bottom. Water is attracted to particles by adhesion, cohesion, and capillary and resist gravity. Matric potential, which is a medium’s ability to hold water through adhesion and cohesion, remains constant throughout the container.

As James Altland, Ph.D of North Willamette Research and Extension Center,
Oregon State University writes in the article, Physical Properties of Container Media: “Because of this gradual decrease in gravitational potential towards the container bottom, matric potential is higher at the container bottom and media particles are able to hold more water. This causes water to form a perched water table at the container bottom. The perched water table is a layer of saturation on the container bottom. In contrast, more air and less water exist towards the top of the container. With the same media, the perched water table occurs at the same height, regardless of the container size. Short containers will have the same perched water table as large containers. Thus, a greater percentage of container volume is filled with water. This explains why a taller 5-gallon container holds less water than a shorter 5-gallon container.”

Media Handling and Porosity

Handling impacts media porosity. Not only is it up to the grower to select the right growing media for their needs, they must also correctly manage it. Many handling and management mistakes can damage media structure and performance, resulting in poor plant growth.

Over mixing and fluffing or using inadequate equipment for fluffing compressed-bale growing media can damage the structure of various components, which can decrease porosity and structure. Pre-wetting media to 48 to 52 percent  moisture at the fluffing stage will help it maintain structure and perform well. On average, media are produced at 35 to 45 percent moisture. Pre-wetting will also help growers avoid irrigation problems, such as water channeling and dry areas in the media.

Media compaction will also decrease the presence of macropores, decrease air space and increase bulk density. Compaction can occur when a medium is manually pressed down at planting time, or when filled pots of flats are stacked. Proper media handling will keep it well aerated will allow it to perform as expected.


When selecting or formulating media for crops, be sure to choose media components carefully and consider the overall formulation with respect to air porosity and water-holding ability. Manage your media well to ensure it maintains the right characteristics and choose the right medium for the right pot size. Making good growing media decisions will ensure plant roots have sufficient porosity for air and water as well as the right balance of available nutrients for success.

Review of Major Growing Media Components

Review of Major Growing Media Components Featured Image

By Dan Jacques and Ron Walden, Sun Gro Horticulture –

The major role of a growing mix is to support the plant, while holding water and nutrients for the plant to use during growth. There are five main components commonly used in making growing media: peat moss, bark, coir, perlite and vermiculite. This article describes the components and outlines desired properties of mixes for various uses in greenhouse production.

Read: Review of Major Growing Media Components