Growing Media and Nutrition

There are numerous factors affecting the growth of vegetable transplants including types of growing media, watering practices and fertilization programs.

Types of Growing Media

Growing media for vegetable transplants in greenhouses contain a variety of soilless ingredients such as peat moss, vermiculite, perlite, shredded coconut husks (coir), or composted materials plus starter nutrients and a wetting agent. Field soils are generally unsatisfactory for the production of plants in containers because soils do not provide the aeration, drainage and water holding capacity that are required. They also need to be pasteurized or fumigated to prevent the development of diseases and germination of weed seeds.

Premixed media is common in the greenhouse industry. Suppliers offer a diversity of mixes that are available prepacked (in bags, bales, super sacks) or in bulk. Growing media are designed to achieve high porosity and water retention while providing adequate aeration.  Recipes are specially formulated for propagation, specific crops or general use. Soilless media purchased in bags does not have to be pasteurized or fumigated before use. Preventative applications of biological fungicides or fungicides may be necessary with vegetable transplants prone to damping off. Growers can also obtain commercially available mixes with different types of biological fungicides added to the mix.

Compost-based mixes are also available commercially as a substitute for traditional soilless media, especially for organic production. See section below on organic vegetable bedding plant fertility.

Steam Treatment

Field soil is not recommended for growing vegetable transplants in greenhouses. However, if it is used, it must be treated to eliminate soilborne plant pathogens, insects, and weed seeds. Once the soil has been treated, care must be taken to avoid reinfestation.

Treatment of soil with steam is very effective and safe so is preferred over chemical fumigation. All pathogens will be killed by steam and only a few of the hardiest weed seeds will survive. Portable steam generators are available.

Ideally, the temperature of the entire soil mass should be raised to 160° to 180°F for 30 minutes. It is important to use several accurate thermometers placed in one or more corners and in the center of the soil mass.

The soil moisture content prior to steaming is important. Overly moist soil will take a long time to reach proper temperature. However, some moisture is necessary for the effective killing of microorganisms as well as the conductance of heat. Proper soil moisture for steaming is approximately the same for good planting conditions; soil squeezed in the hand should crumble easily. If possible, the soil mass should be moistened evenly two to three weeks prior to treatment. This will germinate difficult-to-control weed seeds, such as oxalis and clover, making them susceptible to heat. Prolonged steaming of soil at temperatures higher than 180°F can result in undesirable side effects such as overkill of beneficial soil microflora and accumulation of ammonium and manganese. High levels of ammonium can reduce plant growth (stunted plants), cause yellowing of foliage and death of root tips. Soil high in organic matter should be tested for ammonium after steaming. Several weeks may be necessary to allow for the dissipation or conversion of the ammonium. The incorporation of dolomitic lime and superphosphate, based on a soil test, may reduce ammonium levels.

Plant Nutrition

Soil Testing: Have your soil tested to adjust your fertilizer program and to manage the pH of the growing media to prevent nutritional problems.  Soil samples from soilless mixes are tested differently than samples from field soil. Unlike field soil tests that extract nutrients with weak acid solutions, soilless media is mixed with distilled water at a standard dilution and then analyzed. There are three commonly used methods of testing soilless media using water as an extracting solution: saturated media extract (SME), 1:2 dilution method, and leachate Pour Thru. The values that represent each method of testing are different from each other. Likewise, values for specific nutrients are likely to differ with testing methods.  Always use the interpretative data for the specific soil testing method used to avoid incorrect interpretation of the results. Most soil testing laboratories use the SME method. The 1:2 and Pour Thru are methods that can be used by growers on-site using portable soil testing meters. Since different soil testing labs may use different dilutions, it is not advisable to compare soil test results from one lab to those obtained from another. Use one laboratory for consistent results.

In addition to carrying out a complete soil test, growers should routinely check the electrical conductivity or soluble salts (EC) and pH of their growing media.  These tests can be done on-site using portable testing meters, or samples can be sent to a University soil testing laboratory.

Taking a Soil Sample: Take several samples at root depth from several containers and mix together in a clean container. Sampling several containers is important because a sample from one pot or flat could be an anomaly (values too high or too low) misrepresenting the crop as a whole. Sample about 2 hours after fertilizing or at least on the same day. If slow-release fertilizer pellets are present, carefully pick them out of the sample. If the pellets are left in, they can break during testing and this may result in an overestimation of fertility.

Be consistent in all sampling procedures each time you sample. A lot of variability can be introduced to tests due to inconsistent sampling and this diminishes the value of testing especially if you are trying to track fertility levels.

Take about one cup of the soil mixture and dry at room temperature. Put the dry soil in a sandwich size zip-type bag and close it tightly. Identify each sample on the outside of the bag for your use.

pH: The term pH refers a measurement of the hydrogen ion concentration (how acidic or basic a solution is).  The pH can range from 0 (very acidic) to 14 (very basic). Medium pH drives the chemical reactions that determine whether nutrients are either available for root uptake (soluble) or unavailable for root uptake (insoluble).  Major influences on the media pH include limestone in the growing media, irrigation water pH and alkalinity, and the acid/basic nature of fertilizer solution used.

The optimum pH range for vegetable bedding plants grown in soilless media is 5.5 to 6.5.

Electrical Conductivity (EC) or Soluble Salts: Soluble salts are the total dissolved salts in the root substrate (medium) and are measured by electrical conductivity (EC). Most fertilizers (except urea) are salts and when placed in solution they conduct electricity. Measuring EC or soluble salts provides a general indication of nutrient deficiency or excess.  A high EC reading generally results from too much fertilizer in relation to the plant’s needs, but inadequate watering and leaching or poor drainage are other causes.  Sometimes high EC levels occur when root function is impaired by disease or physical damage. Always check the condition of the root system when sampling soil for testing.

Water Quality and Alkalinity: The quality of water used for irrigation and mixing fertilizers should be tested each year for pH, alkalinity and electrical conductivity. Water containing a large concentration of dissolved salts can cause high soluble salts damage.

Water alkalinity is a measure of the water's capacity to neutralize acids. An alkalinity test measures the level of bicarbonates, carbonates and hydroxides in water. Test results are generally expressed as ppm of calcium carbonate. Irrigation water tests should always include both pH and alkalinity. A pH test itself is not an indication of alkalinity. Water with high alkalinity (i.e., high levels of bicarbonates or carbonates) always has a pH value of 7 or above, but water with high pH does not always have high alkalinity. This is important because high alkalinity exerts the most significant effects on growing medium fertility and plant nutrition.

Water with high alkalinity can result in iron deficiency chlorosis caused by increased root medium pH over time. Water with low alkalinity will have little ability to neutralize acidity. It is advisable to have your water tested prior to the spring growing season. Your greenhouse fertilizer program should be adjusted according to these test results.

Fertilizers and Media pH: Most water-soluble fertilizers will change the potting media pH to some extent. Some are acid such as 20-10-20 (potential acidity 429) while others are mildly basic such as 15-0-15 (potential basicity 420). Potential acidity or basicity is printed on the fertilizer label based on calcium carbonate.  Units are given in terms of acidity or basicity in equivalent pounds of calcium carbonate (which is the main constituent of lime) per ton of fertilizer. For example, if a 20-10-20 has a potential acidity of 429 pounds per ton, then the reaction produced by one ton of fertilizer will neutralize 429 pounds of calcium carbonate. If 15-0-15 has a potential basicity of 420 pounds per ton, then the reaction produced by one ton of the fertilizer will be equivalent to 420 pounds of calcium carbonate.  The effect that a fertilizer has on media pH is determined by the nutrients (especially nitrogen) contained in the fertilizer and is dependent on the reactions that take place after the fertilizer has been applied to the crop.

The type of nitrogen contributes to the acidity of the fertilizer. Ammonium nitrogen is acidic (lowers the pH), urea is converted into ammonium and is also acidic (slow reaction) and nitrate nitrogen can cause the media pH to increase.

Fertilizers can be used to manipulate the pH of the growing media and most growers alternate fertilizers to balance the pH of the growing medium.

Fertilizer Injectors: In conventional greenhouses, nutrients are delivered using various water-soluble fertilizers through a fertilizer injector, through the use of controlled-release fertilizers, or using a combination of these two methods.

Fertilizer injectors are used in liquid feeding systems. These devices inject a small quantity of concentrated fertilizer solution (stock solution) into the irrigation line so that the water leaving the hose (dilute solution) supplies the proper concentration of fertilizer. Rates of fertilization are often given in parts per million (ppm) of nitrogen, which is a way of expressing the fertilizer concentration. The amount of fertilizer to dissolve per gallon of water (stock solution) to make the appropriate concentrate for a specific injector setting needs to be determined. This information is listed on the bag of fertilizer. An injector setting of 1:100 indicates that 1 gallon of fertilizer concentrate delivers 100 gallons of final solution and is not an indication that the injector is delivering 100 ppm.

Choosing Fertilizers: Factors to be considered when choosing fertilizers include the ratio of ammonium to nitrate-N, trace element starter charge, content of calcium and magnesium, and potential acidity or basicity. Commonly used fertilizers include 15-0-15 (Dark Weather Feed), 15-16-17 and 20-10 20 or Cal-Mag 15-5-15.

Peat-Lite Specials (15-16-17, 20-10-20). These fertilizers are among the most popular for routine fertilization of vegetable bedding plants. Both are high (>50%) nitrate fertilizers. However, these fertilizers also have elevated trace element levels which may raise iron (Fe) and manganese (Mn) to toxic levels at low pH. Both are acid-forming fertilizers, but 20-10-20 has the greater potential acidity.

20-20-20 General Purpose. Growers who use this fertilizer with soilless media risk ammonium toxicity because the nitrogen in this fertilizer is 75% ammonium and urea. Some growers who use media containing soil do not appear to have problems. If 20-20-20 is used, the soilless growing medium should be tested frequently for ammonium. 20-20-20 supplies trace elements and has the greatest potential acidity of fertilizers commonly used in New England greenhouses. Tomato, eggplant and pepper plants are especially sensitive to ammonium, reducing plant growth and causing yellowing of the foliage.

Low Phosphorus (P) Fertilizers (20-0-20, 20-1-20, 15-0-15). These fertilizers can be tried as an alternative to chemical growth regulators for vegetable transplants. This technique of growth control is sometimes called "phosphorus starvation." It is generally believed that more P than necessary is being applied to greenhouse crops. Too much P may cause plants to stretch and P is a ground water pollutant. Unfortunately, in terms of height control, these fertilizers may be of no benefit if they are applied to a growth medium containing superphosphate or a high starter charge of P. Also, there is a risk of P deficiency if the fertilizers are used continuously with low P growth media. The low P fertilizers are quite different in many ways. 15-0-15 and 20-0-20 supply Calcium (Ca). 15-0-15 is a basic (raises pH) fertilizer containing about 95% nitrate and 20-0-20 is a neutral fertilizer and is 50% nitrate. 20-1-20 is an acidic fertilizer and it does not supply Ca, but it is about 70% nitrate.

Calcium Nitrate and Potassium Nitrate (15-0-15). Use of this fertilizer combination greatly reduces the chance of trace element toxicities. Some growers alternate its use with the Peat-Lite Specials on a 2 to 3 week basis to supply Ca and to counter the acidic effect of the Peat-Lite fertilizers. However, both superphosphate and a trace element fertilizer must be incorporated in the growing medium if this combination is to be used as the sole fertilizer. However, application of a water-soluble NPK fertilizer every 10 to 14 days or superphosphate must be incorporated in the growing medium if this combination is to be used as the sole fertilizer.

Nitrogen, Phosphorus, Potassium

Nitrogen. Nitrogen concentration in the greenhouse fertilizer program has a greater effect on the growth of transplants in the greenhouse than either phosphorus or potassium. Raising the level of nitrogen results in taller transplants with thicker stem diameters and heavier plant weights, but applying too much nitrogen may result in soft, poor quality transplants. These lush transplants may also be more prone to phloem feeding insects such as aphids, whiteflies and to foliar blights.

Phosphorus: Phosphorus has a limited effect on the growth of bedding plants when compared to nitrogen, but should be included as part of a complete fertilizer. Increasing the phosphorus concentration results in a moderate increase in transplant height, stem diameter, and shoot fresh and dry weight. If phosphorus is restricted to the point at which the plants show extreme phosphorus deficiency (purple leaves and stems, stunted plants), field performance will be reduced.

Potassium: Adequate potassium is applied as part of a complete fertilizer.

Guidelines for Rates and Frequency of Fertilizer

Small, slow-growing plants should receive lower rates or less frequent application until they are well-established. Care should be taken not to over-fertilize vegetable bedding plants to avoid overgrown plants. Young seedlings are especially vulnerable to injury from high soluble salts.

While plants are in the plug or seedling stage, use a complete water-soluble fertilizer at the rate of 50 to 100 ppm N every time plants are watered and use clear water (no fertilizer) every third watering. Use the lower rate (50 ppm) early and the higher rate (100 ppm) later if the seedlings are to be held in the flat or tray three or more weeks before transplanting. Shortly after transplanting, as plants approach rapid growth, increase the rate to 200 ppm N at every watering or 300 ppm N once every 7 days, watering with clear water 2 or 3 times between each fertilization.

Fertilizer Solution Volume: The volume of fertilizer solution applied has a dramatic effect on the growth of the vegetable bedding plants. As the volume of water-soluble fertilizer increases, the quantity of nutrients delivered to the plant also increases resulting in an increase in height, stem diameter and plant weight. Doubling the volume applied also doubles the amount of each nutrient potentially available to the plant.

Plant Growth Rate and Environmental Conditions. In general, nutrient requirements of vegetable bedding plants are greatest during periods of rapid growth. Too much fertilizer during slow growth periods may lead to high soluble salts; failure to provide enough fertilizer during periods of rapid growth will lead to nutrient deficiency.

Nutritional Problems

Early in production serious nutritional problems are: high soluble salts, trace element toxicities, and ammonium toxicity. Late in production, particularly in cell packs, plants may develop nitrogen deficiency symptoms as the earliest indication of insufficient fertility levels.

Soluble Salts. Injury to vegetable bedding plants from excess salts seems to be most common shortly after transplanting. Seedlings are much less tolerant to excess salts than established, rapidly growing plants. Some soilless mixes may contain enough "starter charge" to cause excess salts problems in the first few weeks after transplanting, particularly when a water-soluble fertilizer is also applied. Excessive drying, poor drainage, and uneven watering are factors that can aggravate this problem. Check roots of plants often and conduct regular soil tests to identify and prevent problems. It is difficult to diagnose a soluble salts problem by symptoms alone. Often nutrient deficiencies and root diseases cause the same symptoms. Therefore, a greenhouse (not field) soil media test is advisable.

Trace Element Toxicities. Iron (Fe) and/or manganese (Mn) can be accumulated to toxic levels by tomato plants. Symptoms appear as numerous small dark spots and mottling of the foliage. The potential sources of excess Fe and Mn are: trace element fertilizers in the mix, water-soluble fertilizers with elevated trace elements levels, and sometimes irrigation water. Low growth medium pH aggravates the problem by increasing Fe and Mn availability. Toxicity can be avoided by keeping the pH in the range of 5.8 to 6.0 for susceptible crops and by the use of fertilizers with lower trace element levels.

Ammonium Toxicity.  Tomato, eggplant, and pepper are most sensitive to ammonium nitrogen, but many other vegetable bedding plants can be harmed if ammonium becomes excessive. During the early spring (February or March) with low light and cool media conditions, plant growth may be reduced with yellowing of the foliage.  To prevent ammonium toxicity, use water-soluble fertilizers that supply about 50/50 ammonium/nitrate to fertilize plants in soilless media.

Organic Vegetable Bedding Plant Fertility

Conventional growing media containing synthetic ingredients (wetting agent, starter chemical fertilizer) cannot be used in organic production of field transplants and vegetable bedding plants. However, acceptable growing media can be created from a wide variety of approved materials. These blends for organic production may be purchased off-the-shelf, custom-blended by manufacturers, or produced on-the-farm.

Most commercial potting mixes contain synthetic fertilizers and wetting agents that do not meet organic standards. One alternative is to arrange a special order from a commercial supplier who agrees to exclude starter fertilizers and wetting agents and then, plan to add your own. Purchasing a commercially prepared mix for organic production is the easiest way to get started and most growers choose this option to ensure consistency and reduce the risk of soil-borne diseases. Common components such as peat moss, perlite, vermiculite, and coconut coir are acceptable for organic certification. Compost, being the most renewable, is the preferred material for many organic growers. Commercial mixes for organic production may contain a "starter charge" of organic fertilizer or no starter fertilizer at all. Check with your organic certifier to make sure your mix complies with standards. More information on growing media for organic production can be found in the publication, "Potting Mixes for Certified Organic Production"

Supplementing preplant fertilizers or compost with liquid organic fertilizers is generally required to provide adequate nutrition. One alternative is the use of fish fertilizers made from waste products of the ocean fish processing industry. These materials are thick, heavy liquid which are difficult to use with fertilizer injectors because the concentrate consists of very fine particles in suspension. Fish fertilizers can be a problem to store diluted because they become moldy and develop a strong odor. Growers may prefer to fertilizer less frequently, perhaps every two weeks or once a month because of this strong odor. The rate to apply will depend upong how often they are appled. Different fish fertilizers supply plant nutrients at varying levels of availability. Some may be stabilized with phosphoric acid, resulting in a high concentration of readily available phosphorus. Others contain liquid seaweed resulting in a higher concentration of potassium. Most fish fertilizers contain ammonium nitrogen, which as discussed previously, can be a problem for sensitive transplants such as tomatoes, peppers and eggplants. In New England, the Neptune’s Harvest Brand is the most commonly available fish fertilizer and it is OMRI-approved for organic production.

Some growers use Nature's Source Professional Plant Food 10-4-3 a liquid, “organically-based” fertilizer. The organic portion is oilseed extract. Most of the nutrients, however, are derived from inorganic salts and for this reason it cannot be used for certified organic production.

Several liquid fertilizers used for organic production are derived from plant extracts. The best-known of these has been Nature's Source Organic Plant Food 3-1-1, in which the nutrients are derived from “oilseed extract”. The container has dilutions rates expressed in familiar terms for greenhouse growers and has been recommended based on trials at UMass.

Several other liquid organic products are available, such as Biolink 3-3-3 (also an oilseed extract), Converted Organics 3-2-1 (a byproduct of grain fermentation), 1-1-1 Liquid Compost Concentrate and Verdanta PL-2, 2-0-6 (a liquid made from fermented sugar cane and sugar beet molasses). Verdanta would be used as a supplement to use in combination with other fertilizers used for organic production that are low in N or K.

Verdanta EcoVita 7-5-10 (granular) is composed of bone meal, soybean meal, cocoa shell meal, feather meal and fermented sugar cane and sugar beet molasses.

Mixing and application. The fish fertilizers and plant extract fertilizers are sold as concentrates and they must be diluted in water to be safe for plants. Nature’s Source, Bombardier, and Espartan have a pleasant “beer-like” aroma as concentrates, but within 7 days of being mixed with water they “spoil” and develop an unpleasant odors. The nutrient value of spoiled fertilizer is unknown and the colonies of bacteria that develop may plug irrigation lines, so diluted fertilizer solution should be used as soon as possible after mixing.

Fish fertilizer has the thickest and least consistent solution and should be agitated before mixing with water. Bombardier and Espartan concentrates are “syrupy” but mix well with water.  Nature’s Source is the thinnest concentrate and it mixes well with water and can pass through fertilizer injectors.

Sustane 8-4-4 and EcoVita are granular fertilizers mixed with the growing medium before planting. These are the easiest organic nutrient source to use in combination with the liquid types.

Fertilizer analysis. Some fertilizers used for organic production supply only one or two of the NPK elements; an example is Bombardier which is 8-0-0. A grower using Bombardier would have to use other fertilizer(s) to supply P and K. One possibility would be Sustane with an 8-4-4 analysis or some other complete NPK granular organic fertilizer.

Nutrient disorders. Plants may develop an overall light green or yellowed color caused by a general nutrient deficiency or more likely, N deficiency. For example, if Sustane is used alone the symptoms might occur about 45 days after planting, the end of its release time.  This can be prevented by applying an organic liquid fertilizer supplement about 30 days after planting.

Interveinal chlorosis sometimes occurs about halfway through cropping time if plants are fertilized with some liquid organic fertilizers alone starting at planting. This chlorosis is most likely caused by an accumulation of too much ammonium-nitrogen in the plant, so-called “ammonium toxicity.” Most greenhouse crops do best with a combination of ammonium and nitrate nitrogen. Unfortunately fertilizers used for organic production generally don’t contain nitrate-nitrogen. The best approach is to rely on Sustane as the sole source of nutrients for the first month after planting and then start applying Nature’s Source or another liquid organic fertilizer.

Organic fertilizer effects on growth medium soluble salts (EC). Sustane is a slow-release fertilizer and its use results in low EC, and potentially a deficient level after 45 days. As for the liquid organics, at the same N level the lowest EC results from Nature’s Source (similar to chemical fertilizer) and then Bombardier. Espartan results in an EC significantly higher than the other liquid organic fertilizers and this might be an aggravating factor in ammonium toxicity.

Use fertilizers for organic production with caution, on plants you know have exacting nutrient requirements or those prone to foliar chlorosis. Fertilizers should always be tried first on a small number of plants.


Cox D. 2016 Plant Response to Nature's Source and EcoVita Organic Fertilizers vs Plantex Chemical Fertilizer. July-Aug. Floral Notes 29(1).

Cox D. 2014. Organic Fertilizers - Thoughts on Using Liquid Organic Fertilizers for Greenhouse Plants. Sept.-Oct. Floral Notes 27(2)

Mattson N. Substrates and Fertilizers for Organic Vegetable Transplant Production. Cornell Greenhouse Horticulture, Cornell University.

Organic Greenhouse Vegetable Production, Potting Mixes for Certified Organic Production, Organic Greenhouse Tomato Production, Plug and Transplant Production for Organic Systems, ATTRA - National Sustainable Agriculture Information Service.

The National Organic Program Guidelines:

Organic Potting Mix Basics: eXtension.