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Vegetable Bedding Plant Fertility Program Print E-mail

There are numerous factors affecting the growth of vegetable bedding plants. Two factors which have a dramatic effect on growth are the watering and fertilization programs used in the greenhouse.

Water Quality: It is advisable to have your water tested prior to the spring growing season. Your greenhouse fertilizer program should be adjusted according to test results.

Watering: The amount of water and frequency of watering will vary depending on container size, growing media, greenhouse ventilation and weather conditions. It is important to water thoroughly, to moisten the entire container, which will promote root growth to the bottom of the container. If this is not done, root growth will develop in the upper part of the container and plants with be more prone to drying and drought stress. Allow plants to dry down before watering, but do not let the plant wilt severely, as this will damage roots. Vegetable bedding plants should be watered thoroughly early enough in the day to allow foliage to dry before evening. If foliage remains wet overnight, foliar disease problems will occur.

Choosing Fertilizers

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

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

15-15-15 Geranium Special. "Triple 15" is a good alternative to the Peat-Lite Specials for crops sensitive to trace element toxicities. Trace element levels supplied by this fertilizer are lower than the Peat-lite Specials. Otherwise, at the same rate of N, plant response will be very similar to 15-16-17. This is an acid-forming fertilizer also; the potential acidity is slightly higher than 15-16-17.

20-20-20 General Purpose. Growers who use this fertilizer on soilless media risk ammonium toxicity because the N 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 growing medium should be tested frequently for ammonium. 20-20-20 supplies trace elements and has the highest potential acidity of fertilizers commonly used in New England greenhouses. Note that tomato, eggplant and pepper plants are especially sensitive to ammonium.

Low Phosphorus Fertilizers (20-0-20, 20-1-20, 15-0-15). These fertilizers can be tried as an alternative to chemical growth regulators for vegetable bedding plants. 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 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 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 2-3 week basis to supply Ca and to counter the acidic effect of the Peat-lites. 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.

Nitrogen, Phosphorus, Potassium

Nitrogen. Nitrogen concentration in the greenhouse fertilizer program has a greater effect on the growth of transplants in the greenhouse than the other two major nutrients. Increasing the level of nitrogen results in taller transplants with thicker stem diameters and heavier plant weights. Applying too much nitrogen in the greenhouse results in soft, poor quality transplants. These lush transplants may also be more prone to phloem-feeding insects such as aphids.

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: Potassium has the least effect on the growth of plug tomato transplants of the three major nutrients. Adequate potassium is applied as part of a complete fertilizer.

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 excess soluble salts; failure to provide enough fertilizer during periods of rapid growth will lead to deficiency.

General fertilizer guidelines

Have your soil tested each month to adjust your fertilizer program and to prevent problems.

Optimum pH range for vegetable bedding plants: 5.5 to 6.5.

While plants are in the plug or seedling stage, use a complete water soluble fertilizer at the rate of 50 – 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 in-between each fertilization. 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 because no growth regulators are labeled for use on edible crops.

Nutritional Problems

Vegetable bedding plants are subject to the same nutrient disorders as other plants. Early in production serious 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.

Soluble Salts. Injury to bedding plants from excess salts seems to be most common shortly after transplanting. Seedlings are much less tolerant 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 which 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 soil test is advisable. Check with your Soil Testing Laboratory or Extension Specialist for information on soil testing.

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 – 6.0 for susceptible crops and by the use of fertilizers with lower trace element levels.

Ammonium toxicity. This is less common today because most growers use water-soluble fertilizers that supply about 50/50 ammonium and nitrate to fertilize plants in soilless media. Tomato, eggplant, and pepper are most sensitive to ammonium nitrogen, but many other bedding plants can be harmed if ammonium becomes excessive. Too much ammonium during the early spring (February or March) in low light and cool media conditions can be toxic to plants.

Organic Vegetable Bedding Plant Fertility

The quality of the planting mix is important to insure proper plant health in organic production. Conventional growing media that contains synthetic ingredients cannot be used in organic production of field transplants, container plants, and greenhouse crops. However, acceptable growing media can be composed from a wide variety of approved materials. These organic blends 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 organic mix is the easiest way to get started and most growers choose this option to reduce the risk of soil-borne diseases. Common components such as perlite, vermiculite, and coconut coir are acceptable for organic certification. Compost, being the most renewable, is a preferred material for many organic growers. NOTE: Check with your certifier regarding the status of peat moss.

Dr. John Biernbaum, from Michigan State University, for his research, chooses a 50/50 mixture of peat and compost with a pH of 6.0 as an organic potting media. He makes his own compost from garden waste, straw, hay and sheep/horse manure and screens the compost to provide a uniform product. This is just one of many options, however the finished compost product needs to have good physical, chemical and biological properties.

There are also many fertility management options such as supplementing with liquid organic fertilizers or using a growing media that contains sufficient amounts of major nutrients to grow the transplants while they are in the greenhouse. In his studies, Dr. Biernham used liquid fish emulsion (5-1-1) as a sole fertilizer with a soilless peat based medium and coconut coir successfully for several months. No synthetic fertilizer was used.

Fish emulsion fertilizers are likely to have an odor and fertilizing less often may be preferred, for example, every two weeks or once a month. The rate applied will vary depending upon how often one fertilizes. Plants fertilized with organic fertilizers will not show the rapid growth response seen with synthetic fertilizers. With this method, John suggests beginning with a growing media containing 60-70% peat and 30-40% perlite and/or vermiculite without fertilizer and wetting agent, but limed to a pH of 5.5 to 6.5. Apply the water soluble organic fertilizer as needed, usually soon after seed emergence or at transplanting.

Another option recommended by Eliot Coleman, from Four Season Farm, Maine is to use a blend of peat and compost or peat-based soilless medium and compost mixed with 14 lbs. per cubic yard of equal parts blood meal or alfalfa meal for nitrogen, rock phosphate for phosphorus and green sand or organic approved potassium sulfate for potassium. He lets the blend sit for a month or more before use.

A third option is to use mature, well-balanced compost blended with peat and possibly perlite and/or vermiculite for aeration to supply all the nutrients to grow and finish transplants. Nutrient sources such as alfalfa, alfalfa meal and other organic approved components can be incorporated during the composting process. Compost will mature during the fall and can be stored for use until spring. These last two options add some nutrients to the root medium, so plant nutrition is not dependent on liquid fertilizer. However, these options require purchasing the organic nutrient sources and making sure the rate and method of application are correct. Check with your certifier before using mixes or ingredients.

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