Feature Article

Nutrition of Spring Plants

by Sheilah Graham M.Sc. AGR
US Sales Manager, Plant Products, Co. Ltd.

Despite the cold, blustery, snowy weather we've been experiencing this winter growers all over the country are gearing up for the quickly approaching 2011 spring season by planting in the dead of winter inside greenhouses. Spring annuals are a quick crop, for the most part, and are an overwelmingly welcome sight at many operations. Many of the species and varieties being marketed and grown are received as plugs in February and early March and grow rapidly. Certain varieties may finish in 8-10 weeks based on the pot size and others may take as long as 20 weeks. However, considerations need to be made on how to provide spring annuals with optimal nutrition.

Generally speaking, spring crops may be divided into different categories based on temperature requirements, soilless media pH levels, EC levels and light levels. However, many growers are not equipped to produce one species (monoculture) or group in a specific greenhouse area due to space and time restraints and secondly, some plant species belong to two or more groups based on the criteria above. It’s quite normal to walk into a greenhouse facility with zonal geraniums being grown next to calibrachoa with petunias and mixed baskets hanging overhead. How do we coordinate a feed program to provide optimal nutrition in this arrangement? 

One of the most widely adopted practices is to use a general purpose feed for most of the crops through the main injector, and then use a custom feed program for those plants needing specific nutrients or pH adjustment. This is where a portable injector, such as the Dosatron, comes in handy. Growers prefer to use the fewest number of different analyses to keep the program simple and practical for use. Characteristics of your irrigation water will determine which general purpose feed you should choose.

The main properties of your water that need to be examined are:

• Alkalinity (mg of Calcium Carbonate/l)
• pH of the water
• Concentration of Calcium and Magnesium and their ratio to each other
• levels of Boron and Sodium

One commonly used general purpose analyses is Plantex® 20-10-20 High Nitrate. The advantage of Plantex ®20-10-20 is that it is acidic and will decrease the pH in the root zone if the alkalinity of the water is between 0 and 250 mg CaCO₃/l. It is composed of 12% Nitrate Nitrogen and 8% Ammoniacal Nitrogen. Higher levels of Ammoniacal nitrogen are not recommended for early productions, especially during low light and cool temperatures. Too much ammonia results in a toxicity since nitrifying bacteria in cool temperatures are unable to convert Ammoniacal nitrate to nitrate and plants absorb the excess ammonium. This will be seen as an edge burn on leaves. 

For 20-10-20 to be the optimal general feed, it is recommended that water samples that have over 50 ppm of Calcium dissolved in the water and at least 15 ppm of Magnesium. Plantex® 20-10-20 is not recommended as the general purpose feed when Mg and Ca levels are low. There has recently been discussion about the amount of Phosphorus required by annual plants and the amount being provided in water soluble fertilizers. Additional P is absorbed by plant roots and may result in late season internodal stretch. This may be avoided with the use of blends containing lesser amounts of P such as Plantex® 20-7-20, 21-5-20 or 20-2-10.

Most Popular

The blend most commonly used as a general purpose feed in 2010 was Plantex® Solutions 17-5-17 Complete. 17-5-17 is a good general purpose feed for water alkalinity levels between 75 and 150 mg/l CaCO₃. It is called Complete because it's composed of raw materials that provide Nitrogen, Phosphorus, Potassium, Calcium and Magnesium as well as a chelated micronutrient package. The 3:1 Calcium:Magnesium ratio encourages compact growth, especially during dark weather periods. It has a lower amount of phosphorus than 20-10-20 resulting in more

controlled growth and the nitrogen is provided by 30% ammonium nitrate and 70% nitrate nitrogen. Most Cal-Mag fertilizers are basic in reaction and will increase media pH. However, the acidity potential of this blend is 106 lbs CaCO₃/ton, which means that it will cause the pH of the media to decrease slightly with this water type but not drastically. In most situations, the media pH is not affected by this blend.  One observation to make about this blend is that it comes in our unique patented “duo” bag. The full bag must be used upon opening and the full bag must be dissolved in one container of water.

The third general purpose feed of choice is our Plantex® Solutions Complete Plus 19-8-13. 19-8-13 is recommended for water alkalinity between 150-200 mg/l CaCO₃. It has a 2.5:1 Calcium:Magnesium ratio contributing to compact, short internode growth and has 2% less Phosphorus than 20-10-20.  Complete Plus is more acidic (323 lbs CaCO₃/ton) than Complete 17-5-17 and is recommended for operations where the grower would prefer not to add sulfuric acid to adjust media pH. It also has a higher micronutrient package than 20-10-20 and 17-5-17. 

The Iron is chelated with EDTA, DTPA and EDDHA chelates, allowing for greater absorption at higher pH levels. The amount of Iron is increased to 0.25% and the amount of chelated Manganese and Zinc is elevated to 0.075 ppm from the standard 0.05 ppm. Complete Plus should not be used by growers who have issues with pH dropping on crops such as zonal geraniums, marigolds, lisianthus and New Guinea impatiens. This blend works particularly well for all vegetative annuals which require a higher level of nutrition, including vegetative petunias, calibrachoa and many others.

The Finishing Touch

Once the general analysis has been chosen, the grower can custom feed the crops that have special requirements. For example, petunias, calibrachoa and gerbera all have higher needs for iron to prevent chlorosis and decrease the amount of root disease.  Although some growers refer to this blend as a “petunia feed”, it can be used on most vegetative annuals that are heavy feeders. These crops should be fed with Plantex® Solutions 19-2-19 Fertility Plus at least twice during their production or more, depending on the water quality. 19-2-19 contains a higher amount of Iron chelate than 20-10-20 or 17-5-17 and the element is chelated with three different chelates, EDTA, DTPA and EDDHA. 

This makes the iron available over a broad range of pH values. Manganese and Zinc are also competing with the uptake of Iron and therefore the amount of these elements has been increased in this blend. Magnesium is at the center of the chlorophyll molecule and plants need Mg to stay green. The Mg in 19-2-19 is 1.5%, higher than 20-10-20 and 17-5-17. We recommend that 19-2-19 be applied as the first drench when the plugs are transplanted and then again every 2-3 weeks or more frequently, depending on the water quality involved. Additional iron treatments from other sources are not needed if 19-2-19 is used. These crops will start out green and stay green during production when following these guidelines.

Please visit our AHS Plantex page, the Plant Products site or call your AHS representative for more information to stay green.