Which Type of Fertilizer Should You Choose?

By Dr. Tahir Mahmood

There are several types and brands of liquid and powdered/granular water soluble fertilizers available in the market for plant growth in soil/soiless mediums. These products have different or similar N.P. K. (Nitrogen, Phosphorous, and Potassium) concentrations and are recommended for different or the same plants but with different application rates. This difference in N.P.K. concentrations on labels does not mean anything in terms of quality of that fertilizer. A low N. P. K. number product has recommendations with a high application rate while high in N.P.K. product having a low application rate. Every product claims to increase production without having any negative effect on plant growth. But the quality and selection of a suitable fertilizer depends on several factors such as;

• Composition of product (Nitrogen, Phosphorous, Potassium concentrations along with all other essential nutrients)
• Raw material used in manufacturing
• Ratios of different nutrients
• Ratios of different forms of nutrients

Composition

Composition means whether the fertilizer (One part/Two part/Three part) is a complete fertilizer and has all the major and micro nutrients required by plants. Secondly, it means how much that product has of the concentration of these nutrients in that fertilizers mix. As mentioned earlier, concentration does not really matter because one can increase or decrease the application rates. The important thing that needs to be considered is to look into the composition of the product to see if that product really is a complete fertilizer. Does that product have all the essential nutrients required by plants? If the product selected for use has all the required nutrients essential for plant growth and if it meets the minimum requirement by following the application rate, then that product can be selected for use on crops. To know whether that application rate can fulfill the plants requirement or not, you can determine this by doing a very simple calculation.

Example: If a fertilizer product has 5% Nitrogen and the application rate is three ml/liter, that fertilizer will yield 150 ppm final Nitrogen concentration in one liter of water. This 150 ppm in the final solution does not meet the requirement of a plant in the growing phase because the ideal Nitrogen concentration required during the growing phase is 230-280 ppm. To get the Nitrogen concentration in this range one has to use fertilizer with 5% Nitrogen at five ml/liter of water. This will yield 250ppm of Nitrogen in the final solution.

To calculate how much a fertilizer will yield the final concentration of any nutrient, multiply the percentage of any particular nutrient with 10 and the application rate. This will give you an idea as to how much your fertilizer will yield that particular nutrient in a final solution. This can be done with all the nutrients listed on the label.

Sometimes you will find more ppm in the final solution than the calculated one. This will be because of a couple of reasons. Concentration of nutrients often mentioned on the label are lower than the actual in order to make sure that the grower will get the minimum number of ppm in the final mix.

2. Impurities of the raw materials used and other non listed nutrients on the label will also increase the final ppm number.

Here are the general ideal limits of major and micro nutrients required by plants in hydroponic mediums for your reference.

Raw Material

The materials used in the process of manufacturing a hydroponic fertilizer system is also very important. Most companies use water soluble (horticulture grade) straight or compound fertilizers for the manufacturing of the fertilizers. Even among water soluble there are two categories; one is of best quality with less concentration of toxic heavy metals (Lead, Cadmium, Mercury, Chromium etc) and the other of poor quality with high levels of toxic metals. The presence of toxic metals can have a negative effect on plant growth and especially, their accumulation in fruits and vegetables will result in many health problems in human beings and animals. To avoid using a higher concentration of these heavy metals always confirm the heavy metal contents of the product with the manufacturer before making any decision to purchase the nutrients. Well recognized companies have those metal values available on their website or a person can get these values by request by phone or e-mail.

Secondly, the use of chloride based raw materials for manufacturing will result in toxicity of chloride that will lead to several malfunctions in the plant body. Chloride has an antagonistic relationship with Nitrate Nitrogen. Higher concentration of Chloride in the medium will inhibit the uptake of Nitrate that will immediately lead to poor plant growth. Similarly, for micronutrients, it has been recommended to always select a fertilizer which has chelated micronutrients. Never compromise on this and always use a product that has micronutrients (Iron, Copper, Manganese, and Zinc) derived from a chelated source, no Sulphate. The Sulphate form of these micronutrients can be held by soil or can react with other ions in the solution that lead to unavailability of nutrients for plants. Chelated micronutrients are expensive compared to Sulphate sources of nutrients but their quality and availability to plants is not comparable. Chelated micronutrients can never be bound by the soil particles and never become unavailable by reaction with other ions in the solution. It’s better if you find some products that use Calcium and Magnesium chelates as a source of Calcium and Magnesium and some products that use natural chelating substances for manufacturing of liquid fertilizers like Humic acids.

Ratio of Different Nutrients

As mentioned earlier in the table, (ideal range of nutrient required for plant growth in hydroponics) all the nutrients are not required in equal quantities. Some of them are required in large quantities while other in low quantities. Secondly, it is just not the matter of too much or too little but the relationship of one to another that will ultimately determine whether the plants can take up either of the nutrients. Different ratios between different elements are being recommended in fertilizers used for hydroponics system like K:Ca, K:Mg, Ca:Mg, Fe:Zn etc. the ideal ratio between these nutrients is important in solution used for feeding plants in hydroponics systems. The most important ratio that is often recommended is the Calcium to Magnesium ratio which is of critical importance in fertilizers used for feeding hydroponically grown plants.

In general Ca:Mg ratio of 3:1 is required for better plant growth. This ratio varies a bit for different species of plants but the bottom line is that minimum Calcium to Magnesium ratio in any fertilizer should be 3:1, especially fertilizers being used in hydroponics.

As long as the ratio stays in this range and there is an adequate supply of each nutrient from that fertilizer, plants will continue to pick up both Calcium and Magnesium from the medium. Fertilizers with ratios substantially different from the required ratio will result in some deficiencies of nutrients and will lead the plants to stress conditions, especially when the plant is prone to any stress such as disease or pests.

Ratio Between Two Forms of Same Nutrient (NO3:NH4)

As discussed earlier, the selection of a suitable fertilizer depends on many factors such as concentration of major and minor nutrients, the quality of raw material being used, the ratios of different nutrients as well as the ratio between different forms of a nutrient in the fertilizers etc. All the above mentioned points have their own individual importance, but the most important among these is the ratio between different nutrients and different forms of the same nutrient. (Ratio between different nutrients discussed earlier)

For example the best ratio of NO3-N (Nitrate) VS NH4-N (Ammonical) in any liquid fertilizer used for plant growth is 9:1 because most of the Nitrogen taken up by plants is in a nitrate form and a very small portion is taken up in an Ammonical form. Secondly, fertilization with more NH4-N VS NO3-N can reduce the volume of total plant growth. In general, NH4-N fertilized plants will produce smaller darker green foliage compared to fertilizing with NO3-N. This is due to N form effects on photosynthesis and N assimilation. When NH4-N is taken up by the plants, it must immediately assimilate into amino acid, a process requiring carbohydrates/photosynthates (substances produced as a result of photosynthesis). Without sufficient carbohydrates free NH4-N can be toxic to plants. While on the other hand, when NO3-N is taken up, it is reduced to NH3 and assimilated into amino acid. However, if sufficient carbohydrates are not available then NO3-N is stored in the vacuoles (storage house for salts used for build up of osmotic pressure) of cell until carbohydrates became available. This means NO3-N unlike NH4-N fertilization will never tie up available carbohydrates at the expense of plant growth.

These are some very simple but important points that can help in the selection of a fertilizer product/fertilizer feeding program for the optimum growth of plants. Give it a try and you will see the difference in growth of your plants after selecting such fertilizers.

Dr. Tahir Mahmood

Director Research and Development

Grotek Manufacturing Inc