How Does Fertilizer Work?

In this post we are going to show you how to figure out the nutrient concentration in a bag of fertilizer. At the end you will know what the numbers on the front of a bag mean and how they will influence plant growth. If you would like your own copy of this information, feel free to download our eBook!

Important Nutrients for Plant Growth

Nutrients are classified as either macro or micronutrients. Plants require higher quantities of macronutrients and lower amounts of micronutrients. Examples of macronutrients include nitrogen, which is important for amino acid and protein synthesis, phosphorous, which helps form cell membranes, and potassium, which is important for carbohydrate synthesis and helps plants fight infections. Other important macronutrients include calcium, magnesium, and sulphur. Iron, manganese, copper, zinc, boron, molybdenum, and chlorine are examples of micronutrients.

Straight vs Mixed Fertilizers

Based on the nutrients they contain fertilizers can be classified as either straight or mixed. Straight fertilizers will contain only 1 nutrient, such as urea, which provides the plants with nitrogen only. Complete fertilizers contain all the 3 major nutrients: nitrogen, phosphorous and potassium. Complete fertilizers are most commonly available in dry formulations in bags like this. However, there are so many different mixtures and concentrations, how do you know which fertilizer is best for you?

Interpreting Fertilizer Bags

First, you need to know what the 3 large numbers on the front of the bag mean. These numbers describe the ratio of the nutrients present in the mixture. The first number refers to the nitrogen concentration, the second to phosphorous and the third to potassium. The number in brackets describes the total amount of nutrients present in the bag as a percentage. For example, if the number in brackets is 20, then in 1kg of that fertilizer there is 200g of nitrogen, phosphorous and potassium combined. The remaining weight is made up of other constituents, like lime. 

n p k fertilizer

Which is Best for You?

So, once you know how to interpret the ratios of fertilizers, you need to know which option will suit you best. As we mentioned before, the different elements will influence a plants growth in different ways.

Nitrogen stimulates vegetative growth in a plant and applications thereof will promote leaf development. Therefore, fertilizers that have a higher proportion of nitrogen will be suitable if you want to encourage overall growth in your plants. Nitrogen applications have a greening effect because nitrogen is an important constituent of chlorophyll, which makes plants green. Nitrogen deficiencies therefore commonly show as yellow leaves.

Phosphorous is especially important for root development in plants. Fertilizers with a higher phosphorous concentration are best when you want to encourage root development. These are perfect when laying new lawns so the grass can establish a healthy root system. Phosphorous deficiencies can present as dark green leaves and purplish stems. The pH of a soil will influence the amount of phosphorous available to plants, so it is important to test your soil if you notice any phosphorous deficiency symptoms.

Potassium promotes flower development and encourages overall health in plants. Fertilizers with a higher potassium concentration are commonly applied to established plants after sufficient vegetative growth has occurred. These fertilizers can be applied to flowering plants for high quality, vigorous blooming and can even infer a degree of disease resistance in the plants. Malformed or absent flowers or fruit can be a symptom of potassium deficiencies.

Calculating Nutrient Concentrations

Fertilizer bags will commonly give the exact amount of nutrients in the mixture. However, you may still need to calculate this yourself if they do not provide this information. So how do you do that?

Firstly, add up the 3 numbers on the front of the bag. This will give you the total amount of nitrogen, phosphorous and potassium present. Secondly, for each element divide its number by the total of the 3 numbers. Lastly, multiply this number by the percentage in the brackets. You will then be left with the percentage of that element in the entire bag of fertilizer.

Leave a Comment: