Most of us know that plants create their own food through photosynthesis, in the presence of light and chlorophyll, carbon dioxide and water are combined to make sugar and oxygen. Sugar is then converted to starch in order to store the energy. Plants also need various macro and micronutrients derived from the soil in order to thrive, bloom and reproduce. The three most important macronutrients are nitrogen, phosphorus and potassium or N-P-K, the nutrient analysis ratio you see on all fertilizers.
Nitrogen, the first number in the ratio, is necessary for chlorophyll formation, leaf growth and gives plants their green colours. Micro-organisms in the soil also use nitrogen to break down organic matter. That is why you will see a high N number on turf fertilizers. Nitrogen stimulates leaf growth over flower production so you don’t want to fertilize your roses with a high N content or you will get lovely big bushes with no flowers! If you like to use a lot of mulch on your garden beds, make sure to add some extra nitrogen because as the mulch breaks down, it uses up nitrogen and can cause a nitrogen deficiency in the soil. This is seen by the leaves turning a pale green, then yellowing and dropping off.
Phosphorus, the second number in the ratio, is needed for root development, and flower and fruit formation. All transplant solutions will have a high P number to reduce transplant shock and stimulate healthy root growth. Flowering plant and shrub fertilizers should also have a high P number than N or K for optimum flowering. A phosphorus deficiency show up as dark green to purplish colouring on leaves and stunted flowers.
Potassium, the third number in the ratio, promotes plant vigour and disease resistance, and helps plants harden off to better withstand heat, drought and especially winter cold. Fall fertilizers for both lawns and plants will often contain a high K number or only potassium to slow plant growth and prepare for winter dormancy. A potassium deficiency shows up as dead spots along the edges of leaves and in between the leaf veins. Other macronutrients include calcium, magnesium and sulfur and are usually listed on the bag.
Micronutrients such as molybdenum, copper, zinc, magnanese, boron, iron and chlorine are needed in much smaller amounts and are supplied through the breakdown of organic matter such as compost and manures. They are only available once the organic matter has broken down into very fine particles so never add fresh manure to your garden. It will burn your plants and your neighbours will hate you! A healthy soil rich in organic material will contain all the necessary micronutrients, smaller amounts of the macronutrients and should be dark, friable, retain moisture and have that wonderful earthy smell 🙂
Next blog: synthetic fertilizers vs. organic fertilizers and how to use