22/09/2025
Spring is coming and thoughts have turned towards the garden. I am often asked about which fertilizer is best for what plant so, I thought I’d give a brief introduction to the different nutrients contained in fertilisers to help you decide on what to buy!
Plant nutrients can be split into macro, secondary and micronutrients. The macro nutrients supplied though the soil are Nitrogen ( N) Phosphorus (P) and potassium (K). All of these must be supplied in fairly decent quantities to maintain healthy growth. These nutrients are listed on fertilizer packets as NPK values. They can be supplied chemically (made in a laboratory) or organically from animal, plant and mineral resources such as manures, composts, blood and bone, fishmeal and naturally occurring mined minerals such as sulphate of potash. In addition, these inputs can be ‘Certified Organic’ which guarantees that the inputs used are free of chemicals such as wormers and herbicides, or contaminants such as cadmium. Organic fertilisers also add food to the soil for beneficial fungi and bacteria which also helps plants and soil to remain productive and healthy.
The secondary nutrients are Calcium (Ca), Magnesium (Mg) and Sulphur (S) and are needed in moderate quantities. Again, they can be supplied chemically or organically. Dolomite lime, mined here in the Golden Bay, is a combination of magnesium and calcium carbonate and AgLime is just calcium carbonate. Which kind of lime you need can be determined by a soil test, but if you don’t have a test, I usually recommend Dolomite. Lime is also used to raise the pH of acidic soils. Most plants prefer a pH of around 6.5 to grow well. Sulphur is most commonly applied as part of another compound such as sulphate of potash, but it can be applied on its own as a powder.
Iron, manganese, boron, zinc, copper and molybdenum are micronutrients which are needed in very small quantities by plants, much like we need vitamins to function correctly. Most of the micronutrients are essential for forming various enzymes within the plant which perform chemical reactions within cells, without them plants cannot grow properly. Again, these can be supplied chemically (laboratory synthesized) or organically from mined sources. They need to be applied very carefully as excess application can poison soils. Most soils in Golden Bay are deficient in all of these micronutrients, except for iron, which in some areas is very high (Ironworks Road)! Seaweed is a good way to add micronutrients, but it is not a complete fertilizer as it is lacking in most of the macro nutrients listed above.
We stock a range of chemical and certified organic fertilisers plus various composts, soil conditioners and liquid feeds, including chloride free formulas suitable for Avocados. Pop in and have a chat with me to see how we can help your garden grow!
Jay :)