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Growing Guide

Fertilising your garden

Plants need to be fed just like we do so check out our handy guide to getting it right for your garden!

​Fertilizing your garden.

Written by Emma Urquhart

As I explained to my kids one day when they questioned why I was sprinkling 'blue dots' on the garden, plants need to be fed just like we do, but they can't go to the supermarket to get what they need, instead we get it for them.

If you have added lots of compost and our slow release fertiliser when you prepared your garden bed then you may find that your plants have all they need until harvesting. However this isn't always the case because your soil type, climate and your planting and watering habits will affect how many nutrients are available in your soil. Light, sandy soils often leach out nutrients quickly, especially when lots of water is applied during hot days. Densely planted garden beds will also use up more nutrients from the soil.

I am a huge believer in monitoring your plants as they often can tell us what is happening in the soil and what they are lacking. Most nutrient deficiencies can appear on the leaves. Plants lacking in Nitrogen get pale green-yellow leaves and stunted growth. Potassium deficiencies show browning or purple tones on the leaf and Phosphorus deficiencies show yellowing around the leaf veins. Do take note that there are other factors that can cause the same or similar discolouring of the leaves, such as sap-sucking insects, viruses, cold weather or wind damage, so take this all into account before deciding that you have nutrient deficiencies. Soil tests are the most accurate way to find out what you may be lacking, taking out all the guesswork and can save you a lot of time and money.

You will often see N-P-K on your fertiliser packets, this refers to the ratio of Nitrogen, Phosphorus and Potassium respectively. These three elements are responsible for plant growth and are needed in large quantities. Nitrogen is found in all green materials and manure which is all often added to your compost. Adding Nitrogen-rich compost or fertiliser has the most dramatic effect on crops by producing lots of green leafy growth, making it ideal for all your leaf vegetables. Phosphorus is also present in manures, especially cow, or can be applied as a chemical additive – superphosphate. Phosphorus is important for new shoot and root growth. Potassium controls water uptake and is important for photosynthesis. Root vegetables require higher quantities of potassium than other vegetables. Pot ash, wood ash and manures also contain high amounts of potassium.

Trace elements, Zinc, Magnesium, Sulfur, Boron, Manganese, Molybdenum are all required for plants to build vitamins. All these elements are present in manures and seaweed, so a well composted soil shouldn't lack trace elements.

Now, there is a catch with all these nutrients, you may have applied them all as required and still have poor looking plants. Your soil pH is the key! Available minerals/nutrients get locked up in the soil if the pH is too acidic or alkaline, the optimum range for nutrient availability is a pH 5.5 -8. Outside of this range most of your applied mineral will be unavailable to plants. (Note that there are plants that thrive in pH levels outside this range, like blueberries that prefer a pH of 4.5, they still get what they need at this pH level). pH testers are available at most garden stores and are far more accurate than putting soil in a cup with vinegar or backing soda.

Lime (Calcium) is a great soil conditioner, it creates soil structure in heavy clay soils, releases locked nutrients and plays a vital part in plant metabolic activities. Lime is effectively added to the soil to increase pH levels.

You can ensure your garden is getting everything it needs by creating a well-balanced compost. For this you can add lime for Ca, wood ash of K, manures for N-P-K and trace minerals and Epsom salts for Mg and S.

We also have a couple of different fertilisers that contain different ratios of N-P-K depending on what part of the garden you are wanting to apply it to. Our fruit and flower fertiliser has a higher percentage of Potassium to help the plant flower, and set fruit. Whereas our vegetable and ornamental fertiliser has a higher percentage of Nitrogen to increase the leaf quality.

There is no one way to fertilise your garden because as I mentioned above, soil type and climates affect your nutrient loss. For example, I don't add fertiliser to my main vegetable beds or my flowering gardens during the growing season because I use large amount of lucerne mulch, manure and compost (which has some fertiliser granules tossed every few months already). My soils are heavy with very little rain. An accumulation of salts is more of a problem than leaching. However in all my plant pots and tubs I fertilise every two weeks with a liquid fertiliser because these get a lot more watering and have free draining soil.

20 June 2024