Watermelon: Magnesium will make your watermelon crop sweeter! | Happy Valley Seeds

Magnesium will make your watermelon crop sweeter! 

Preparing your soil

I always say that the number one mistake that beginner gardeners make is insufficient watering. Most people underestimate the amount of water than plants need.  When it comes to growing watermelon, I cannot emphasise the amount of water you need to feed your plants. It's called a water-melon after all! 

To assist you, I recommend your garden soil is made up of one third coconut coir. It's nature's water retainer, and will absorb moisture. This prevents the water you feed your plants from draining away and makes it accessible to the roots over the day. 

Improve your soil with manure, compost and rock minerals. Blood and bone is another good source of nutrients for your garden.  You need to add a good amount of potassium to the soil as this is the nutrient which encourages fruiting. Add banana peels under where you will plant your Watermelon, and they will break down over the season and feed your plant. Alternatively you can purchase potassium fertiliser which is in a form which the plant can feed on immediately.  

Mulch straight away. If you plant with seeds, do not cover the area above your seed with mulch, or it may prevent the seed from sprouting. However, cover the soil around the seed, and once sprouted, mulch again. Mulch should be a minimum of 3cm thick. You can also mulch with compost or coconut coir.  

Water in your new watermelon with a seaweed solution and magnesium water (details below). 

Companion planting

The three main groups of plants which a watermelon benefits from being planted near are alliums, flowers and herbs.  The allium family which includes onions, spring onions and garlic, are a strong scented plant which deter pests. I often advise people to plant these around the border of a garden bed.  Flowers, such as marigolds, nasturtiums and lavender, and herbs such as dill, mint and oregano, have flowers which attract beneficial insects especially bees which help the watermelon to pollinate.  Some of these like the nasturtiums have the added benefit of attracting pests to eat them instead of your crops, and are also edible themselves.  Do not plant your watermelon with other cucurbits or nightshades (tomato and potatoes) as they compete for the same nutrients.  


Your watermelon will benefit from a seaweed concentrate and a fish emulsion alternating fortnightly.  I recommend this for all vegetables and have reminders on Facebook and Instagram for Fish Friday and Seaweed Saturday.  They provide essential nutrients for plants including nitrogen and phosphorous.  

Magnesium is also an essential nutrient that all plants need and it makes your watermelons sweeter! It is easy to supply them with it by adding half a cup of Epsom salts to 10L of water and watering your garden once a month.  


Stop watering your watermelon two weeks before harvest for a sweeter fruit.  There are two ways to tell when your watermelon is ripe.  The first way is to tap the fruit. If it makes a hollow sound it is a sure sign your Watermelon is ready to be harvested. But some varieties do not make a hollow sound. Therefore the second way to know is to harvest when the vine has died off. This means the watermelon is no longer being nourished by the vine and so can be picked.


Author: Diana Barnes, Growing Vegetable Down Under

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