Seaweed vs Kelp Fertilizer – The Complete Lowdown

Fertilizer can be expensive, and choosing the wrong thing not only leaves you out of pocket but might also not do your garden much good. If you prefer a natural fertilizer, you might have considered the use of seaweed – or kelp – help! Just what is the difference, anyway? Here’s our complete guide to seaweed and kelp natural fertilizers. 

What are fertilizers?

Fertilizers can help to correct the balance of nutrients in the soil. Image: Jing/Pixabay

Fertilizers are concentrated sources of plant nutrients, like giving your garden a vitamin pill. The commercial kind come in some sort of easily distributed medium such as pellets, liquid, powder or granules. They’re used to improve yields and can help to correct soil imbalance. 

Just like humans, plants need a certain ‘diet’ that’s nutritious enough to support growth. Usually, they need three nutrients in large amounts – they are nitrogen (N), which promotes leafy growth; phosphorus (P) which encourages healthy roots and potassium (K) for blooms and fruits. Commercial fertilisers should display their N:P:K ratio on the packaging. A good all purpose fertilizer will have equal portions of N, P and K. Specialist fertilizers, such as those for roses, will have different ratios. 

You can choose from two main types of fertilizer – organic and inorganic. Inorganic fertilizers are man-made and are normally very efficient. Organic fertilizers are made from various types of organic material such as bone meal or animal manure…and, of course, seaweed. They tend to be slower-acting, but many gardeners prefer them to using a chemical solution. 

Seaweed fertilizer

Coastal gardeners have been using seaweed to improve soil and aid plant health for generations. Seaweed contains the three major essential nutrients (nitrogen, potassium and phospurus) required for healthy plant growth, plus a number of trace nutrients. Fertilizer products made from seaweed are often referred to as growth regulators, as they help boost the growth rate of plants. 

Seaweed is becoming an increasingly popular choice as it’s natural, renewable, sustainable and contains no animal products. You can buy pure seaweed extract in various forms in the garden centre, or choose a fertiliser with seaweed as an added ingredient. 

Kelp fertilizer

'Kelp' is also a generic name, but for a sub group of marine plants. Image: Monicore/Pixabay
‘Kelp’ is also a generic name, but for a sub group of marine plants. Image: Monicore/Pixabay

OK, here’s where it starts getting a bit tricky – while all kelp is seaweed, not all seaweed is kelp! ‘Seaweed’ is a generic name, while ‘kelp’ refers to a specific group of marine plant. 

Kelp is a type of brown seaweed that grows on rocky coastlines and tends to be higher in trace minerals, such as calcium and iodine, than other seaweed. Plus, it contains biostimulants which may help to encourage plant growth. Most commercial seaweed products don’t state the type of seaweed used – if you’re keen to use a pure kelp fertilizer, you’ll probably be looking at a premium organic product with a price tag to match. As kelp is a natural product, the N:P:K content won’t be standardised as it will be on a commercial product. 

Should I use seaweed fertilizer?

Tomatoes in particular love seaweed fertilisers. Image: kie-ker/Pixabay

Seaweed (and kelp) is safe to use on almost all plants – tomatoes in particular love it. Unless you live by the sea and have access to your own stretch of beach, it’s probably best to buy it in a commercial form – with the added advantage that the manufacturer will have checked and adjusted the N:P:K content so it has the best levels of trace elements to aid the plant hormones. 

What forms are kelp and seaweed fertilizer available as?

Kelp and seaweed garden fertilizers are commonly available in a choice of formats, so you can choose the one most appropriate to your gardening needs. For example, your local garden center may stock these types of fertilizers:

  • Kelp powder fertilizer 
  • Liquid kelp fertilizer 
  • Dried seaweed
  • ​Seaweed-based fertilizers in pellet form 
  • Seaweed powder 
  • Liquid seaweed extract 
  • Cold processed liquid fertilizer

If you’re unsure which seaweed or kelp products to try, ask the garden centre staff for advice. If you don’t get on with one type of product, you can always try a different form of seaweed or kelp fertilizer. 

How to use seaweed and kelp fertilizers

Mix seaweed pellets with potting compost when potting on young plants. Image: jag2020/Pixabay

The answer to this one lies in the form of the product you have. Liquid seaweed can be diluted and watered straight onto plant roots. Pellets or dried seaweed can be spread on the soil around the base of the plant, or mixed into potting compost before sowing or planting out.

The liquid form of seaweed or kelp fertilizer can be diluted and used as a foliar spray. This means it’s sprayed onto the foliage of your plants, and then around their base. Foliar application may help the plants absorb the fertilizer more efficiently. Just take care to always ensure the liquid seaweed fertilizer is properly diluted before doing this. 

Whatever type of fertilizer you’re applying, always read the instructions on the packet first and follow them for the best results. 


Both seaweed and help fertilisers are organic and natural products that can be effective used on plants during the growing season. Commercially prepared fertilisers are available in a variety of different forms, but for the best blend of micro-nutrients, kelp fertilizer comes out on top. 


Here are some answers to commonly asked questions about seaweed vs kelp fertiliser. 

Kelp vs seaweed fertiliser – which is better?

Both kelp and seaweed are an excellent choice of fertilizer for your garden plants. However, if it’s important for you to have high levels of trace elements and calcium in your fertilizer, then the benefits of kelp make it a better choice. 

Can kelp help root growth?

If you use seaweed fertiliser such as kelp on your plants, it can help root growth. That’s because kelp naturally contains growth hormones that can stimulate the growth of plants, including their stems, leaves, flowers and roots. 

What is fish emulsion and how does it compare to seaweed fertilizer?

Fish emulsion is a form of plant fertilizer that is made from ground up, decomposed fish products. Fish fertilizer is high in nitrogen that can help promote leafy growth in plants so it can be a good product to use for organic gardens where you’re trying to grow healthier plants. The main differences from seaweed fertiliser is that seaweed contains more micro-organisms and is known to aid soil health in a way that fish emulsion doesn’t. 

What plants can you use kelp and seaweed fertiliser on?

Both kelp and seaweed fertilisers are ideal to use with a variety of plants. They are a great product to use with vegetables, flowers, fruits and indoor plants and the nutrients are absorbed efficiently. In fact, some say that the fruits and vegetables grown may last longer after they are picked. 

Can seaweed fertiliser burn plants?

Seaweed and kelp are mild fertilisers, so when used properly, are kind to plants. Like any fertilizer though, if you use too much or fail to dilute it properly, it could result in accidentally burning your plants. In order to ensure this doesn’t occur, always use a small amount of seaweed or kelp liquid and follow the instructions on the product packaging. 

Main image: HealthyBacon/Pixabay

By .