Best Fertiliser for Potatoes in Australia: Grow Bigger, Healthier Spuds

Potato 🥔

Transform Your Garden into a Personal Paradise!

Understanding the Nutritional Needs of Potato Plants

Potatoes are heavy feeders, meaning they require consistent access to nutrients from planting through to harvest. The most effective way to ensure high yields and good tuber quality is by using a fertiliser specifically designed for root vegetables. The best fertiliser for potatoes will have a lower nitrogen level and higher phosphorus and potassium levels to encourage strong root development and starchy, well-formed tubers. Unlike leafy greens, potatoes don’t benefit from excessive nitrogen. In fact, too much nitrogen causes excessive foliage growth at the expense of tubers, often resulting in a lush plant with very few usable potatoes.

The recommended fertiliser profile for potatoes is an NPK ratio of approximately 4-18-38. This is ideal for all varieties including white potatoes, Irish potatoes, and sweet potatoes. High phosphorus (P) promotes strong root formation, while potassium (K) improves tuber size, flavour, and disease resistance. The inclusion of calcium and magnesium further supports plant cell integrity and nutrient uptake.

A top-performing option for Australian growers is the Complete Grow 4-18-38 + CaMg Powder Fertiliser. This is a professional-grade formula that dissolves quickly and delivers immediate results, making it especially suitable for both field crops and container-grown potatoes. It is free from harmful fillers and provides clean, consistent nutrients that support high yields.

Potatoes prefer slightly acidic soil, with an ideal pH between 5.5 and 6.5. Before planting, it’s recommended to test your soil and adjust the pH if necessary using garden lime or sulphur. In poor soils, pre-mixing fertiliser into the soil before planting is essential. Mix approximately 20–30g of fertiliser per square metre into the top 10–15 cm of soil. For pots or grow bags, add 5–10g per 10L of soil mix at the start of the season.

In summary, successful potato growing begins with choosing the right fertiliser. Aim for a high-quality NPK blend such as 4-18-38, avoid overfeeding with nitrogen, and always maintain optimal soil conditions. This sets the stage for strong early growth, better tuber set, and a healthier plant overall. All potato types—from Sebago and Desiree to sweet and Irish cultivars—benefit from the same foundational feeding strategy based on this nutrient model.

How to Fertilise Potatoes Through the Growing Season

Timing is critical when it comes to fertilising potatoes. Whether you’re growing in raised beds, garden rows, or containers, the plant’s nutrient needs change throughout the season. A structured feeding plan helps maximise yield and tuber quality while preventing common issues like small or misshapen potatoes.

Here is a complete fertilisation schedule designed for Australian conditions using Complete Grow’s 4-18-38 + CaMg Fertiliser:

Pre-Planting Soil Preparation

About 1–2 weeks before planting, mix the fertiliser into your soil at a rate of 20–30 grams per square metre. This ensures a rich base layer of nutrients that your seed potatoes can tap into immediately. If you are growing in pots or bags, incorporate 5–10g per 10L of soil or potting mix. Water in well to activate nutrients and allow them to distribute evenly.

After Sprouting (2–3 Weeks Post Planting)

Once your potato shoots have emerged and reached 15–20 cm tall, side-dress with 10–15g per plant or 30g per metre row. Apply the fertiliser around the base of the plant, not directly on stems or leaves. Then gently mound the soil (a process known as “hilling”) to cover the new growth and protect tuber development.

Tuber Initiation Phase (Around Flowering)

At the flowering stage, potatoes begin to set tubers underground. This is the most critical phase to support with potassium-rich fertiliser. Apply another round of 10–15g per plant to promote large, firm tubers and strengthen disease resistance. Always water after feeding to help nutrients penetrate deeply.

Late Season (4–5 Weeks Before Harvest)

Discontinue fertilising once the foliage begins to yellow and die back. This signals that the plant is preparing for harvest and nutrients are being withdrawn from leaves to harden tuber skins. Continuing to fertilise during this time risks soft potatoes and shortened storage life.

This feeding schedule is especially effective for container-grown potatoes, which are more dependent on external nutrient applications due to limited root space. Adjust quantities according to container size, and always ensure adequate drainage to prevent fertiliser burn or rot.

Fertilising Irish Potatoes, and Common Mistakes

While many fertilising principles apply across all types of potatoes, there are a few considerations for specific varieties such as sweet potatoes and Irish potatoes. These varieties still require balanced nutrition, but small adjustments in timing and dosage can improve your results significantly.

Feeding Sweet Potatoes (Ipomoea batatas)

Sweet potatoes are not true potatoes botanically, but they benefit from a similar NPK approach—low nitrogen, high potassium. The best fertiliser for sweet potatoes is still NPK 4-18-38 + CaMg, applied at slightly lower rates:

Pre-planting: 15–20g per square metre mixed into the topsoil
At vine growth: 8–10g per plant every 3–4 weeks

Avoid high nitrogen at all costs—this leads to rampant vine growth and poor root production. Sweet potatoes prefer warm, sandy soil and need at least 4–5 months of frost-free growth. Regular watering and mulch retention are essential to keep nutrients available and prevent root cracking.

Feeding Irish Potatoes

The term “Irish potatoes” generally refers to white-fleshed cultivars like Kennebec, Coliban, or Atlantic. These are traditional cropping varieties in Australia and follow the standard feeding regime discussed earlier. However, Irish varieties are slightly more prone to scab, especially in alkaline soils. Maintaining slightly acidic conditions (pH 5.5–6.0) and avoiding excess calcium or lime in the soil will help minimise the risk.

Use the standard application of 4-18-38 fertiliser every 3–4 weeks throughout the season, but water consistently to keep soil pH and salt levels balanced.

Fertilising Mistakes to Avoid

Using lawn fertiliser: High nitrogen blends will result in lush foliage but poor tubers
Fertilising dry soil: Always water before and after feeding to protect roots
Late-season feeding: This delays tuber hardening and reduces storage quality
Neglecting micronutrients: Calcium and magnesium play a key role in tuber development and preventing hollow core or cracking

By following these best practices and selecting a professionally balanced fertiliser like the Complete Grow 4-18-38 + CaMg, you’ll give your potato crop the best chance to thrive. Whether you’re planting Dutch Cream in raised beds, Kipflers in containers, or sweet potatoes along the fence line, consistent nutrient management is the key to a bumper harvest.