Tony O’Neill, gardener and author of the popular “Composting Masterclass” and “Your First Vegetable Garden,” combines lifelong passion and expert knowledge to simplify the art of gardening. His mission? Helping you cultivate a thriving garden. More on Tony O’Neill
Potatoes need more phosphate and potassium than nitrogen for maximum yields. Both play an important role in tuber yield, size, and quality.
N-P-K fertilizer, nitrogen, phosphate, and potassium, respectively, feed foliage(N), roots(P), and flowers(K). Still, potato plants need healthy foliage for photosynthesis and plant health, a product of timeous nitrogen applications.
The Right Fertilizer for Potatoes in Containers
If you watch my YouTube Channel, you may have noticed that I mainly use organic fertilizers to grow potatoes in containers. The main reason is that it works, and I’ve repeatedly proven that.
I did a short video on the topic, which you can find here, Container Potatoes Thrive with These Fertilizers.
In the 30-liter (8-gallon) bucket I use, the ones from Oakland Gardens, I use three ounces (85g) of a blood, fish, and bone meal blend sprinkled on top of the four inches (10 cm) of the soil layer at the bottom of the bucket that will be directly accessible to the potato plant roots.
I also add two ounces (57g) of a pelleted organic potato-specific fertilizer. If finding a potato-specific organic fertilizer is challenging, use an organic mix with an NPK ratio of about 6-16-20.
Choosing a Fertilizer Format
- A soluble fertilizer dissolves rapidly in water, giving potato plants immediate access to nutrients.
- Soluble fertilizers are simple to apply and are ideal for container plants, where the rooting area is limited and nutrients are frequently lost due to frequent soaking.
- When nutrients are needed rapidly, such as when the lowest leaves of your tomato plants turn yellow due to a lack of nitrogen, a soluble fertilizer is an ideal choice for plant health.
- Slow-release fertilizers can also be used efficiently in containers, where watering gradually releases modest amounts of nutrients.
- Large, fast-growing plants may require more nutrients than slow-release fertilizers can supply, so keep an eye on them and supplement them if necessary.
Whatever fertilizer you use, you must follow the label to avoid over-fertilizing, which can harm plants and release excess fertilizer into the environment.
To avoid nutrient loss due to water drainage, try applying fertilizer at a lower rate more frequently. For example, if a fertilizer requires one scoop per gallon and you fertilize every two weeks, try using half a scoop per gallon and fertilizing once a week instead.
What Does Soil Fertility Mean?
It is essential to understand that added fertilizer cannot compensate for unproductive soil because it is excessively stony or has a subsoil layer that restricts average root growth and development. Soil fertility is a component of soil productivity strongly influenced by management practices.
Other components of soil productivity, especially soil depth, generally remain unchanged. Together with climate, these components set the soil productivity limits, above which yields cannot be obtained even with the correct use of natural fertilizers or synthetic fertilizers.
Environmental Effect on Nutrient Availability
Soil temperature, water availability, and acidity all affect the well-being of growing potatoes. Growing potatoes with a pH between 4.8 and 6.2 will help prevent scab formation but reduces access to potassium. Up the pH by adding some lime, but keep checking for scabs and adjust soil pH accordingly.
Potatoes have fibrous root systems, and these roots are a medium depth (between 24 and 36 inches/60 – 91cm). In pots, gardeners need to ensure enough room for root development without compaction – between 4 and 6 inches (10 – 15 cm). Potted roots do not need to grow deep as all the nutrition and water are localized.
While root growth occurs when soil temperatures range from 45 to 80 degrees (7.2 – 26.7°C), the best, most active root development occurs when soil temperatures range in the lower sixties (16 to 18°C).
Leaf (haulm) growth occurs at temperatures ranging from 45 to 86°F (7 to 30°C), with optimal growth occurring at temperatures ranging from 68 to 77°F (20 to 25°C). The optimal temperatures for stolon development are comparable.
The tuber of the potato is an expanded part of the rhizomes. The colder the soil temperature, the faster tuber formation and the bigger the number of tubers formed. The ideal soil temperature for tuber initiation is 15 to 20 degrees Celsius (59 to 68 degrees Fahrenheit).
The potato plant will develop short rhizomes and branches under these conditions. Longer day durations delay tuber start and promote stolon and shoot growth.
Low nitrogen and high carb levels in the plant promote tuber development. Tubers develop quickly once formed, reaching their peak in temperate climates. Late variants (indeterminate potatoes) are more vulnerable to longer days or high temperatures.
Moisture is essential for root growth and nutrient absorption. Adequate moisture improves nutrient uptake by diffusion, root contact, and organic matter decomposition, which releases N, P, and K. Low moisture levels can produce insoluble nutrient-containing molecules.
pH influences nutrient availability by modifying the nutritional form. For instance, the different forms of N (influenced by pH) have distinct leaching capabilities. Other nutrients may become adsorbed, desorbed, precipitated, mineralized, or immobilized at different pH levels.
Many nutrients are readily available in slightly acidic soils, and potash is most accessible at a neutral pH (about 6.5). Because soil microorganisms are pH-sensitive, pH is critical in nitrogen transformations such as nitrogen fixation.
Essential Potato Nutrients and Their Role
Container plants have a difficult time keeping even moisture levels.
Depending upon the size and composition of the container, you may need to water it more than once every day during hot, dry weather. It would be best if you watered your plants at least once a day.
Keep your containers from drying out for too long. Plants can get highly stressed after being without water for an extended period.
Most of the plant’s nutritional requirements are absorbed from the soil via the roots. The importance of each element varies, but all play a role in plant growth, and each one’s availability varies.
|Nitrogen||NO3– (gas) and NA4+ (ammonium – a product of ammonia and water)||Air and soil – deficiency related to low organic matter||Protein, chlorophyll, cell formation|
|Phosphorus||H2PO4-; HPO42-; PO43- (dihydrogen phosphate and phosphate)||Roots||Fat, carbohydrates, and protein metabolism, cell formation|
|Potassium||K+ (Potassium Ion)||Roots||Enzyme activity and water regulation|
|Calcium||Ca2+ (Calcium Ion)||Roots||Enzyme activity, root permeability|
|Magnesium||Mg2+ (Magnesium Ion)||Roots||Metabolism, fat formation, chlorophyll|
|Sulfur||SO42- ( Sulfate Cation)||Roots||Protein, lipids, and vitamin formation|
Organic Fertilizer Alternatives
Natural fertilizers are organic materials with high natural levels of nitrogen, phosphorus, potassium, or other essential plant nutrition elements—the list below from the National Center for Appropriate Technology and USDA’s Rural Business-Cooperative Services are common natural fertilizer examples [source].
|Alfalfa Meal||2 – 3%||1 – 2%||2%||Source of Micronutrients|
|Bat Guano||0 – 8%||0 – 10%||0 – 1%||Quality depends on the source|
|Blood Meal||12%||0 – 2%||0 – 1%|
|Bone Meal||2 – 4%||14 – 16%||0||Rich in calcium (20 to 24%)|
|Coffee Grounds||3%||0||0||Will reduce nitrogen availability due to high carbon content (11:1)|
|Cottonseed Meal||4 – 6%||2 – 3%||1%||Acidic|
|Crab Shell Meal||4%||3 – 4%||0||Rich in calcium (14 to 18%)|
|Fish Hydrolysis||2 – 5%||3 – 4%||1%||promotes beneficial fungi microorganisms|
|Fish Meal||9 – 10%||3 – 7%||0 – 1%|
|Greensand||0||0||0 – 5%||Source of K, Mg, Fe, Si, and trace minerals|
|Kelp Meal||1%||0 – 1%||2%||Source of micronutrients|
|Rock Dust||0||0||3 – 6%||Source of Ca, Si, and trace elements|
|Rock Phosphate||0||3%||0||20% P2O5 with 20% Ca and trace elements|
|Sul-Po-Mag||0||0||22%||22% sulfur, 11% magnesium|
|Vermicompost||2 – 3%||2%||2%||Adds essential soil microbes|
|Wood Ash||0||1 – 2%||4 – 10%||Do not add ash to nitrogen; it produces ammonia gas|
This list shows that natural fertilizers are organic and made from renewable carbonaceous plant or animal products. In contrast, mineral fertilizers are inorganic salts from mining or chemical industrial processes.
Not all chemical industrial processed fertilizers are inorganic, though. A byproduct of biogas production is an organic residue, also known as digestate.
Anaerobic Digestate (AD)
Most off-the-shelf organic fertilizers are a byproduct of biogas production. Food waste, maize, and manure biodigesters use anaerobic bacteria to decompose the organic materials in a sealed environment, producing methane gas and a nutrient-rich residue.
During the breakdown of the organic matter, the essential plant nutrients (N, P, and K) remain in the digestate. Anaerobic digestion is a remarkable process as it is achieved without substantially reducing the fertilizing properties of the feedstock material.
However, unlike aerobic composting, anaerobic digestion does not reach temperatures sufficiently high to destroy pathogens and weed seeds. There is, therefore, a possibility that AD can contain some disease contaminants.
What is the best type of compost to use on potatoes?
Firstly, let’s establish that there are several types of compost: aerobic compost, anaerobic compost, leaf mold compost, vermicompost, and actively aerated compost tea, to mention the most popular varieties. Another factor to consider is the final product’s fungi-to-bacteria ratio – read on and learn.
Fungi populations are essential for plants with rhizomes—like potato plants. Potato plants do well with compost slightly higher than a 2-to-1 fungi-to-bacteria ratio. Getting that ratio right during composting is challenging, but ratios are more easily manipulated by using the compost to make an actively aerated compost tea.
Actively aerated compost tea (AACT) is made by submerging a horticulture tea bag filled with composts into a pail of water and pumping air through the mix for 24 hours – the microorganisms don’t drown, humic acid is produced, and if you add a protein like fish hydrolysis, the fungi population is boosted.
Is Epsom Salts a Natural Fertilizer?
Epsom salt was found in the English town of Epsom. It’s not the kind of salt we’re used to seeing. Epsom salt is made up of magnesium, sulfur, and oxygen and is a mineral salt, and it is very soluble in water and swiftly breaks down into magnesium and sulfur.
While it’s a naturally occurring material, it is mined or produced and is not an organic fertilizer. Like other mineral fertilizers, it can contribute to a build-up of salts in the soil, and the added sulfur can decrease the soil’s pH (more acidic). Remember the rhyme; sulfur suppresses, lime lifts.
It is exceptionally leachable in sandy soils due to its soluble nature, and it may quickly leach past the root zone and become inaccessible to the plants. Adding compost can help reduce the loss by increasing the cation exchange capacity.
What is the Advantage of Using Natural Fertilizer for Potato Plants
Focusing on boosting nature’s regenerating ability is always a good idea. Any interaction with nature should be transactional using nature’s currency, and natural fertilizer is nature’s currency, providing your soil with the means to self-improve its health.
Soil should be a vibrant ecosystem of living organisms, roots, hyphae, and plant nutrients. Healthy soil smells good because of the life in it, marked by the presence of geosmin [from the Greek, geo (earth) and some (odor)].
Added synthetic chemicals generally target plants, ignoring the impact they may have on the surrounding fragile ecosystem – a system responsible for the symbiotic sustainability of everything that benefits plants.
Potato plants are not heavy feeders and will get every nutrient from a bone meal in powder or liquid form. Coffee grounds are an excellent slug repellent as a thin mulch (less than an inch) will boost the presence of other beneficial microorganisms.
What Are the Disadvantages of Using Natural Fertilizers on Potato Plants
The disadvantages of using natural fertilizers are minimal, specifically on potato plants.
List of Natural Fertilizers That Need Close Monitoring with Potato Plants
Coffee grounds are great as a compost additive, but adding them directly to the soil will bind the nitrogen as microorganisms use it to decompose the carbon. However, as a thin mulch, coffee grounds are excellent slug repellents that prevent weeds, attract earthworms, and more.
Coffee grounds also contain compounds that help suppress some plant disease-causing microbes.
You may have heard that wood ashes may be utilized as a potassium source for plants in the garden. However, because wood ashes raise the pH of the soil and necessitate additional measures, they should be used in the garden with caution.
Whether you burn hardwoods (such as oak) or softwoods (such as pine) affects the fertilizer value and limits the impact of wood ash (e.g., pine). The potassium content of wood ash is typically 4 to 10 percent, while the phosphorus content is 1 to 2 percent.
Hardwood ashes contain more significant percentages of nutrients and have a more substantial liming impact on soil than softwood ashes.
FAQs on How to Choose the Best Container Fertilizer for Potatoes
What is NPK fertilizer for potatoes?
NPK fertilizer for potatoes contains three essential nutrients: nitrogen (N), phosphorus (P), and potassium (K), which are important for plant growth and development. Nitrogen helps with foliage growth, while phosphorus and potassium are crucial in tuber yield, size, and quality. The right balance of these nutrients is essential for maximum potato yields.
What are the best natural fertilizers for potatoes?
Natural fertilizers are effective for growing potatoes, including compost, aged manure, bone meal, and kelp meal. These fertilizers provide essential nutrients such as nitrogen, phosphorus, potassium, and micronutrients for potato growth and development. It’s important to use the right amount of fertilizer and to apply it at the right time to avoid over-fertilizing and damaging the plants.
Is urea good for potatoes?
Yes, urea can benefit potatoes as it is a good source of nitrogen, an essential nutrient for plant growth and development. However, excessive use of urea or other nitrogen-based fertilizer can lead to imbalanced soil nutrient levels and may negatively impact plant health and yield. Using urea or any fertilizer according to recommended rates and monitoring soil nutrient levels regularly is important.
The recommended fertilizer at the beginning of this article has served me well for years. Feel free to try some of the other organic options suggested—gardening is an art as much as it’s a science.