Potato Fertilizer [What Works and What Doesn’t]
A lot of the advice from universities regarding potato fertilizer seems to mostly apply to commercial farming where soil fertility is low and more acidic. I’ve talked to home gardeners who use all kinds of different things in their garden and some don’t fertilize at all.
I view fertilizer as taking vitamins. Will you be ok without them? Probably. But could you be healthier with them? Definitely. It’s the same with potatoes.
This year I decided to do an experiment. I separated my garden into 3 sections. In the first two sections, I applied a blend of organic materials such as blood and bone meal mixed with a natural mineral called langbeinite.
In one section I applied it when my plants were 6-8” tall and then again monthly after that. In the second section, I applied it only when planting and right before they started flowering.
In the third section, I used Old Cobblers Farm Potato Fertilizer and applied it only at planting and before flowering.
This article contains everything I’ve learned about potato fertilizer and the outcome of the experiment.
Fertilizer for Potatoes
Potatoes are heavy feeders but they require different nutrients at different stages of their growth. Here’s a breakdown of the main nutrients that potatoes need.
Nitrogen (N)
Potatoes do need nitrogen, but not as much as other plants. Nitrogen promotes the growth of the vines and leaves on top of the soil.
In the beginning, it’s important that potato plants have access to nitrogen to form healthy foliage. This foliage is what will help the plant send energy to develop tubers later.
As the potato plant ages, it requires less and less nitrogen.
If I’m growing in a raised bed or container with fresh potting mix, I don’t add any nitrogen fertilizers prior to planting. I find that a good blend like Fox Farm’s Happy Frog has all the nitrogen my plants need to get started.
If I’m recycling soil or growing in the ground, I usually either mix in compost or I’ll add some blood meal to the planting hole prior to planting my potatoes.
When I apply my mid-season fertilizer, usually around the 30-40 day mark, I cut the amount of blood meal in half. I’ll talk more about this later.
I’ve found that too much nitrogen causes lush foliage and reduces the number of tubers and their quality. So don’t overdo it.
Phosphorus (P)
Phosphorus is essential for cell division and is directly related to the number of tubers your plant will set. I typically double the amount of phosphorus compared to the amount of nitrogen I use. In other words, if I add 6% nitrogen, I add around 12% phosphorus.
Unlike nitrogen, this doesn’t change through the potato’s growth stages. I feed my potatoes a consistent amount of phosphorus during each application.
I will usually add bone meal to my soil, which is an organic fertilizer at planting and again mid-season.
Potassium (K)
Potassium helps the potato plant to regular water and carbohydrates stored in tissues. This is really important for potatoes because it impacts tuber quantity, size, and quality of the skin. It also helps with disease resistance.
Most agronomists will tell you that potassium is the number one nutrient for potatoes and that they use more of it than any other plant nutrient. Potatoes have a higher demand for potassium than any other vegetable and that’s proven through research at Oregon State University.
Some research has shown that potassium can incre ase tuber yield (total weight) by 2,000 pounds per 1/10 of an acre. (Farm Progress)
I typically apply potassium in equal amounts as I do phosphorus (double the nitrogen amount) and that seems to work well for me. Too much potassium can mess with tuber water absorption and that’s a problem.
So far, I’ve found the best natural potassium fertilizer to be langbeinite. I apply it when I apply phosphorus in the form of bone meal.
Dry Fertilizer vs Liquid Fertilizers
I always use dry or granular fertilizers for my potatoes. There are liquid fertilizers that you can use, some of them foliar even, but I do everything I can not to put water on the leaves of my potato plants.
I’ve had problems in the past with potato blights and other diseases which I think fostered because of the way I was watering and adding these fertilizers.
When it comes to potatoes, I exclusively use dry fertilizers. I will mix it into the soil at planting and then side-dress plants when they need it throughout the growing season.
When to Fertilize Potatoes
I fertilize my potatoes two times. First, prior to planting, I will add compost or a natural slow-release fertilizer to the soil. Note that I don’t do this if I’m using fresh potting mix because I find that it already has enough nutrients to get the potato plant started. I like to use the Happy Frog blend by Fox Farms. It has enough nutrients, but not too much.
Once the plants emerge from the ground and are about 6 to 8 inches tall, I will side-dress them again with a second round of fertilizer right before they bloom.
That’s it. Two times I find is plenty for potatoes. Depending on the weather, this is usually about 30 to 40 days.
Potatoes are really easy to overfertilize so I tend to err on the side of caution. Too much and your plant will yellow or wilt.
One final note that I’ve discovered is that potato plants don’t do well with fertilization during peak heat. I always try to avoid fertilizing during heat waves whenever possible. Sometimes fertilizers raise the temperature of the soil and potatoes don’t like it hot.
How to Fertilize Potatoes
If you’re new to using fertilizers, both organic or synthetic, I would follow the directions on the bag in terms of how much to use at any given time. I use slow-release fertilizers and try to pick organic ones whenever possible because they’re less likely to burn my plants.
How Much Fertilizer to Use
For most organic and mineral fertilizers, 1 to 3 pounds is applied per 100 square feet if that helps give you a reference point. That could be anywhere from 1 cup to 3 cups depending on what you’re using.
For example, blood meal (nitrogen) is 3 cups to equal 1 pound and it takes about 2-3 pounds to cover a 100 square foot garden.
Langbeinite, which is a mineral heavy in potassium and magnesium, is 1 cup to 1 pound and only requires 1-2 pounds to cover a 100-square-foot garden.
I always check the box or bag for application rates and err on the side of less when I’m unsure.
How to Fertilize Potatoes Before Planting
It’s always best to do a soil test first to find out what nutrients your soil is lacking. I prefer the Luster Leaf Rapitest. It’s fairly easy to use and you can do it at home. It includes 4 tests, one for each N, P, and K, plus one for pH; and it’s super easy to read the test results.
Depending on the soil, I usually add fresh compost to my soil by mixing it into the top 6 inches of soil, no more than 50/50 ratio.
If I don’t have any compost handy, I use a blend of blood meal, bone meal, and langbeinite. All are natural and organic. Sometimes I mix it into the soil and sometimes I put a little bit at the bottom of each hole before I plant a seed potato.
If I put it in the hole, I will dig the hole a little deeper, add the fertilizer, and then backfill it with an inch or two of soil before planting my potato and backfilling the entire hole. I never let fertilizers come in direct contact with my plants or seed potatoes.
How to Fertilize Potatoes Before Blooming
When the potato plant is ready to bloom, it’s also setting tubers. This is when I apply a second application of fertilizer. I side-dress each potato plant with 1 tsp of blood meal, 1 tsp of bone meal, and 1 Tbsp of langbeinite. That’s a general rule of thumb that I’ve come to follow; however, if the plant is showing signs of any deficiency, I will increase or decrease those amounts.
I’ll talk about signs of under and overfertilization a little later in this article.
After side-dressing the plant, I water the nutrients into the soil as well as possible.
What is the Best Fertilizer for Potatoes
When it comes to choosing fertilizers, there are really three options.
Homemade Fertilizer for Potatoes
Some people make their own homemade fertilizers which are usually liquids. I’ve experimented with this a little bit. I’ve found it to be too much of a hassle for the way I like to garden and I’m pretty sure I’ve introduced diseases to my garden before by doing so.
I’ve tried grass clippings before, but I have a huge problem with rolly-pollies and this led to an outbreak of them and they ate nearly all of my seedlings one year. If you don’t have a problem with these bugs, give it a try. Grass is a great source of nitrogen.
I’ve also tried banana compost tea, but I don’t know how well it worked. Potatoes need a lot of potassium so I’m not sure it was sufficient.
I’ve also heard others use eggshells, but those take so long to break down, if your potatoes need nutrients now, it’s not going to do much good.
Needless to say, I’m not a huge fan of homemade fertilizers, except compost of course. Everyone should have an at-home composter.
Organic
This is my preferred type of fertilizer for potatoes. Organic fertilizers are slow-release and because they’re made with natural stuff they don’t have some of the harmful side effects that synthetic ones have. I’ve never had an organic fertilizer burn my plants before for example.
My go-to organic fertilizers for potatoes include:
Blood Meal for Nitrogen
Bone Meal for Phosphorus
Langbeinite for Potassium
The downside to organic fertilizers is that most aren’t pre-balanced for you. You can buy synthetic ones that might be a 5-10-10 NPK or an 8-24-24 NPK. Those already have the appropriate ratio for potatoes and there’s no math involved.
With organic fertilizers, you’ll have to figure out how much of each to apply. But I did give you my cheat sheet earlier if that makes it easier.
Other organic fertilizers that are good include wood ash and seaweed.
I do not use manure anymore. It has way too much nitrogen in it for potatoes and unless you’re applying it months before you plant to dematerialize some of that, it’s not a good idea. I’ve also had manure create an environment favorable for scab.
Synthetic
Synthetic fertilizers do offer a benefit in that they are almost always (in my experience) premade with set ratios of NPK for you.
The best synthetic potato fertilizer I’ve used is Old Cobblers Farm Seed Potato Fertilizer. It has an NPK of 8-16-16 which is an excellent ratio for potatoes.
I like to think of synthetic fertilizers like a shot of espresso in the morning. They give plants a quick shot of nutrients. Organic fertilizers on the other hand are like a home-cooked meal. They help improve soil structure, improve water retention, and are beneficial to microbial activity.
Both work depending on your garden preferences.
Signs of Nutrient Deficiency in Potato Plants
Besides having a regular fertilizing routine for your garden, it’s also important to pay attention to what the plants are telling you.
If the leaves at the bottom of the plant start to yellow and the ones toward the top are light green, that could be a sign of nitrogen deficiency. In that case, side-dress them with some bone meal.
If the leaves have yellow edges, that could be a potassium deficiency. Side-dress them with some langbeinite.
If the leaves are yellow in between the veins, that could be a magnesium deficiency. Langbeinite is also a good source of this mineral.
If your plants don’t begin to bud and bloom when they should, that too is a sign that they are hungry. Give them a well-rounded dose of NPK, with the phosphorus and potassium levels being higher than the nitrogen.
Signs of Overfertilizing Potatoes
Here are the most common signs that your potatoes might have too much of that good stuff.
If you have gigantic potato leaves, that is a sign that they have too much food and you can ease up on the fertilizer levels.
Yellowing or wilting can be a result of overfertilizing.
If there is a white crust on the soil surface, that is likely excess minerals that the plant can’t absorb.
Brown leaf tips and blackened roots can also be signs of an overdose.
If your plant’s symptoms are severe, you can try to give them a deep soak and leach some of the excess minerals out of the soil. I would only do this if you have really good drainage. Waterlogging potato plants will lead to a whole host of other problems.
Another thing that I have done in the past with side-dressed plants is to gently scoop some of the fertilizer off the surface if it hasn’t all been washed into the soil. Just be careful not to damage the plant and the roots. I also don’t know how effective this really is, but it’s worth a try.
Potato Fertilizer FAQ
When to Fertilize Potatoes in the Bag
If you’re growing potatoes in a bag or container, you can skip pre-planting fertilizers. Most likely, you’re growing in potting soil that is already high in nutrients, and that will provide enough for the potato plant to get started.
Once the potato plant starts to form buds (right before blooming), I would apply NPK in a ratio of 8-16-16 or 6-12-11, then add soil on top and water it well into the soil.
Are Potatoes Fertilized When Green Sprouts Appear
I do not fertilize potatoes when sprouts appear. I wait until my plants are at least 6 to 8 inches tall before side-dressing with fertilizer after emergence.
Can I Use Tomato Feed on Potatoes
Tomatoes and potatoes are from the same family. They are both nightshades. Most tomato fertilizers I’ve seen, such as a 3-8-7 will work fine for potatoes too. Most of them have a higher phosphorus and potassium ratio compared to nitrogen which is what you want for potatoes.
Avoid any tomato fertilizers that are high in nitrogen, especially later in the potato plant’s growth stage.
Are There Fertilizers That I Should Avoid for Potatoes
Yes. Avoid lime because it can also promote some plant diseases and will affect the soil pH. Because of this, I don’t use lime in my potato garden.
Also, I’m cautious with any mineral fertilizers. It’s really easy to overfertilize potatoes and many mineral fertilizers have salts that can decrease nitrogen-fixing bacteria and earthworms which are good for soil structure.
I still use minerals like Langbeinite, which is a natural mineral, but it has a relatively low overall salt index so I find it a bit safer.
