It took probably 10 years for me to really start figuring out fertilizers. Now I get it.
Most fields and gardens should be fertilized around planting time, then again about a month later. Sandy or very wet soils may need additional fertilization. Low-fertility soil may need fertilization every month through the growing season to reach peak performance. Very good soils need little fertilizer.
Now, there’s quite a lot to it and every patch of soil is a bit different. let’s break it down.
When do You Need to Fertilize a Garden?
Gardens need to be fertilized every year from either organic or synthetic sources. As nutrients are pulled out of the soil in the form of plant matter and by exchanging into gasses, more nutrients need to be added. The more nutrients that are removed, the more need to be added. It’s simple in concept.
There are tons of variables at play here. Different plants need different amounts of fertility and different soils hold on to those fertile nutrients in a different manner. For most garden soils, you should add an actual fertilizer of some sort at least once a year. There are a few exceptions.
In overworked soils, there is very little fertility remaining in the soil by spring. between summer growth and seasonal rains, most soils have little left to offer. That’s why most soils are fertilized heavily in the spring. Farm fields are the worst culprit of this nutrition deficiency.
After a simple soil test to determine your dirt’s fertility (a simple DIY kit is sufficient), you calculate the amount of fertilizer needed. It takes about a month for most fertilizers to start working in the soil and become available to plants. Organic fertilizer tends to take longer.
Usually, to make sure the fertilizer is available on time, you would add 1/2 to 2/3 of the fertilizer at planting and again a month later, that gives it time to break down and avoids putting down a heavy application. Heavy fertilizer applications can hurt soil biology, damage plants, and increase fertilizer runoff.
Some folks spread out the application into weekly or bi-weekly applications until the last month of the season. That works too, it just means you have to spend more time fertilizing and be much more careful about over-fertilizing.
Recognizing Signs of Fertility Issues
Yellowing or light-colored foliage are the most common signs of a severe lack of fertility. Light-colored or yellowed foliage means low chlorophyll production, which usually comes from a lack of available fertility. Stunted plant growth often indicates a moderate fertility shortfall.
Plants with a sudden wilting or burnt appearance, particularly on new leaf growth or along the edges of leaves tend to indicate an over-fertilization or a sudden rapid creation of available nitrogen in the soil. Moderate over-fertility can express as plants with quick green growth but little flowering and weak stems.
The best way to identify fertility issues is with a DIY soil test in the off-season, and again just into the growing season. It can be really fun and interesting to watch just how your soil changes throughout the year. The soil test kit I use and recommend is the Luster Leaf 6101.
It’s a $20 kit and will test major nutrients 10 times. None of the do-it-yourself tests will check for the micronutrients, just the macronutrients: nitrogen, phosphorus, and potassium. If anything is lacking, it’s almost always one of those. But, not always.
For a non-production garden, results in the middle of the test range are acceptable. For high production crops, results closer to the top of the test scale are needed.
The best way to ensure the micronutrients and trace elements are in adequate supply is to look at the color of your topsoil. Basically, if it’s got a decent amount of dark, organic matter, it’s going to be alright with the lesser-needed minerals.
Macronutrient deficiencies tend to happen every year. That’s why we usually benefit from adding a basic NPK fertilizer of some sort (nitrogen, phosphorus, potassium). It’s a much bigger issue in light-colored and heavily tilled soils. Very sandy or volcanic till soils also tend to have unbalanced micronutrients.
Soils that have been used in heavy production or that have had the same crop growing for many years do tend to have more concern about micronutrient deficiencies. In any case, the best way to balance plant micronutrients is by adding a good amount of organic matter. It really fixes everything.
How to Make Soil Naturally Fertile
It takes a lot of time and material to fix soil on a large scale. Fortunately, it’s pretty simple in a garden. I have exceptional soil for my area. In spring, it usually has about half the nutrients needed for our produce growing operation. The rest I can add at or just before plating time and be okay, most of the time.
Before I get any deeper, I want to talk about something called cations and Cation Exchange Capacity. I’ll keep it basic. Cations are positively charged mineral particles and are attracted to negatively charged particles. Fertilizer compounds, whether organic or synthetic, are cations. Good soil is negatively charged.
It holds nutrients for plants’ roots to take in. Fine textured soil like clay is somewhat negatively charged. that helps it to hold on to those nutrients. Sand is not and holds nutrients very poorly. Soil organic matter has a very strong negative charge and holds soil nutrients like a magnet on a fridge.
Soil with little organic matter has a poor cation (fertility) holding and exchanging capacity. Basically, good black dirt holds onto a lot of nutrients, so fertilization can be spread further apart. If you sprinkle fertilizer on bad soil, as little as 20 percent of it may actually get to the plants.
Good soil gets closer to 90 percent efficiency of fertilizer, and it’s naturally spread out through the growing season. In really good soil, I only need to fertilize once for even the highest feeding crops but in bad soil, I would probably need to fertilize bi-weekly, or even weekly.
In a natural system, as plant matter dies back it breaks down and is converted by bugs and microbes into both a small amount of fertilizer and into an organic material called humus. Humus acts as a sponge that chemically attracts and holds on to that fertility so it doesn’t wash out or convert to gas quickly.
We’re talking very small amounts of actual fertility here being created for the most part. The more important part is in the storing up of fertility. The chemical attraction between humus and the fertile agents break down very slowly, allowing you to build up and increase fertility over time.
How Often to Fertilize?
I recommend fertilizing two or three times during the growing season if you have a good, black topsoil with a high organic matter content. If you have moderate topsoil, fertilize monthly. With poor topsoil, fertilize every two weeks during the growing season and amend with organic matter.
I have good soil. Besides rotating chickens through the garden in the off-season, applying rabbit manure, and adding compost yearly, I fertilize in early spring, then again in early summer as needed depending on soil tests and what I’m growing.
With poor soil, you have to fertilize quite often to have a chance of growing anything half-decent. Of course, the solution is to create a nice, dark layer of topsoil by adding a lot of organic material to the soil. Do that, but also fertilize often the first year.
The problem with fertilizing often is that it’s very easy to way over-fertilize and hurt your plants. Too much fertilizer causes extreme drought-like symptoms and burns in leaves and roots. Be careful. I would suggest mixing up a liquid solution and applying as a foliar spray, which can be applied weekly if needed.
A foliar spray is just fertilizer diluted in water that you spray on the plants. Commercially, they are mixed quite strong and lightly misted on, but a low-dosage solution works poured right on the plant from a watering can. A basic recipe is 1 tablespoon of fertilizer to a gallon of water and briefly shower the plant.
That solution is good for up to 10-15 row-feet of garden space. The more often you fertilize, the less you need per application. When you’re using a more mild, organic option like compost, manure, or alfalfa meal, you can be a lot less specific and precise. Just add a heavy dose a few times and see how the garden responds.
It’s a game of experimentation really. Your soil is different than my soil. Every garden is different and responds differently to fertilization. It will probably take a few years of careful observation for you to really start figuring it out. You need to notice both how your plants respond and how the soil responds.
Then you will know about how much and how often to fertilize your garden to match the type of plants you grow, the quality and condition of your soil, and the particular growing methods you use.
How Much Fertilizer to use?
The standard recommendation for fertilizing gardens is an even NPK or a nitrogen-high NPK mix at a rate of 2-1/2 pounds N per 1,000 square feet, or 100 pounds N per acre. Heavy-feeding crops or a more production-based garden model may need twice as much.
The better your soil and soil tending methods, the less loss and more holdover of fertility you will have the following season. Limiting soil disturbance by avoiding bare soil, minimizing tilling, and leaving plant residue in the garden, and avoiding over-fertilization; that’s the pillar of good soil management practice.
Is Fertilizer Bad for the Environment?
Fertilizer runoff may cause fertility to build up in naturally non-fertile soil such as prairie or savanna land. The concern is that it could then cause a change in the wild ecosystem. Fertilizer is impactful on the environment in potentially good and bad ways. Phosphorus is the main concern here.
Fertilizer itself is detrimental to soil biology and microbiology. It is a salt. salt is used to kill germs and soft-bodied bugs. When you sprinkle fertilizer in the garden, it kills the beneficial nematodes, bacteria, and amoebas in the soil. It can also kill earthworms.
Heavily fertilized soils tend to be fairly devoid of worms. That’s something I discovered myself years ago. Turns out, fertilizer draws moisture out of things. It’s about like salting a ham to dry it for aging. Fertilizer draws moisture out from the little soil critters. It draws moisture out of plant roots too.
That’s why fertilizer “burns” plants. When applied too heavily, it actually draws out water from the plants creating a type of severe drought-like symptoms. in any situation where you fertilize heavily, it’s pretty bad on soil biology, and soil biology is what makes every component of fertility and growing work.
You can midigate that by spreading out fertiliztion into several applications, and by irrigating imediately after fertilizing. I like to just use a liquid fertilizer solution, dry fertilizer disolved in water. It midigates most of that.
Can you Fertilize with Manure or Compost?
You can fertilize with just manure but it’s hard to manage unless you are growing lower-feeding plants. Compost is harder to use as a sole fertilizer because it’s usually lower in nitrogen than manure. The issue with both compost and manure is that they take a while to work and are often both low in nitrogen.
Old manure is usually quite devoid of nitrogen. Manure will seldom work as a stand alone fertilizer and usually needs nitrogen added to it. But, it can work. Yo’d have to use a lot of it. The issue with that is when you add a ton of organic matter, you can over do it.
Organic matter sucks up fertility like a sponge ubtill it’s full, then it slowly releases it to your plants. Too much organic mater will soak up a ton of fertility before it releases it to your garden. This locking of nutrients can be a serious problem the first few years after adding a lot of organic matter.
If you fertilize with just compost or manure, It’s almost impossible to grow a good crop of high-yielding tomatoes, a nice stand of corn or a hearty potato patch. But, you could grow some acceptable beans, asparagus, or turnips. It’s be fine for most flowers too.
I wrote a more detailed article on fertilizing with manure. Here’s a link to it.
Like I said, I use a ton (several tons acually) of manure each year and my garden is doing swell. I also get the manure very fresh and work to limit the nitrogen loss so it’s more potent. Even after a hairly heavy dose of manure, I still have to amend my soild with fertilizer, often just urea, for my production crops.
But for things like the rhubard, elderberries, flower beds, and perenial herds, I usually need very little other than manure.