Fertilizers give food crops and ornamental plants the nutrients they need to thrive, but when used unnecessarily or incorrectly, they can do more harm than good. In this week’s encore episode, I share the basics you need to know for the most efficient and effective use of fertilizer.
Fertilizer is considered to be any organic or synthetic material added to soil to supply one or more nutrients that plants need for growth. The chemical makeup of organic and synthetic nutrients is exactly the same — plants can’t tell the difference. Where organic fertilizers and synthetic fertilizers differ is in when the nutrients became available to plants and what happens to all the excess nutrients that the plants could not use up right away. When conventional gardeners begin to understand the detriments that come with using synthetics, they often come around to organic gardening.
For a comprehensive recap of the information in this episode, you can view the show notes from the original airing.
Before proceeding with this important information on understanding and using fertilizer, a quick reminder that my new Online Gardening Academy™ course, Growing Epic Tomatoes, co-led by Craig LeHoullier, will open for enrollment on Tuesday, May 11. This course will turn you into a tomato-growing expert in no time. You can sign up to be notified the moment the enrollment window opens.
The Upsides and Many Downsides of Synthetic Fertilizer
Synthetic, or inorganic, fertilizers are man-made and engineered to deliver nutrients rapidly. They are generally water-soluble (they dissolve in water) and therefore can feed plants quickly upon application. This is considered an upside — but most plants rarely need or benefit from a sudden jolt of a high volume of nutrients. Appropriate applications of synthetic fertilizers can include their use on containerized plants, which grow in conditions where nutrients leach away quickly, or on heavy feeders like tomatoes
Some water-soluble synthetics are mixed with water and sprayed on plants while others are granular and dissolve upon watering or rainfall. After an application, plants may be quick to green up and grow, but they may also turn brown at the margins of leaves. This is known as nutrient burn, which happens when plants take up too many nutrients too quickly. The excess nutrients are trying to escape through the leaves, and in extreme cases, the plants can die.
Any synthetic nutrients that are not taken up right away are flushed out as water moves over the soil surface and down into groundwater, ultimately to watersheds and aquifers. Nutrients in streams, lakes and rivers eventually make their way downstream and lead to algae blooms in ponds, lakes and bays. The algae blooms result in all the dissolved oxygen in the water bodies being depleted or to a state of toxicity, leading to dead zones where fish and other marine life can’t survive.
Synthetic fertilizers often contain salt, but not the salt you are thinking of. It’s important to note that fertilizer salt is not like table salt, which is sodium chloride. The salt we’re talking about is ammonium nitrate, a high-nitrogen fertilizer. Salt will damage beneficial microbes and plant roots that it comes into direct contact with, but ammonium nitrate dissolved in water separates into ammonium ions and nitrate ions and does not cause this problem.
While you don’t need to be concerned about salt accumulation in soil, fertilizer salts do become an issue when applied in large doses. In such cases, synthetic fertilizers can create toxic concentrations of salt before they are dissolved.
Aside from nutrient burn, an abundance of nutrients taken up to quickly can lead plants to become disease-prone, with an overabundance of foliage that root growth can’t catch up to. It can also mean less fruiting.
Synthetic fertilizers also often lack the micronutrients that plants need in addition to the big three, and they add no organic matter to soil and do nothing to support beneficial microbial life that supports plants.
The Benefits of Organic Fertilizer
In most situations, the best approach to feeding plants is slow and steady, with an organic, insoluble, slow-release fertilizer.
Organic fertilizer can have a plant, animal or mineral origin. A few examples are manure, compost and blood meal. Unlike synthetic fertilizers, organics make nutrients available to plants in a much slower process that involves bacteria, fungi and algae in soil digesting the organic material into plant-available nutrients. These nutrients are not water-soluble, so they will not leach away when it rains or when the garden is watered.
Because organics do not wash away and are not taken up quickly by plants, they accumulate in soil, improving the soil texture as they do. Regular applications of organic material lead to a growing stockpile of nutrients, so plants are never wanting.
Organic fertilizer options that are high in nitrogen include blood meal (dried blood), cottonseed meal, fish emulsion and seaweed extract. For phosphorus, bone meal and rock phosphate are the way to go. And for potassium, greensand and potash are the common organic options.
NPK: Understanding the Big Three Nutrients
While there are many nutrients that plants require, the big three that they need in the greatest abundance are nitrogen, phosphorus and potassium, also known by their periodic table symbols, N, P and K, respectively. The NPK ratio on fertilizer labels gives you an idea of whether that particular fertilizer will meet your garden’s needs.
Nitrogen helps with plant growth above ground (the green, leafy foliage). Phosphorus helps below-ground growth (the roots) and is most responsible for bloom and fruit production. Potassium is important for all-around plant health due to its ability to help plants build strong cell walls.
The NPK ratio of general-purpose synthetic fertilizers is often 10-10-10. A lawn fertilizer typically has a higher first number than the rest because nitrogen is what greens up a lawn.
For a further breakdown of what an NPK ratio really means and an explanation of how much to apply, I recommend you check out “Understanding A Turf Fertilizer Label” from UMass Extension. It’s a great resource for those who don’t mind getting a bit science-y in their garden research.
Get a Soil Test Before Applying Fertilizer
To get your soil in line with ideal conditions for plant growth, get a soil test from your local county extension service or an online company. The lab will send back recommendations for what soil amendments to make and how much fertilizer to apply. The results will include details on the soil’s nitrogen, phosphorus and potassium content, as well as the many micronutrients that are less talked about. It will also identify the soil’s pH level (a measure of acidity versus alkalinity) and advise how to adjust it for the best growing results.
I hope you now have a greater understanding of fertilizer and how and when to use it. If you haven’t already done so, you can listen to this episode now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
What is your fertilizer of choice? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Tomato expert Craig LeHoullier joins me in leading this course on how to grow healthier, productive tomato plants and how to overcome tomato-growing challenges. You can sign up to be notified when enrollment opens on May 11, 2021.
Disclosure: Some product links in this guide are affiliate links, which means we would get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us. None of the items included in this list have any bearing on any compensation being an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Exmark, Greenhouse Megastore, High Mowing Organic Seeds, Territorial Seed Company, Wild Alaskan Seafood Box and TerraThrive. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.