Plant Fertilizers vs Soil Amendments – What’s The Difference?

Fertilizers and soil amendments are totally different things – one is meant to improve your soil, the other is meant to feed your plants. But in some cases, a soil amendment is also a fertilizer which contains very specific nutrients and micronutrients.

soil amendments, fertilizers

Improve your garden with soil amendments like compost before adding fertilizer.

For organic gardeners, the goal always is to improve your soil with amendments to build a rich environment of humus, microorganisms and micronutrients. This in turn encourages deep root growth, strong, disease-resistant plants, beautiful blooms, and abundant fruits and vegetables. The fertility and viability of the soil is the goal, not the quick fix (and sometimes useless application) of fertilizers to stimulate growth.

A soil amendment is any kind of organic or nonorganic material that improves the condition of your garden soil. The primary purpose is to improve the texture of the soil to make water and air pockets readily available to plant roots. For instance, you can amend clay soils to loosen them and improve drainage or add amendments to sandy soils to retain nutrients and water and provide food for microorganisms. Soil amendments can include animal manures, worm castings, fall leaves, perlite, compost, straw, grass clippings, greensand, gypsum, hay, cover crops , or other materials. Manures, compost and leaves may also be considered slow release fertilizers as they contain many or all of the nutrients your plants need, especially in combination.

A soil amendment is defined as: any material added to a soil to improve its physical properties, such as water retention, permeability, water infiltration, drainage, aeration and structure. The goal is to provide a better environment for roots.  
Colorado State Extension Service

Chemical or organic fertilizers  are concentrated nutrients added to the soil to stimulate plant growth. Chemical fertilizers tend to be very high in concentrations and organic fertilizers are somewhat gentler. Fertilizers are sold in ratios, which are marked on their bags, such as 5-3-2: 5 parts nitrogen, 3 parts phosphorous, 2 parts potassium. But one should be aware that a plant will never take up more of any element than it can use.

Fertilizers alone do not help improve a soil’s structure. If your plants are suffering from a lack of air or water in the root zone, or too much water in a poorly draining or compacted soil, all the fertilizer in the world won’t help. In fact, if your soil is compacted, fertilizers may just run off the surface and will never be taken up by your plants’ roots.

Should you add a fertilizer or a soil amendment to your garden or lawn?

This is the big question which faces every gardener and anyone who has a lawn. Rule of thumb, garden and lawn problems are frequently with the soil, not a lack of fertilization. The soil is always the first place to look.

How do you know what the soil problem is?

If you live in an area with heavy clay soil, I’ll bet you donuts to dollars that your soil isn’t draining well. Poor drainage allows water to collect around plant roots for much too long, which encourages disease and limits available oxygen for the roots. A tell-tale sign of this is a lawn which doesn’t drain after a storm or a garden with too much mud or puddles. To get water moving through the soil, add lots of compost across the entire garden or lawn and work peat moss into the soil in very dense clay areas. You’ll also want to aerate your lawn once a year.

How do you know when enough is enough? Dig a hole about 2 feet wide and 2 feet deep. Fill it with water. If the water is still there after 15 minutes, add more compost and peat moss to your soil. Peat moss is outstanding in separating clay particles to improve drainage. But don’t be concerned that adding peat moss to clay soil will make it highly acidic – it probably won’t, unless you add truckloads, or are adding it to raised beds. Adding bricks of peat moss occasionally garden-wide (in an area with clay soil) won’t move the dial on pH more than 0.5-1.0 points. I’ve been adding it to my Pennsylvania garden soil for years, with little change to pH.

If you live in an area with friable soil – soil that drains well – you may have the opposite problem: water and nutrients are moving through the soil too quickly, faster than the plants can take them up. In this case, once again, the answer is add compost to introduce more organic material to retain water and build soil.

If you’ve added sufficient soil amendments and you’re still experiencing growth or bloom problems, then it’s time to consider fertilizers. But as all fertilizers are sold with different nutrient ratios, you need to know what your soil needs instead of embarking on a willy-nilly application of whatever is closest at hand at the garden center. Test your soil either with a good quality test kit or better yet, send it to a local lab for testing. The results will tell you what’s needed and what’s not.

Having said that….

Fertilizers may or may not be required annually for growing fruits and vegetables, as these plants are generally more demanding than your perennial garden or lawn. Many fruits and vegetables are annuals and not native to your area, so many require assistance for optimal production. As above, amend the soil first (compost, compost, compost) and learn which elements your particular food crops require before adding any fertilizers. For instance, root vegetables like carrots have very different requirements than raspberries. As above, a liberal dressing of compost in the spring, then midseason around the plant’s root zone as they mature, and once again in the fall, is sufficient for most. Top dressings of compost year after year will improve soil and provide most of the nutrients your veggies and fruit require, and won’t pollute local waterways with fertilizer runoff.

More about organic fertilizers from Rodale

Partial list of soil amendments:

About Todd Heft

Todd Heft is an organic gardener and freelance garden writer who lives in the Lehigh Valley, PA and has gardened for most of his life. When he isn’t writing or reading about organic gardening, he’s gardening. His first book, “Homegrown Tomatoes: The Step-By-Step Guide to Growing Delicious Organic Tomatoes In Your Garden” is available on Amazon now.
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2 Responses to Plant Fertilizers vs Soil Amendments – What’s The Difference?

  1. weedmanmpls2 says:

    Great information on soil amendments vs plant fertilizers. I know I have seen organic top dressing, that would be I suppose an amendment and a fertilizer. The biggest thing is a lot of people just think they are the same thing and then the expectations are wrong. Though they are both good and serve a purpose, it is important to use the correct solution for what you are doing in the yard.

  2. Sai Bharath says:

    Nice post. Found it informative and very well written. Got to know a lot about soil amendments. Will surely try it for my lawn.