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Garden Fertilizers vs. Soil Amendments - What's the Difference?

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Fertilizers and soil amendments are very 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 that contains very specific nutrients and micronutrients.

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

What is a soil amendment?

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

What is a fertilizer?

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(N), 3 parts phosphorus(P), 2 parts potassium(K). But one should be aware that a plant will never take up more of any nutrient than it can immediately 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 that faces every gardener and anyone who has a lawn. As a 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, and you’re having significant plant problems, 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 the 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 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 to quickly test if your soil drains well

To get an idea of the speed at which your soil drains, dig a hole about 2 feet wide and 2 feet deep. Fill it with water. If the water is still there after 15 minutes, it’s slow. You’ll need to add a sufficient amount of compost and peat moss to get it draining. 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 do the above test in an area with friable soil you’ll see the water drain almost immediately. In this case water and nutrients are moving through the soil too quickly, faster than the plants can take them up. Once again, the answer is to add compost to introduce more organic material. But in this case the compost slows the drainage and helps to build the soil.

When to consider adding fertilizers

Improve your garden with soil amendments like compost before adding fertilizer. © Provided by Big Blog Of Gardening Improve your garden with soil amendments like compost before adding fertilizer.

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 a soil sample to a local lab for testing. The results will tell you what’s needed and what’s not.

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 they may 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. A liberal dressing of compost around the root zone in the spring, then midseason as fruits and vegetables mature, and once again in the fall, is usually sufficient feeding. Top dressings of compost year after year will improve the soil and provide most of the nutrients your veggies and fruit require, and won’t pollute local waterways with fertilizer runoff.

A partial list of soil amendments:

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Todd Heft

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