All About Organic Garden Fertilizers

Natural sources of soil nutrients are available for free, yet in some instances you might want to buy supplemental fertilizers. Use this guide to build your soil’s nutrient content and save money over high-priced commercial fertilizers.


Pesticide-free grass clippings are one of the best fertilizer options.

Photo by GAP Photos

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As more and more people recognize the benefits of organic gardening methods, a fresh crop of organic fertilizers is sprouting up on garden-store shelves. Many are overpriced, and some are stunning rip-offs that reputable stores and catalogs shouldn’t sell. The truly amazing thing is that two of the best organic fertilizers are easily available to most of us absolutely free. If you’re not careful, you could pay five, 10 or 4,000 times more than necessary to get the nitrogen and other nutrients you need. Garden expert Barbara Pleasant and Mother Earth News editor-in-chief Cheryl Long explain the best ways to use organic fertilizers (some of them free!) to build your best garden soil and grow the happiest, most productive garden plants.

The Best Free Fertilizers

All products labeled “fertilizer” must display their content of the three major plant nutrients: nitrogen, phosphorous and potassium (N-P-K). Most organic fertilizers are bulkier than synthetic chemical products, so their N-P-K percentages are typically lower and their application rates are higher.

Also, because organic products are biologically active, their N-P-K numbers may change somewhat from batch to batch and over time. This can make it hard for producers to comply with the labeling laws. As a result, some excellent organic fertilizer options, such as compost, often are not even labeled as a “fertilizer.” One of the best free fertilizers, grass clippings, breaks down so quickly it can’t be bagged and sold.

But make no mistake, compost and grass clippings do what fertilizers are supposed to do: They enrich garden soil with nutrients that plants and microscopic soil life-forms are eager to use. In most areas, you can easily collect grass clippings from your neighborhood, bagged and ready to bring home—to make sure you’re not collecting clippings treated with chemicals, chat with your neighbors about their lawns. Many communities make yard-waste compost (made mainly from grass clippings and leaves) available for free or a small fee. Contact your city’s waste management department to ask if they offer this service.

If you can get free clippings or compost, how much should you use? We prepared these guidelines with help from soil scientists at Woods End Laboratories in Maine.

Grass Clippings: Grass clippings are one of the best organic fertilizers, and it’s easy to find free local sources. You can mix clippings directly into your garden soil; or apply them as a surface mulch where they will do double duty as a combo fertilizer-mulch, helping to prevent weeds and conserve soil moisture. The nitrogen content of clippings will vary, with fresh grass collected in spring from fertilized lawns topping five percent nitrogen, while clippings from later in the year or from unfertilized lawns will likely contain around two percent nitrogen. Again, be sure to avoid clippings from those “perfect” lawns that have been treated with herbicides.

In most regions, plants can get all the nutrients they need for a full season of growth with these application rates, applied in spring: If mixing into garden soil, apply a 1⁄2-inch layer of fresh clippings (that’s about six 5-gallon buckets per 100 square feet); if using as a surface mulch, apply a 1- to 2-inch layer.

Compost: It’s easy to make compost from your yard, garden and kitchen waste. (Learn all about composting.) If you have a large garden, however, you may want more compost than you can make from your own home. Many communities offer free yard-waste compost, or you can look for compost made by local farmers at LocalHarvest.

Yard-waste compost typically contains about one percent nitrogen, and composted manure is closer to three percent nitrogen. One of the advantages of either kind of compost is that it releases nutrients very slowly, over a period of years rather than weeks or months. All the while, bacteria and fungi introduced to the soil from the compost form partnerships with plant roots, helping them to absorb or actually manufacture even more nitrogen, phosphorus and other nutrients. Compost also helps garden soil hold more moisture.

Each time a crop is finished, spread a 1⁄2-inch layer of compost over the soil.  Twice that much is better, but even a scant 1⁄4-inch blanket of compost will help maintain your garden soil’s fertility.

Mulches and Cover Crops

As soil’s organic-matter content increases, so too does its ability to hold nutrients and retain moisture. Organic matter also plays a role in suppressing soil-borne diseases. Using mulches of shredded leaves, old hay and grass clippings will help boost soil’s organic-matter content as the mulches slowly decompose into compost. Cover crops planted during periods when you’re not growing food crops also help increase the soil’s fertility. (Read more about cover crops at Using Cover Crops in Your Garden.)

After three years of regularly adding compost and mulch, the soil’s organic-matter content will increase by several percent. Use nitrogen-rich grass clippings as mulch, and you’ll have plenty of nutrients to meet the needs of most crops.

When to Add Extra

A survey of soil testing labs across the U.S. revealed that garden soils have too much fertilizer more often than too little. Adding too much can be just as bad for your crops as not applying enough. (A soil test every few years is a good idea.) If you apply grass clippings and/or compost according to our guidelines, you will only need to use more concentrated—and costly—organic fertilizers in a few special circumstances. When you start with balanced soil and apply grass clippings, compost and mulch regularly, necessary nutrients usually will be replenished in the correct proportions.

If fertilizers sold in bags or bottles are easier for you to use than grass clippings or compost, use the nutrient nitrogen to guide your application rate because it’s the nutrient most likely to be depleted as you harvest your crops each season. Plants need the right amount of nitrogen to grow new stems, leaves and other parts. If they don’t get enough, they stay small and spindly, and never come close to their productive potential. If they get too much, they grow into huge plants that produce way behind schedule.

But, to complicate matters, if your soil doesn’t get regular additions of compost or organic mulch, it will have trouble retaining the nitrogen you add. Nitrogen is a slippery nutrient, prone to volatilizing into thin air or washing away—one of the reasons organic matter that holds nitrogen is so helpful.

Yet in some situations, even gardeners with high levels of soil organic matter may want to apply supplemental fertilizers. Usually the reasons have to do with time and temperature.

To spur early spring plants: In spring and fall, when soil temperatures are low, the biological processes that release nutrients from organic matter slow to a crawl. A fertilizer that quickly releases nitrogen (such as fish emulsion or blood meal) helps support strong, early growth of hungry cool-weather crops. Before planting big Brassicas such as broccoli, cabbage and kale, and to help spring peas get off to a strong start, mix half of the amount recommended on the bottle of one of these products into the soil before planting.

To maintain productivity: Tomatoes, peppers and other crops that stay in the ground all summer sometimes exhaust the soil’s supply of available nutrients by midsummer, just when they need it most. When the plants load up with fruit, you can prevent temporary shortfalls by mixing a light application of fertilizer into the top inch of soil over the plants’ root zones, topped off by a fresh helping of grass clippings for mulch. This “side dressing” of fertilizer and mulch work together to lengthen plants’ productivity.

To feed growing seedlings: Seedlings started indoors often benefit from a light feeding starting two to three weeks after the seeds sprout. By then, they have used up food reserves provided by the seed, yet they are not free to forage for nutrients beyond the confines of their containers. A half ration of fish-based fertilizer, mixed into room-temperature water, helps satisfy their nutrient needs until they are ready to be transplanted outdoors.

You can also use a drenching of fish fertilizer to help wake up overwintered spinach, which usually is ready to produce a flush of leaves before the soil is warm enough to release its nutrients.

To feed sweet corn: Sweet corn is famous for its heavy nitrogen needs, and that one crop can take a big bite from your soil’s nitrogen supply. To be sure your corn plants don’t run short of nitrogen, you have three options:

1. Mix in a concentrated organic fertilizer before you plant, and then side dress with more as the plants grow.

2. Precede corn with a winter cover crop of hairy vetch, alfalfa or another nitrogen-fixing legume. When the plants are chopped down as they begin to bloom in late spring, the roots left behind in the soil will release enough nitrogen to get sweet corn off to a good start. More will become available as the surface mulch decomposes.

3. Your best bet is to apply compost every year. In a three-year study done at The Connecticut Agricultural Experiment Station, a 1-inch layer of leaf compost applied to soil yearly made it possible to reduce sweet corn’s fertilizer needs by half.

This article originally appeared in our sister publication Mother Earth News as Build Better Garden Soil with Free Organic Fertilizers!

Weigh Your Options

Consider the pros and cons of these various organic fertilizer options.

Manure-based fertilizers: Usually composted or processed to reduce odors, high-quality, manure-based fertilizers produced in your area don’t carry the environmental weight of long-distance shipping.

Alfalfa, soy, cottonseed or other plant meals (alone or in blended products): Meal-based fertilizers often include 12 or more ingredients to balance fast-release nitrogen sources such as meat, fish, alfalfa or cottonseed meal with other minerals and micronutrients. These fertilizers often are less bulky compared with manure-based products, and most provide a broad array of nutrients.

Organic fertilizers: The best deals come in big packages. The prices for blended organic fertilizers in garden stores tend to cost more than a bag of soy or alfalfa meal at a farm-supply store. Dry fertilizers are almost always a better buy than liquid products. Compare the value of various products according to their nitrogen content, as nitrogen is the major nutrient most likely to become deficient in garden soils. For handy charts that compare the value of nationally available products according to nitrogen content, visit Mother Earth News.

Products dressed up with microbes, enzymes, humic acids and other substances: These extras might be helpful if you are trying to bring dead soil to life, but in a garden that is well-nourished with compost and organic mulches, they are a waste of time and money. Microbes come along for free in compost, and earthworms never charge a cent for producing enzymes and humic acids. The diverse soil life beneath your feet produces everything your crops need; all you need to do is feed it what it wants—a steady diet of organic matter.