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Vegetable plants to gardener: “feed me and I’ll feed you”

You’re getting ready to plant vegetable seeds and transplants for the first time and trying to make sense of the conflicting advice you’ve been getting from HGIC, a neighbor, and your brother-in-law. You want to grow your vegetables organically but now realize that you don’t have a clue about fertilizing. Are there enough nutrients in the not-so-great soil or in the “potting media” used to fill a raised bed? What type of fertilizer should you use? How much and when? Take a deep breath and relax. You and your plants are going to get through this together.

Step 1: Test your soil

Your plants will get most of the nutrients they need from air and water, and the minerals and organic matter in the soil. Soils vary quite a bit, and soil testing is the surest way to get important baseline data on soil pH (affects nutrient availability), nutrient levels, organic matter content, and the amount of lead (Pb) present. After carbon, oxygen, and hydrogen, the nutrients needed in the greatest amounts are nitrogen, phosphorus, potassium (the primary macro-nutrients), and calcium, magnesium, and sulfur (the secondary macro-nutrients). Basic soil tests also include some micro-nutrients required in very small quantities, such as molybdenum, copper, zinc, and boron.

The lab sends you a report showing which nutrients are in the medium-excessive range (no worries) and which are deficient (you’ll get fertilizer recommendations).  Labs don’t test for nitrogen because it changes quickly, moving between organic forms (immobilized inside living organisms) and inorganic forms (mineralized as ammonium and nitrate).

It’s ok to start gardening if you missed Step 1, but try to test your soil sometime between now and the fall.

Step 2: Feed the soil to feed your plants

Soil organic matter is made up of living and dead organisms- plants, bacteria, fungi, earthworms, and countless others. Nutrients, like the nitrogen needed to build proteins, are locked up in organic compounds in living organisms. When plants and animals decompose, these nutrients are transformed into inorganic forms, available for use by plants and soil microbes.

So, soils really do feed plants. Adding organic matter in the form of plant residues, compost, organic mulches, and cover crops will increase soil organic matter levels and ensure a slow and steady supply of plant-available nutrients. Organic matter also improves the structure of the soil, allowing for better movement of air and water, and a better home for plant roots and soil critters.

Vegetable crops, as a group, are “heavy feeders” compared to annual flowers and perennials and compete poorly against scrappy weed species for soil nutrients. They need our help to ensure strong, continuous growth. After carbon, oxygen, and hydrogen, nitrogen is the nutrient required by plants in the greatest amount. For each 1% of soil organic matter, about 0.4 lbs. of nitrogen/1,000 sq. ft. is available for plants (conservative estimate). A soil with a 5% level would release about 2 lbs. of nitrogen/1,000 sq. ft. which is a typical nitrogen recommendation for vegetable gardens. The problem is that organic matter may not be able to supply sufficient nutrients at particular times of the season and at particular stages of plant development. Nevertheless, many people with well-established, high organic matter gardens, forego supplemental fertilizers and get large harvests.

Compost is mixed with soil before planting chile pepper. The compost will supply a portion of the nutrients needed for high yields.

Step 3: Types of organic fertilizers

Fertilizers are regulated materials that contains at least one plant nutrient. The nutrient content is guaranteed by the three numbers (e.g., 3-4-3) found on a fertilizer bag or container. Also known as the nutrient analysis, the numbers represent the percentage, by weight, of nitrogen (N), phosphate (P205), and potash (K2O), respectively.

A complete fertilizer contains all three of the primary macro-nutrients. Some fertilizers contain only one or two of the three major nutrients, such as nitrate of soda (16-0-0), a good choice when your soil test indicates high levels of P and K.

Commercial organic fertilizers are relatively low and variable in nutrient content, and typically release nutrients more slowly than synthetic fertilizers. They are also more expensive to buy on a per pound of nutrient basis. Many are made from composted or processed animal and plant waste products, such as fish fertilizers, composted manure, and cottonseed meal. A number of products are blends of several organic ingredients. Some organic fertilizers are inorganic materials (lack a carbon backbone), such as rock phosphate, and sodium nitrate.

Vegetarian or vegan fertilizers are all plant-based, like alfalfa meal and yard waste compost. Some organic fertilizers sold to home gardeners carry OMRI (Organic Materials Review Institute) certification. Farmers participating in the USDA’s National Organic Program can only use OMRI-certified fertilizers. Gardeners are not restricted in this way.

Some Organic fertilizers               Nutrient analysis             Nutrient release rate

Fish emulsion                                    5-1-1                                      fast

Bloodmeal                                          15-1-0                                   med-fast

Cottonseed meal                              6-2-1                                      med

Alfalfa meal                                        3-1-2                                      med

Nitrate of soda                                  16-0-0                                   med-fast

Dried poultry litter                           4-3-3                                      med-fast

Mushroom compost                       2.5-1.5-1.5                           slow

Rock phosphate                                0-14-0                                   very slow

Muriate of potash                            0-0-60                                   med-fast

Nitrate of soda is a mined mineral. Organic farmers can use this particular, single nutrient fertilizer because it’s OMRI-certified.
This is a complete liquid organic fertilizer that is not OMRI-certified but is safe and effective for organic gardeners.

Step 4: Let’s fertilize!

 How much?


Where and how?

Step 5: Fertilizer recommendations from soil test reports

This U. of DEL soil test report recommend only nitrogen for a vegetable garden that tested “excessive” for other macro-nutrients. Note the very high % of soil organic matter. Labs do not credit the expected nitrogen release from soil organic matter when making fertilizer recommendations.

The soil test report above recommends 1 lbs. of N/1,000 sq. ft. which equals about 2.5 lbs. of urea.

If you decide to use bloodmeal you will need 7.6 lbs. (46% divided by 15% X 2.5).

Cottonseed meal, soybean meal, and alfalfa meal are dry, organic fertilizers with a relatively high N content.

Example: the recommendation is to apply 20 lbs. of 10-10-10/1,000 sq. ft. and you wish to substitute cottonseed meal (6-2-1).

Divide the percentage of N in the synthetic fertilizer by the percentage of N in the organic fertilizer and multiply by 20:

0.10/0.06 (or 10%/6%) X 20 = 33.3 lbs. of cottonseed meal

Calculate the amount of the fertilizer product needed by dividing the pounds of N needed by the percentage of N in the product.

Let’s assume we need 2 lbs. of nitrogen per 1,000 sq. ft. or 0.2 lbs. of nitrogen per 100 sq. ft.

0.2 (lbs. of N) divided by 0.06 (% of N in cottonseed meal) = 3.33 lbs./100 sq. ft.

Step 6: Observe, engage, enjoy!

Take notes on your garden this year and record your successes, failures, and ideas.

Send us your questions!

By Jon Traunfeld, Extension Specialist. Read more by Jon.

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