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.
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
Step 4: Let’s fertilize!
- Use test results, crop conditions, and product instructions to guide your decisions on how much fertilizer to apply. For example, a new garden with only a moderate amount of organic matter and without the benefit of a soil test report, should be fertilized according to the label directions. You could reduce the amount somewhat if you added a large amount (>2 inches) of compost.
- When factoring in nitrogen contributions from compost take into account that only 5-10% of the N is available the first year following application.
- In a relatively small garden with just a few plants of several vegetables it is fine to fertilize all crops the same.
- Sweet potato, Southern peas, cucumber, okra, squash, and beans need less nitrogen than crops like tomato, pepper, eggplant, broccoli, leafy greens, lettuce, spinach, and sweet corn.
- Raised beds and containers, filled with soilless growing media and/or compost, need to be fertilized more regularly depending on the crop and growing conditions.
- Experiment with reducing fertilizer amounts and the number of applications once you reach a 5% soil organic matter level.
- Think before fertilizing
- Excess nitrogen can reduce flowering and fruiting and cause plants to be overly succulent and more vulnerable to sucking insect pests.
- Organic gardeners contribute to water pollution when they use excessive amounts of fertilizer and don’t prevent stormwater run-off and soil erosion.
- The nutrients in synthetic and organic fertilizers come in the form of salts. If a large amount of any fertilizer remains in direct contact with plant roots, leaves, or stems, you may see some injury symptoms. This “burning” is known as phytotoxicity.
- Fertilizer can’t bring a sick plant back to health if the problem is not a nutrient deficiency. Symptoms such as slow growth and low productivity could be caused by many factors including, lack of light, poor soil conditions, limited root system, and weather extremes.
- For long-season crops that produce fruits (tomato, pepper) or heads (cabbage, broccoli) you can apply half of the recommended amount of fertilizer right before planting and the other half when fruits start to form.
- Early season crops benefit from complete, quick-acting liquid fertilizers, or compost tea. “Starter fertilizers” high in phosphorus (P) are not beneficial because most plants need 4X more nitrogen than phosphorus.
- Perennial crops, like asparagus and rhubarb, are fertilized in early spring and after harvest.
Where and how?
- Sprinkle dry fertilizer evenly over the area where plants will be growing and not in walkways. Mix it into the top 4 inches of soil.
- For vegetables that are spaced far apart, like eggplant, tomato, and squash, mix the fertilizer into the planting hole soil or the area where seeds are planted.
- Water the area after fertilizing, especially when fertilizing during hot, dry weather.
- “Side-dressing” is applying fertilizer around or alongside established plants (pull back organic mulches first). If possible, gently incorporate the fertilizer into the top 1-2 inches of soil.
Step 5: Fertilizer recommendations from soil test reports
- Labs will often recommend urea (46-0-0) as a nitrogen source when phosphorus and potassium levels are high. You can select an organic fertilizer that only contains N (nitrate of soda) or one with an N content much higher than the P and K content (bloodmeal, cottonseed meal).
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).
- Use simple algebra to convert a synthetic fertilizer recommendation to an organic recommendation.
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
- Without the benefit of a soil test how do you fertilize a 100 sq. ft. garden with cottonseed meal, a complete organic fertilizer (6-2-1)?
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.
- Having a soil very low in P can be a challenge for a strict organic gardener. Rock phosphate (the primary organic P source) has a low P content and is very slow to break down. Superphosphate (0-20-0) and triple superphosphate (0-46-0) fertilizers are made by treating rock phosphate with sulfuric acid and are much faster-acting. These products are available for gardeners but are not allowed for use by organic farmers.
Step 6: Observe, engage, enjoy!
Take notes on your garden this year and record your successes, failures, and ideas.
By Jon Traunfeld, Extension Specialist. Read more by Jon.
Thanks for the very complete discussion of fertilizers for the home garden. Really appreciated the analysis of how much nitrogen you can expect to be released from a certain level of compost in the soil.
This is such a helpful overview. Thank you!