How Much Nitrogen Shall I Add?

Farmers love nitrogen because they love to see green and vigorous plants, and beginning gardeners do too. However, too much nitrogen results in weak plants susceptible to insect and disease attack. And, if you want to grow nutrient dense plants it is important to hit the “sweet spot”. Too much nitrogen will decrease nutrient density. Garden-farmers may have to go through a stage where they enjoy the vigorous growth and large sized plants high nitrogen applications bring in order to appreciate smaller, more robust vegetables.

OrganiCalc uses 150 lbs/acre of N at the default target, however this may be changed by utilizing the pull-down menu or by entering a specific amount.

Many growers rely on observation to determine when and how much nitrogen to add. While plants don’t require many nutrients when they are small, it is easier to incorporate N pre-plant. But, this comes at a cost in materials. While sidedressing during the growing season is more efficient of materials, it adds labor.

What am I growing?

• Different crops have different nitrogen demands. For vegetables, rates vary from less than 65 up to almost 400 lbs/acre N. For precision, it is best to do an internet search for nitrogen demand of your crop in your area. Accessing this information can be confusing. It is fairly easy to find crop removal rates, but these are less than the total demand for the crop. The following table is for Plant Utilization – what it takes to grow a crop. These figures are not really what we need for organic growing as much of our nitrogen can be already in the soil, but it is a good starting point.

Partial listing from: A&L Laboratories Agronomy Handbook. Highly Recommended.  A&L credits The Potash and Phosphorus Institute, The Fertilizer Institute, California Fertility Association.

How much N will be released from my soil organic matter?

• This is a difficult question to answer. The OM% value from the lab tells us nothing about the quality of the organic matter.
• N release depends on the percentage of organic matter in the soil, it’s state of decomposition, soil texture, temperature and moisture levels. Higher oxygen levels (for example, following tillage) will cause a spike in the decomposition rate and flush of available nitrogen, at the expense of soil carbon.
• If you have been routinely adding good quality compost or manure, chances are you will have a higher level of nitrogen available for release.
• If you live in a rainy climate, the anion forms of nitrogen, nitrate and nitrite, may have been lost due to leaching.
• According to the NRCS/USDA, each percent of soil organic matter in the 6” of a medium textured soil releases 10-20 lbs/acre N per year. We have more about testing for available nitrogen below.
• Not all SOM is created equal! Raised bed “soil” mixes may include unfinished high carbon composts that will not supply nitrogen. They may have high levels of organic matter but little plant available nitrogen. These soils need nitrogen amendments.

How much N will be available from decomposing crop residue and cover crops?

• Vegetable residues contain anywhere from 45-240 lbs/ac N depending on the biomass and type of crop. High throughput organic garden-farms may choose to compost these rather than incorporate them into the field.
• When cover crops or crop residue are incorporated into the soil, decomposition can release substances harmful to plants for the next 3 weeks or so depending on soil temperature and moisture.
• Cover crops may release 0 – 85 lbs/ac N after incorporation into the soil. The amount of N they release depends on the proportion of legumes, overall biomass, and the stage at which it is incorporated into the soil.
• It is entirely possible that cover crops will require nitrogen from the soil in order to decompose. Non-leguminous cover crops past the bud stage are especially likely.
• The figure above is from this source. PAN is Plant Available Nitrogen.

Can I test the nitrogen content of my soil?

• Yes and no. The Soil, Paste and More test includes a test for ammonium and nitrate. For organic growers, there is little to no ammonium in the soil because it is quickly converted to nitrate by microbes. Nitrate is easily leached and so is not a stable figure if you are in a rainy or over-irrigated situation.
• Horiba makes a nitrate tester for in-field testing.
• Soil bioactivity tests (like the Haney test from Logan) are popular and can be an indication of available nitrogen.

Which nitrogen fertilizers are available to me?

• Composted manures are good nitrogen sources but may build up potassium and phosphorus to unacceptably high levels with excess use. Only about a third of the nitrogen in composted manure will be available in the 12 weeks after it is applied, even under good conditions. However, if you consistently apply compost, nitrogen levels may build up but phosphorus and potassium levels will build up more. We wrote a handy calculator to help you determine what you are applying with your compost.
• Yard trimming compost, such as from a city green waste recycling program, is a great mulch but will tie up nitrogen if incorporated into the soil.

• Granular fertilizers such as pelleted feather meal or seed meals are good nitrogen sources to incorporate at planting. They tend to release slowly over the season. Be aware that soybean meal needs to be incorporated 2 weeks before planting. See this study.
• If you know the protein content (of soybean meal, for example) you can multiply protein by 0.16 and convert protein percent to nitrogen percent.
• Blood meal, powdered feather meal and guanos release quickly and are good for side dressing during times the plants are growing quickly.
• Liquid fertilizers such as hydrolyzed fish products or soluble amino acids have readily available N and are good to apply when the plants are actively growing.
• Recently amino acid soy hydrolysates (about 16-0-0) have become available. These supply readily available N to the plant, saving 20-some percent of the plant’s energy as they don’t need to convert nitrate to amino acids.
• If you get your soil ecosystem right, free-living nitrogen-fixing microbes may be able to supply all your nitrogen needs (please let us know if you have been able to set up the conditions for these to flourish).

To determine how much nitrogen to apply using amendments:

• Find the total amount the crop will use. For example, 120 lbs/ac for lettuce.
• OrganiCalc assumes that all the nitrogen listed on the label of the amendment will be available. This works if you have been consistently applying high quality composts and nitrogen amendments, since extra N will be available for release from your soil organic matter.
• If your soil is depleted or you are on a new plot, increase the amount of nitrogen to amend.
• Subtract the amount of N released from a cover crop. For example, if we had a 20% legume cover crop turned in at bud, it might release 20 lbs/ac N. 120 – 20 = 100 #/ac N remaining to be supplied by amendments.

Consider when and how you will be applying amendments.

• A high dose of highly soluble nitrogen at planting is a recipe for aphids and other sucking insects. Blood meal and guanos should only be applied to rapidly growing plants.
• Likewise, it is tempting to force overwintered plants with a nitrogen application. Resist this temptation!
• A handful of slow-release fertilizer and vermicompost under transplants will get them off to a good start.
• Broadcast amendments will encourage roots to spread out.
• Plant-based compost is a great mulch. Don’t rely on it for nitrogen.