Organic farmers, like any others, need to provide enough nitrogen
(N) for their crops to maintain good yields, product quality, and profitability.
But organic farmers rarely rely on bagged fertilizers alone to feed their
crops. That approach would be costly as well as inconsistent with the organic
standards, which emphasize the importance of crop rotation with legume
cover crops and application of compost or manure. However, on many organic
farms there may be a need to supplement those practices with N fertilizer.
Applying the right amount of fertilizer N is just as important organically
as it is conventionally, although it can be a bit more challenging.
How much N is needed? The first step in any fertilizer
decision is to figure out what quantity of nutrient is necessary to grow
a particular crop. Your regional or state Extension Crop Management Guide
may provide that information. If not, you can use a text like Knott’s Handbook
for Vegetable Growers. It contains a table which shows that vegetable crops
generally remove between 100 to 200 pounds of N per acre. Crops that produce
a lot of biomass, like potato or Brussels sprouts, take up the most N.
Crops of small stature, like spinach and lettuce, take up the least. A
good sweet corn crop uses about 150 pounds of N per acre.
How much N is already there? Soil organic matter, manure,
cover crops, and compost all contribute to the available N supply. These
sources need to be accounted for in order to avoid over-applying N, which
wastes money and can lead to pollution.
To figure out your N ‘credits’ start by estimating how much N will
be released from soil organic matter over the season. Next, add up the
N that will be provided by animal manure, green manure, plus compost, and
subtract that. What’s left is the fertilizer N requirement.
Don’t ignore soil organic matter. The total amount of
N in the plow layer of agricultural soils can be surprisingly large. A
soil with 4% organic matter contains about 4,000 pounds of total N per
acre. However, the amount of N that becomes available to plants in any
one year is relatively small. In most soils, it’s just 1% to 4% of the
total N. So, in a soil with 4,000 pounds of total N per acre, anywhere
from 40 to 160 pounds of N could be released annually for plant use. On
well-managed soils with good structure, a reasonable estimate is that for
every 1% of soil organic matter, 20 to 30 pounds of N per acre will become
available during the growing season. In warm seasons with optimal soil
moisture levels this can increase to over 40 pounds of N.
Do the manure math. Up to 50% of the total N in fresh cow
manure is available to crops in the year of application. That’s about 5
pounds of N per ton applied, or about 100 pounds of N from a 20-ton application.
To maximize the availability of N from fresh manure it should be incorporated
immediately after spreading, shortly before growing a crop. However, the
use of fresh manure on organic farms is limited by the requirement to wait
90 to 120 days after application until harvest of the crop. (Ninety days
if the harvested portion of the crop does not come in contact with the
soil, 120 days if it does.)
In the year after fresh manure has been applied, much less N is released.
Between 5% and 10% of the total N it originally contained may become available.
In the year following a 20 ton per acre application of fresh cow manure
that’s about 15 pounds of available N per acre. In the next year, two years
after application, the release of N declines again, by at least half, and
the year after that the release declines again, and so forth, until the
old manure is releasing N at the same low rate as soil organic matter.
In fields where manure has been applied consistently over many years, this
all adds up to a lot of N being available, so little or no fertilizer N
may be needed.
Poultry manure can supply roughly 3 to 6 times more N per ton than
fresh cow manure, or about 15 to 30 pounds of available N per ton per acre
in the year of application, depending on how much moisture it contains.
With poultry manure, a higher percentage of the total N is converted to
plant-available forms in the year of application, so there is relatively
less carry-over of N to crops in succeeding years.
Green manures are cover crops that are turned under to improve
soil fertility. Legume cover crops such as alfalfa, cowpeas, or hairy vetch
can fix 150 pounds of N or more per acre under good conditions. Clovers
and field peas generally fix about half as much. To provide the most N
to a subsequent crop, these legumes should be incorporated just before
or at flowering. At that stage, leafy annual legumes tend to contain 3%
to 4% nitrogen, and about 80% water.
Here’s how you can estimate the N contribution of a legume green
manure. Clip four square feet of typical growth and weigh it. Multiply
by 11,000 to get the fresh weight yield per acre. Divide that number by
5 to estimate dry weight yield per acre. Multiply that by 3.5% (.035) to
get total N per acre. If the soil is warm and well-aerated, about 50% of
that total N in the legume will be available in the summer following incorporation.
Several factors can reduce the contribution of N from legume green
manures. If the seed is not properly inoculated, a legume may not form
the root nodules necessary to fix N from the air. A fair stand will provide
significantly less N than a good stand. As legumes age and get woodier,
the foliage will contain a lower percentage of available N. And, if conditions
are wet or cool, microbial activity will be reduced and so will the amount
of N release via decomposition.
Legume green manures are a great way to obtain N for several reasons.
They fit into a good crop rotation plan, helping to maintain soil structure.
By growing your own N, they can help avoid the need to purchase more expensive
N sources. And, they allow you to add N to the farm without adding other
nutrients. This is especially important when soil P levels are already
Mature compost: is a soil conditioner. Mature compost
is good for the physical and biological health of soil, but it is not a
strong fertilizer. It generally contains about 1% total N, with some variation
depending on how it’s made. As compost ages, the availability of the N
it contains tends to decrease. The majority of the N in finished compost
is tied up in organic compounds that slowly decompose over a period of
years. A rough estimate is that only 10 to 15% of the total N in compost
will become available to a crop in the year of application. That’s about
2 to 3 pounds per ton.
Use the lab. Soil testing labs can help you manage N
in several ways. Soil organic matter percentage is often reported as part
of normal soil test results, or it may be available upon request for an
additional fee, depending on the lab you use. Compost and manure tests
report the total N and the available N content, as nitrate and ammonium.
The pre-sidedress nitrate test is available in some areas, for specific
crops, to estimate how much available N the soil will supply over the growing
season, and whether supplemental N fertilizer should be applied.
Organic fertilizers are a last resort, to be used only if
the sources described above will not fulfill your crop’s N needs. Organic
fertilizers may be made of animal by-products, plant-derived materials,
mined minerals, or a combination. As with conventional materials, organic
fertilizers can provide primarily N, or a blend of nutrients. All fertilizers
are less expensive per unit of nutrient when purchased in bulk.
Animal-derived N fertilizers include: blood meal, crab meal, feather
meal, fish meal, and pelletized, composted chicken manure, to name a few.
Plant-derived N fertilizers include alfalfa meal, peanut meal, and soy
meal, to name a few. Be aware that GMO issues may affect the acceptability
of certain plant by-products in organic agriculture.
Chilean nitrate is a mineral source of N for organic farms, but its
use is limited to providing up to 20% of a crop’s N needs, and it may be
further restricted by some certifying agencies. It can be helpful in providing
available N to crops early in the season when soils are cool, microbes
are sleepy, and the release of N from other sources is low.
If you are an organic farmer, before purchasing or applying any fertilizer
make sure it is complies with the organic standards! Look for the OMRI
seal, or check with your certifying agency to get assurance that the brand
of fertilizer, not just the generic ingredient, is allowed for use on organic