Organic farmers, like any others, need to provide enough nitrogen (N) for crops to maintain good yields, product quality and profitability. But unlike conventional farmers, organic farmers rarely rely primarily on bagged N fertilizers. That would be costly and inconsistent with the organic approach to soil fertility, which emphasizes rotation with leguminous cover crops and application of compost or manure. However, many organic farmers do need to supplement these sources with N fertilizer, and there are many different sources to choose from. Determining which organic fertilizer material to use, how much to apply and when to apply it is just as important organically as it is conventionally, although it’s a bit more challenging.

First, you need to know how much available N is recommended for the crop. The New England Vegetable Management Guide and Knott’s Handbook for Vegetable Growers provide these numbers for specific crops. A reasonable average for mixed vegetables in the northeast is 125 pounds per acre. Small, short season crops like lettuce obviously require less; high-yield long season crops like potatoes may require more. Next, subtract how much N will be released from your soil’s organic matter (OM) over the season. A conservative estimate is 10 pounds for each percent of soil OM, up to 4 percent. Finally, calculate how much N will be provided by cover crops, compost and manure, and subtract that. What’s left is the fertilizer N requirement.

Legume cover crops, or green manures, can be an excellent source of N when grown in a crop rotation system that includes them on enough land and allows adequate time to produce sufficient biomass. Incorporating a vigorous stand of alfalfa, red clover or hairy vetch early in the season can provide most if not all the N needed by a subsequent vegetable crop. These crops may contain 100-200 lb of N per acre. About half the N in a green manure will be released during decomposition following incorporation. The Northeast Cover Crop Handbook explains how to estimate the N content of various cover crops by multiplying percent dry matter by N content.

Manures and composts contain and release N in varying amounts. Fresh dairy manure promptly incorporated should provide at least 5 pounds of available N per ton. Fresh poultry manure provides about 3 times that amount. Mature compost generally contains about 1% total N, but that varies depending on how it’s made. As compost ages, the availability of the N it contains tends to decrease. In most cases only about 10% of the total N in compost will be available to a crop in the year of application. That’s about 2 lb per ton, or 3/4 lb per cubic yard.  Laboratory testing helps determine the nutrient content and availability of manure or compost.

Organic fertilizers include animal by-products, plant-derived materials and mined minerals. These can be purchased individually or as fertilizer blends. Many of these materials also contain other nutrients, and some contain carbon, which will help maintain soil OM and soil structure. As with conventional fertilizers, cost is lowest when purchasing in bulk.

ORGANIC                       % N    C/N    LB N/TON     EXAMPLE         $/LB N    RELEASE        COMMENTS
SOURCE OF  N                                                        FOB COST/TON                    RATE

fresh dairy manure             0.5        18        11                $ 8                      $0.72      medium             0.5-0.1-0.5, has weed seeds, consistency varies
fresh cage layer manure     1.5          7        30                $15                      $0.50     rapid                 1.5-1-0.5, may be hard to handle, can burn
poultry manure compost     4          15        80               $251                    $3.13      slow                  3-4-3 analysis varies, may be pelletized
finished ‘field’ compost     1.2        17        24                 $25                     $1.04      slow                   1-1-1 analysis varies, aids soil ‘health’
legume hay                       2.5        16        50             you grow             you grow     medium             strong stand with tops provides most N
grass hay                         1.2         32        25             you grow             you grow     medium             releases N when young; old growth ties N up
alfalfa meal                      2.7         15        54                 $348                    $6.44      medium             3-0.5-3, feed grade by the ton
soybean meal                  6              7       120                $314                     $2.62     medium             6-1-2, feed grade by the ton
blood meal                    12             3        260             $1,146                    $4.41      rapid                 13-2-0, mad cow risk similair to eating meat
cottonseed meal              6             7        120              $ 736                      $6.13     slow                  6-2-2 analysis, 85% water insoluble N
crab meal                        5             4        100              $  628                     $6.28     medium             5-2-0.5  analysis, 15% Ca
fish meal                         9              4        180             $1,157                     $6.42     rapid                 9-3-0, smelly, dusty, may contain high salts
feather meal                 10              4         200             $  968                      $4.84     vy. slow            contains protein slow to break down
Chilean nitrate              16             no C    320             $  500                      $1.56     rapid                 organic standards limit usage
‘Pro-Gro’5-3-4            5                3        100             $  340                      $3.40     medium             balanced organic fertilizer blend
‘Pro-Booster’10-0-0    10             2         200             $  375                      $1.87     medium              vegetable and plant meals plus 1/3 Chilean

References

Bowman, G., C.Shirley and C. Cramer. 1998. Managing Cover Crops Profitably. 2nd edition. Sustainable Agriculture Network, Hills Building, Room 10, University of Vermont, Burlington VT 05405-0082. Phone:(802) 656-0471. E-mail: nesare@zoo.uvm.edu

Ferro, D.N. (ed). 2000-2001 New England Vegetable Management Guide. University of Massachusetts Extension Bookstore, Draper Hall, 40 Campus Center Way, Amherst, MA 01003-9244. Phone: (413) 545-2717. E-mail: books@umext.umass.edu

Gershuny, G. and J. Smillie. 1986. The Soul of Soil: A Guide to Ecological Soil Management. 4th edition.Chelsea Green Publishing Company, P.O. Box 428, White River Junction, Vermont 05001. Phone (800-639-4099) www.chelseagreen.com

Kuepper, G. 2000. Manures for Organic Crop Production. ATTRA, P.O. Box 3657, Fayetteville AR 72702. Phone: (800) 346-9140. www.attra.org/attra-pub/manures.html

Mangan, F.., A. Barker, S. Bodine and P. Borten. 2000. Compost Use and Soil Fertility. In:
UMass Extension Vegetable Notes, Volume 10 No. 1. UMass Extension Vegetable Program, Ag. Engineering Building, 250 Natural Resources Rd., Amherst MA 01003. www.umass.edu/umext/programs/agro/

Maynard, D.N and G.J. Hochmuth.1997. Knott’s Handbook for Vegetable Growers, 4th edition,  John Wiley and Sons, 605 Third Ave., New York NY 10158. www.wiley.com

Magdoff, F. and H. van Es. 2000. Building Soils for Better Crops. 2nd edition. Sustainable Agriculture Network, Hills Building, Room 10, University of Vermont, Burlington VT 05405-0082. Phone:(802) 656-0471. E-mail: nesare@zoo.uvm.edu

Parnes, R. 1990. Fertile Soil, A Grower's Guide to Organic and Inorganic Fertilizers. AgAcess. Fertile Ground Books, 3912 Vale Ave., Oakland, CA 94619.. www.agribooks.com

Sachs, P.D. 1993. Edaphos - Dynamics of a Natural Soil System, Edaphic Press, PO Box 107,  Newbury VT 05051. Phone: (802) 222-4277

Sarrantonio, M. 1994. Northeast Cover Crop Handbook, Rodale Institute, 611 Seigfriedale Rd.,  Kutztown PA 19530. Phone (610) 683-6009

(The Northeast Organic Farming Association of Vermont offers many books on organic farming for sale, including some of those listed above. NOFA-VT, P.O. Box 697, Richmond VT 05477. Phone: (802) 434-4122, E-Mail: info@nofavt.org).