How much manure should be added to a vegetable garden?

Gardens & Landscapes January 07, 2008|Print
Not all manure is alike. Many things change the nutrient content of manures:
  • Animal type, age, and condition
  • Feed type
  • Bedding type
  • Manure age, degree of rotting, and moisture content
  • The following table gives rough estimates of the amount of nitrogen (N), phosphorus (P2O5), and potassium (K2O) in several fresh manures. A moderate application rate is suggested, and the approximate quantity of each nutrient applied at this moderate rate is calculated. Cut this rate in half for light feeders (e.g., peas, beans), and double it for heavy feeders (e.g., onion, sweet corn, potato).

    Nutrients in fresh animal manures (All values rounded and approximate):

    Average Nutrient Composition (%)

    Moderate Application Rate

    Nutrients Applied (lb per acre)

    Source

    Water content

    N

    P2O5

    K2O

    (lb per 100 sq ft)

    (tons per acre)

    N

    P2O5

    K2O

    Beef cattle

    80

    0.7

    0.5

    0.6

    30

    6.5

    90

    65

    80

    Dairy cattle

    84

    0.6

    0.3

    0.6

    35

    7.5

    90

    45

    90

    Horses

    60

    0.6

    0.3

    0.5

    35

    7.5

    90

    45

    75

    Hogs

    75

    0.5

    0.4

    0.7

    40

    9

    90

    70

    120

    Sheep

    65

    1.0

    0.4

    1.0

    20

    4.5

    90

    30

    90

    Laying Hens

    75

    1.0

    1.3

    0.5

    20

    4.5

    90

    115

    45

    Broilers (litter)*

    30

    3.0

    2.8

    1.9

    7

    1.5

    90

    85

    60

    *Some broiler producers use medications, such as Roxarson and Nitarsone, in their feed mixes. Produce grown using manure from chickens receiving such medication cannot be sold as "organic."

    Supplemental phosphorus

    The suggested application rates should provide about the same amount of nitrogen from each type of manure, but may leave crops deficient in phosphorus. If your soil is low in phosphorus, or you regularly apply manures that are relatively low in phosphorus (e.g., sheep, horse), you can replenish the nutrient by applying 10 lb of rock phosphate or raw bonemeal per 100 square feet every three to five years.

    Organic matter

    Manure is produced by living things, so it contains much more than just nitrogen, phosphorus, and potassium. It also contains trace elements, which plants need in small quantities, and plenty of organic matter. Organic matter feeds the soil-dwelling organisms that make nutrients available to plants.

    Using raw manure

    • Incorporate manure into the soil as soon as it is applied to avoid losing nitrogen.
    • Avoid manure application to bare, frozen ground, or just before heavy rain to avoid losing nutrients and polluting surface waters.
    • Do not apply raw manure close to harvest. If you plan to sell products as "organic" you must leave at least 120 days between application and harvest if the edible portion of the crop comes into direct contact with the soil, or 90 days if the edible portion never touches the soil. Manure is best applied in fall, just before planting winter cover crops.
    • Manure may contain weed seeds.
    • Raw manure may burn plants.

    Advantages of composting manure

    • Composting converts soluble nutrients into more stable organic forms, which are slowly released to plants, and less easily lost.
    • Composted manure tends to have a better nutrient balance than raw manure.
    • Composting kills many weed seeds.
    • Composting breaks down most hormones, antibiotics, and pesticides found in manure (but concentrates any that aren't broken down).
    • Composting kills most disease-causing organisms.