How much manure per acre?

From Tiny Farm Wiki

We're starting a new market garden this season, about 1 acre, and I've been looking into how much manure to spread. We don't have on-farm manure, so the quantity and quality of purchased inputs are critical considerations, for cost and for soil health. The field was last used for hay that was allowed to grow out to orchard grass and some timothy. What I'm looking for is a tons per acre application rate for whatever type of manure is available, as a starting point.

[edit] Manure quality

Manure varies widely, depending on:

• animal source and what its been eating (livestock/feed)
• whether bedding is mixed in
• age of the manure, from fresh to fully composted
• moisture content

These factors determine the amount of nutrients and organic matter a manure contains per given weight.

Besides the strictly nutritional component, what else is in the manure is important. Two important considerations are:

• drugs and hormones from animal treatments
• weed seed

Manure Management (basics) - A good page going into more detail about the factors affecting manure quality from the University of Minnesota.

[edit] Application rate estimates

One basic approach to calculation is to determine the amount of nitrogen (N), phosphorus (P) and potassium (K), generally considered the three most important plant nutrients, contained in a particular manure. This in turn is applied to the needs of various vegetable crops, for example, so many pounds of N, P and K per acre. Veggie crops can be broadly divided into two classes: heavy feeders and light feeders, based on their NPK requirements.

A variation of this is to calculate requirements based on nitrogen requirements alone. N is often considered the most critical nutrient, and available nitrogen is easily leached from the soil. There's a clear and detailed discussion of this in: Managing Nitrogen on Organic Farms.

[edit] Example: Estimating fresh manure using NPK requirements

Here's a sample chart illustrating this calculation for various types of fresh manure. For example, a moderate application rate for heavy feeders is indicated at around 7 tons of fresh cow manure per acre.

Numbers and estimates vary greatly.

According to Fertilizing Your Vegetable Garden (John M. Swiader, University of Illinois):

Adequate nitrogen, phosphorus, and potassium can also be supplied without using concentrated fertilizers. If you prefer to use an organic fertilizer, 4 bushels of fresh cow manure or 1 bushel of chicken manure per 100 square feet will provide adequate nitrogen.

Calculation: 4 bu fresh cow manure = 200 lbs. 1 acre = 43,560 sq ft = 435.6 x 100 sq ft. 200 lbs x 435.6 = 87,120 lbs = 43.56 tons fresh cow manure per acre

Rodale's Garden Problem Solver (Jeff Ball, Rodale Press) states:

On a new garden, with questionable soil, you can add from 200 to 400 pounds of aged (3 to 6 months) undried cow or horse manure, or 4 to 8 bushel baskets (30 to 60 gallons) per 100 square feet. That is equivalent to an inch of manure over the whole surface of the garden. If you use commercial dried cow manure, use less, or about half as much as the fresh.

This recommendation translates to about 43-87 tons of 3-6 month aged cow or horse manure per acre for a new garden. For subsequent years, only 40lbs/100 sq ft (8.5 tons/acre) is recommended.

[edit] Calibrating a manure spreader

Handy ways to determine how much a spreader actually holds, and how much coverage it gives per load here.