How much manure effect on the soil ?

Manure is a valuable fertilizer for any farming operation and has been used for centuries to supply needed nutrients for crop growth. The use of manure has generally declined on many farms over the past 50 years due to: 1) Farm specialization with increasing separation of crop and livestock production, 2) Cost of transporting manure, which is a bulky, relatively low analysis nutrient source, and 3) Increased availability of high analysis synthetic fertilizers that usually provide a cheaper source per unit of nutrient than manure. Despite these limitations, manure (and other organic nutrient sources) produced on or near a vegetable farm provide many benefits and should be beneficially utilized whenever possible.

Nutrient Availability from Manure

It is important to remember that nutrient contents in manures vary widely according to age of the animals, feed used, moisture content, degree of decomposition, and the amount of litter or bedding material mixed in with the manure. The only really accurate way of determining the nutrient content of the manure you are using is through laboratory analysis.

There are three main sources of variability and uncertainty when using manure:
1. Nutrient and moisture content of the manure.
2. Material heterogeneity and application variability.
3. Availability of nutrients to crops.

Manure or compost provides total nutrient content, but availability of the nutrients for plant growth will depend on their breakdown and release from the organic components. Generally, 70 to 80% of the phosphorus (P) and 80 to 90% of the potassium (K) will be available from manure the first year after application. Numbers from a table or from an analysis report should to be multiplied by these factors to obtain the amount of P2O5 and K2O available to crops from a manure or compost application.

Calculating N availability is more complex than determining P and K availability. Most of the N in manure is in the organic form and essentially all of the N in compost is organic. Organic N is unavailable for uptake until microorganisms degrade the organic compounds that contain it. A smaller fraction of the N in manure is in the ammonium/ammonia or inorganic form. The ammonium-N form is a readily available fraction. Other inorganic forms such as nitrate and nitrite can also exist, but their quantities are usually very low. Estimated levels of ammonium-N and organic N and other nutrient content of manure are shown in the following table 1.

Table 1 :Typical nutrient content of manure (from Koelsch and Shapiro, 2006). Because of variability between farms, individual manure analysis is preferable to the estimates below.

When applied to soil, manure, compost, and other organic amendments undergo microbial transformations that release plant-available N over time. Volatilization, denitrification, and leaching result in N losses from the soil that reduce the amount of N that can be used by crops.

Calculating the Amount of Manure or Compost to Apply

Methods for calculating the amount of manure or compost to apply have been adapted and summarized from Livestock Waste Facilities Handbook, 2nd ed., 1985, Midwest Plan Service. Composts can be thought of as similar to manure, but with little or no ammonium-N present. The amount of compost required to meet crop nutrient demands can be very large. For these situations, more readily available nutrients from other sources may be required to supplement compost additions, especially early in the growing season.

Use the following steps to determine the manure or compost rate needed for a particular crop:

Step 1
Determine the nutrient needs of the crop – Base nutrient needs on soil test recommendations.

Step 2
Determine the total nutrient content of the manure or compost – Chemical analysis of the actual product is strongly recommended; a general estimate can be obtained from Table 1 above.

Step 3
Determine the available nutrient content– Use 80% availability for P2O5 and 90% availability for K2O. Calculate N availability using the following equation:

Available N = (Organic N x Km) + Ammonium-N* , where:

Organic N = Total N – Ammonium-N (lb/ton) (from manure compost analysis or Table 1)

Km = Fraction of organic N released (% available/100)

Ammonium-N* = Ammonium-N in lb/ton (from manure analysis or Table 1）

*Note: if manure is not incorporated within 12 hours after application, reduce the value for ammonium-N using Table 3 to account for volatilization losses; reduce ammonium-N in the Available N equation, but use the full value in the equation for Organic N.

Table 2 Percent of the Ammonium-N Available to a Crop When the Time Between Application and Incorporation Is More Than 12 Hours.

Step 4
Calculate the rates of application needed to supply the recommended amounts of N, P2O5, and K2O – Divide the recommended nutrient needs from Step 1 by the pounds of available nutrients per ton of manure or compost determined in Step 3.

Step 5
Select the rate of manure or compost to apply– Frequently, manure and compost application rates are based on the N need of the crop. If manure or compost is applied on a regular basis, you may need to base rates on P to avoid excessive buildup of P in the soil, and supplement with other N sources to meet the total crop N requirement. For legumes, either P2O5 or K2O can be used as a basis for rates, depending on crop needs and soil test levels.

Step 6
Determine the amount of available nutrients applied with the manure or compost – multiply the application rate of manure or compost determined in Step 5 (in tons/A) times the estimated available nutrients (in lb/ton) determined in Step 3. The amounts calculated can be compared with crop needs (from Step 1) to determine if supplemental nutrients are needed (next Step).

Step 7
Determine whether application of additional nutrients is needed–Subtract the amount of nutrients needed by the crop (based on the soil test in Step 1) from the amounts of available nutrients applied with the manure or compost (calculated in Step 6). If the number obtained for a nutrient is zero or negative, then no further application is necessary. A positive number indicates the amount of that nutrient (in lb/A) that needs to be applied from another nutrient source to meet crop demands.