Using Manure as Fertilizer for Vegetable Crops
Carl J. Rosen
Department of Soil, Water, and Climate
University of Minnesota
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 for vegetable crops, however, has generally declined over the past 50 years and can be attributed to 1) lack of animals being raised on many specialized vegetable farms and 2) high analysis synthetic fertilizers providing a cheaper source of nutrients than relatively low analysis manure. Increased interest in organically grown vegetables has prompted a renewed interest in using manure as a fertilizer source. Manure not only supplies many nutrients for crop production including micronutrients, but is also a valuable source of organic matter. Increasing soil organic matter improves soil structure or tilth, increases the water holding capacity of coarse-textured sandy soils, improves drainage in fine-textured clay soils, provides a source of slow release nutrients, reduces wind and water erosion, and promotes growth of earthworms and other beneficial organisms.
Proper use of manure is essential from both a production and environmental standpoint. Too low a rate of applied manure can lead to nutrient deficiency and low yields. On the other hand, too high a rate can lead to nitrate leaching, nutrient runoff, and excessive vegetative growth. Thus, understanding how to manage manure is important for any vegetable operation that relies on manure as a major source of nutrients. This report will address differences between composition of fresh and composted manure, nutrient availability from manure, and calculation of how much manure to apply.
Nutrient Composition of Manure
Many different types of manure are available for crop production. For this discussion, it is assumed than most vegetable growers will be using solid manure with or without bedding. The nutrient content of these manures will vary with animal, bedding, storage, and processing. The approximate nutrient composition of manures is presented in Table 1. While this table provides a general analysis of manure nutrient content, it is strongly recommended that if routine applications are made for crop production the manure be tested by a laboratory for moisture and nutrient content. Accurate manure analysis requires a representative sample be submitted and that the volume/weight of manure is known. Several subsamples from the pile should be collected to make up the sample. If manure or compost is being purchased, request a nutrient analysis for N, P2O5, and K2O content.
Fresh vs. composted manure: Fresh noncomposted manure will generally have a higher nitrogen content than composted manure (Table 1). However, the use of composted manure will contribute more to the organic matter content of the soil and is considered the preferred amendment for organic crop production. Fresh manure has high soluble forms of nitrogen, which can lead to salt build-up, leaching losses, and nitrate accumulation in leafy vegetable plants if over applied. In addition, fresh manure may contain high amounts of viable weed seeds, which can lead to a weed problem. The microbially mediated composting process will lower the amount of soluble nitrogen forms by stabilizing the nitrogen in larger organic humus-like compounds. Some of the ammonia-N will be lost as a gas during composting. Heat generated during the composting process will kill most weed seeds. Unfortunately, many vegetable crops have relatively shallow root systems, high nutrient demands, and short maturities. Compost alone, in some cases, may not be able to supply all the nutrients, particularly nitrogen, for fast growing vegetable crops.
Table 1. Approximate nutrient composition of various types of animal manure/compost. All values are on a fresh weight basis.
|
Manure type |
Dry matter % |
Ammonium-N lb/ton |
Total Na lb/ton |
P2O5 lb/ton |
K2O lb/ton |
|
Swine, no bedding |
18 |
6 |
10 |
9 |
8 |
|
Swine, with bedding |
18 |
5 |
6 |
7 |
7 |
|
Beef, no bedding |
52 |
7 |
21 |
14 |
23 |
|
Beef, with bedding |
50 |
8 |
21 |
18 |
26 |
|
Dairy, no bedding |
18 |
4 |
9 |
4 |
10 |
|
Dairy, with bedding |
21 |
5 |
9 |
4 |
10 |
|
Sheep, no bedding |
28 |
5 |
18 |
11 |
26 |
|
Sheep, with bedding |
28 |
5 |
14 |
9 |
25 |
|
Poultry, no litter |
45 |
26 |
33 |
48 |
34 |
|
Poultry, with litter |
75 |
36 |
56 |
45 |
34 |
|
Turkey, no litter |
22 |
17 |
27 |
20 |
17 |
|
Turkey, with litter |
29 |
13 |
20 |
16 |
13 |
|
Horse, with bedding |
46 |
4 |
14 |
4 |
14 |
|
Poultry compost |
45 |
1 |
17 |
39 |
23 |
|
Dairy compost |
45 |
<1 |
12 |
12 |
26 |
|
Mixed compost Dairy/Swine/Poultry |
43 |
<1 |
11 |
11 |
10 |
a
Total N = Ammonium-N plus organic NSources: Midwest Plan Service - Second ed., 1985.
Organic Soil Amendments and Fertilizers Univ. of Calif. #21505, 1992.
The use of noncomposted manure for organic vegetable production is discouraged by most organic certification associations. Noncomposted manure that has been turned and free of internal frost for six months prior to application is permitted. General restrictions for application of noncomposted manure based on standards set by the Organic Crop Improvement Association are as follows: "Fresh, aerated, anaerobic, or sheet composted manures are permitted on perennials or crops not for human consumption, or when a crop for human consumption is not to be harvested for at least four months following the application. At application, the soil temperature must be at least 50 F and moist to ensure active microbial digestion. All manure sources and management techniques must be clearly documented as part of the certification process". The decision on what form of manure to use will ultimately depend on certification requirements, availability, and cost. Composted manure is usually more expensive than fresh or partially aged manure.
Nutrient Availability from Manure
The analysis of manure or compost provides total nutrient content; however, availability of the nutrients for plant growth will depend on the breakdown and release from of the organic components. Generally, 70 to 80% of the phosphorus and 80 to 90% of the potassium 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 the manure application.
Calculating nitrogen availability is more complex than for phosphorus and potassium. Most of the nitrogen in manure is in the organic form. This form is considered unavailable for uptake until it is degraded by microorganisms. A smaller fraction of the nitrogen in manure is in the ammonium/ammonia form or inorganic form. The ammonium form is considered 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 total N in fresh and composted manure are shown in Table 1.
When applied to soil, manure undergoes microbial transformations that release nitrogen over time. The stepsof nitrogen transformation in manure are as follows:
Volatilization (NH3)
^
| denitrificaton (N2)
| /
organic N -----> NH3 <-----> NH4 -----> NO2 ------>NO3 -------->
unavailable (ammonia) (ammonium) (nitrite) (nitrate) \
available available leaching (NO3)
|----- mineralization ------| |-- nitrification --|
Shown in Table 2 are estimates of N availability from manure the first growing season after application. The actual amount available is dependent on manure type, bedding, and whether the manure has been composted. Usually 25 to 50% of the organic-N in fresh manure is available the first year. In addition to the organic fraction, N available from manure also has to take into account the amount of ammonium-N present. This form of N is readily available for plant uptake, but is prone to losses as ammonia if not incorporated within 12 hours after application. Assuming direct manure incorporation after application, 45 to 75% of the total N (organic-N + ammonium-N) is available the first year. Note that for composted manure, the amount of N available is much lower than for fresh manure. Since there is very little ammonium-N in composted manure, the organic N fraction is basically the same as the total N fraction.
Table 2. Estimated nitrogen availability from manure the first season after application. Total N refers to organic N plus ammonium-N.
|
Manure Type |
Organic N % available |
Total N % availablea |
|
Swine, fresh |
50 |
65 |
|
Beef, no bedding |
35 |
60 |
|
Beef, with bedding |
25 |
55 |
|
Dairy, no bedding |
35 |
60 |
|
Dairy, with bedding |
25 |
60 |
|
Sheep, solid |
25 |
60 |
|
Poultry, no litter |
50 |
75 |
|
Poultry, with litter |
45 |
65 |
|
Horse, with bedding |
20 |
45 |
|
Composted poultry |
30 |
35 |
|
Composted dairy |
14 |
14 |
a
Assumes incorporation within 12 hours after application
Type of bedding or litter used will usually decrease the nutrient content by dilution. If straw is used as the bedding, nitrogen availability may be lowered by increasing the C/N ratio of the product. High carbon relative to nitrogen will lead to a tie-up of nitrogen, potentially causing nitrogen deficiency in the crop. A C/N ratio of 25/1 or greater will lead to nitrogen tie up. A C/N ratio of less than 25/1 will release nitrogen to the crop. The C/N ratio is also an important consideration in the use of various composts.
Manure Application
As discussed above, some of the nitrogen in fresh manure will be lost to the atmosphere during application in the form of ammonia gas. The higher the ammonium-N fraction is in manure the more prone it is to ammonia losses. For vegetable crops, manure should be applied in the spring and incorporated within 12 hours. To avoid salt injury from fresh manure, seeding operations should take place about 3-4 weeks after application. Fall and winter applications are not recommended. Because of lack of incorporation, manure used for established perennial vegetables such as asparagus should will supply the organic N fraction and at most 20% of the ammonium-N fraction. Without incorporation, most of the ammonia-N will be lost through volatilization. Incorporation of composted manure is not as critical because the nitrogen is stabilized in organic compounds with little free ammonium present. In order to obtain full benefit from compost, incorporation is recommended whenever possible.
Residual Nutrients in Soil from Manure Application
The residual effects of the manure or compost are important. Some benefit from manure will be obtained in the second and third year following application. When manure and compost are used to fertilize crops, soil organic matter will increase over time and subsequent rates of application can generally be reduced. Continuous use of manure or compost can lead to high levels of residual nitrogen and other nutrients, which can potentially runoff or pollute the groundwater. Taking into account residual release of nitrogen in subsequent years should help to avoid excessive applications. General rules of thumb for nitrogen are that organic N released during the second and third cropping years after initial application will be 50% and 25% respectively of that mineralized during the first cropping season. It should be remembered that some manure contains high levels of phosphorus, so while crop nitrogen demands are met, soil levels of phosphorus may be building up. Use of soil and tissue tests, and monitoring of crop growth will help in determining the amount of residual nitrogen and other nutrients in the soil as well as the need for further applications.
Calculating the Amount of Manure or Compost to Apply
The amount of manure to apply has been adapted and summarized from "Manure Management in Minnesota" - M.A. Schmitt, Minnesota Extension Service, FO-3553, 1988. Consider compost as a form of manure with little or no ammonium-N present. The amount of compost to apply in many cases can be very high in order to meet crop nutrient demands. For these situations, more readily available nutrients from other sources may be applied to supplement compost additions. The following steps can be used to determine the manure rate needed for a particular crop:
Step 1.
Determine nutrient needs of the crop - Base on soil test recommendations.
Step 2.
Determine the total nutrient content of the manure - Manure analysis is strongly recommended to obtain these values; however, an estimate can be obtained from the Table 1 above.
Step 3.
Determine the available nutrient content - Use 70-80% availability for P2O5 and 90% availability for K2O. For nitrogen availability, two options are available:
a) if ammonium-N and organic N content are known:
Available N = (Organic N x Km) + Ammonium-N
where:
Organic N = Total N - Ammonium-N (lb/ton) (from manure analysis or Table 1)
Km = Fraction of organic N released (%/100) (from Table 2)
Ammonium-N = Ammonium-N in lb/ton (from manure analysis or Table 1)
Note: ammonium-N values assume incorporation within 12 hours after application. Reduce the value of ammonium-N as follows if incorporation is not done within 12 hours:
|
Days Until Incorporation |
% of Ammonia-N Available to Crop |
|
0.5-2 |
80 |
|
2-4 |
60 |
|
4-7 |
40 |
|
>7 |
20 |
b) if only total N is known:
Available N = (Total N x Km)
Where:
Total N = Organic N + Ammonium-N (lbs/ton) (from manure analysis or Table 1)
Km = fraction of total N released (%/100) (from Table 2)
Note: the Km values in Table 2 assume incorporation of manure within 12 hours after application.
Step 4.
Calculate the rates of application needed to supply the recommended amounts of N, P2O5, and K2O - Divide the recommended nutrient values from Step 1 by the lb of available nutrients per ton from Step 3.
Step 5.
Select the rate of manure to be applied - For most fruit and vegetables, base the manure application rate on the nitrogen need. The exception for this would be for legumes such as peas or beans where either P or K should be used depending on need.
Step 6.
Determine the amount of available nutrients applied with the manure - multiply the rate of manure applied from Step 5 in tons/A times the estimated available nutrients in lb/ton calculated in Step 3. The amounts calculated can be compared with crop needs from Step 1 to determine if supplemental nutrients are needed.
Step 7.
Determine whether application of additional nutrients are needed - Subtract the amount of nutrients needed based on a soil test in Step 1 from the amount of available nutrients applied with the manure in Step 6. If the number obtained is zero or negative then no further application is necessary. A positive number indicates the amount of nutrient in (lb/A) that would need to be applied to meet crop demands.
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