Horse manure can provide a great source of nutrients to your pastures and forage. There are a number of benefits to spreading manure compost when compared to raw manure. Nitrogen and other plant nutrients in composted manure are in their organic form and some are not immediately soluble in water. These nutrients are released gradually into the pasture soil providing a slow release fertilizer source. This decreases the risk of immediate leaching and extends the availability of nitrogen throughout the growing season.
Some of the other benefits of adding compost to pastures include:
- Increased water infiltration
- Increased water-holding capacity
- Increased aeration and permeability
- Increased soil aggregation and rooting depth
- Decreased soil crusting
- Decreased soil bulk density
- Decreased runoff by erosion
Before spreading composted manure onto your pastures it is a good idea to sample it and have it tested in a nearby soil lab. Testing the manure for total nitrogen (N), phosphorus (P), potassium (K), ammonium- (NH4+-N) and moisture content will allow you to accurately match the manure application with the pasture needs. Before sampling your manure compost make sure that you check with your local soil lab to see if they have special sampling guidelines they would like you to follow.
In general, you should obtain a representative sample by collecting from 6 to 8 different locations within your manure pile. Make sure that you take samples from both the exterior and the interior of the pile. Next, thoroughly mix the manure samples to break up any clumps and combine them well. Collect a minimum of 3 sub-samples from your representative sample and label well. Ideally, the samples should be analyzed within 24 hours of collection. It is also possible to test the nitrogen, phosphorous and potassium content with a simple soil kit from a local garden centre, however, these are not always accurate and soil labs will be able to give you a better idea of appropriate application rates. An over-application of compost can lead to leaching and surface or groundwater pollution, while too little can reduce pasture growth.
Application rates are based on: (1) nutrients required by the plant for optimum growth, (2) nutrients present in the soil, and (3) nutrients available in the compost. The application rate will also depend on field topography, climatic region and soil type so these should be taken into account when spreading. According to the BC Ministry of Agriculture and Lands, manure or a manure/bedding mixture from 3 to 4 horses can be spread on each acre of productive pasture on your land.
Most soil and manure testing labs will supply crop requirement information and application rates in easy to read formats. Once you determine the needs of your pasture grasses the following example calculation will help you determine the appropriate compost spreading rates for your land.
Example for calculating compost application rates
Determine: Compost application rate to supplying 200 kg of N. Given the following compost analysis, which can be obtained through a soil test:
Total N (TKN) 1.59% (or 15,900 ppm)
Mineral N Ammonia (NH4) 1562 ppm
Nitrate (NO3) 672 ppm
Bulk density of compost is 400 kg/m3
Assume: 50% loss of ammonia (NH4-N) (based on research with manures, composts and biosolids)
20% of organic N is available in year of application (based on a range of 10 to 30% from research on composts and biosolids)
Calculation: Total available N in the first year is: available organic + remaining ammonia + nitrate
Total organic N = (TKN - NH4) = 15,900 ppm - 1562 ppm = 13,666 ppm
Available organic N = 13,666 ppm x 20% = 2733 ppm
Remaining NH4 = 1562 x 50% = 781 ppm
Nitrate (NO3) = 672 ppm
Total available N = 2733 ppm + 781 ppm + 672 ppm = 4186 ppm (or 4.18 kg/tonne)
To obtain 200 kg N per ha apply 47.8 tonnes compost per hectare (200 kg N/ha / 4.18 kg N/t) or 47.8 tonnes/ha x 1000 kg/tonne x 1 m3/400 kg = 120 m3/ha
Answer: Therefore, 120 m3/ha of compost can be applied to supply the 200 kg N/ha.
Source: Using Compost, Composting Factsheet (1996), BC Ministry of Agriculture and Food
When spreading manure compost it is important to remember that it should only be spread during the growing season from April to September. Up to 1/3 of fall and winter applied nitrogen in manure may be lost by denitrification, volatilization, leaching and surface runoff during the spring thaw. Only apply a ¼ inch of compost at a time and no more than 3 to 4 applications per year. Reapplication should only occur when the previous layer has worked its way into the soil. For this reason good record-keeping is important.
Additionally, composted manure is lighter and more uniform than raw manure so it is easier to spread. However, there may be some clumping so it is important to harrow or cultivate your pasture after spreading. Seeding can take place shortly after spreading as compost provides a great medium for seed germination.
BC Ministry of Agriculture and Food, 1996, Using Compost, Composting Factsheet,