Washington State University Puyallup
  Organic Farming Systems and Nutrient Management

Horse Manure and Soil Nitrogen

Horse manure and soil nitrogen.  Horse manure is an abundant, locally available source of organic matter for soils. A major concern about horse manure is that it can cause a nitrogen deficiency when added to soils, leading to stunted, yellowed crops. Undiluted horse manure contains modest levels of nitrogen (about 1 to 2% of dry weight), and supplies small amounts of N to plants via slow release. When horse manure is mixed with woody bedding, the resulting product often ties up nitrogen in the soil through microbial immobilization, leading to N deficiency in the crop. Photo of Horse Barn

We sampled horse manure from 36 farms and stables in King County, Washington, and determined N concentration and C:N (carbon:nitrogen) ratio in each sample (Tables 1 and 2, Figures 1 and 2). All but one of the undiluted materials had a C:N < 30:1 and half had a C:N 20:1, while most of the samples with bedding had C:N >30:1. In general, materials with a C:N < 20:1 are expected to supply N to plants, and C:N > 30:1 are expected to create N deficiency. Materials with intermediate C:N often tie up some N the first few weeks after application and supply N later in the season.

Table of C to N ratio of horse bedding
Figure1.

Table of nitrogen content of various horse bedding types
Figure 2.
Table 1.  Averages.

Bedding Type Moisture % Total N % C:N Ratio
None 75 1.59 20.2
Paper 66 1.60 25.4
Wood Shavings 67 1.31 34.4

 

 

Table 2. Ranges.

Bedding Type Moisture % Total N % C:N Ratio
None 47-81 0.93-2.23 16-45
Paper 61-81 0.74-1.70 16-64
Wood Shavings 52-78 0.55-1.90 23-61



The data indicate that horse manure without bedding will supply a modest amount of N to plants. Horse manure with bedding will often tie up N, and supplemental N is needed when using this material as a source of organic matter for soils. Information on managing N when using horse manure is found in the following link: A Horse Owner's Guide to Good Stewardship (pdf file). 

 

 

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