Nitrogen Deficiency and Carbon:Nitrogen Ratios of Organic Ammendments

"Carbon to nitrogen ratio inbalance" is a term used to describe a type of nitrogen deficiency. Farmers from my grandfather's generation called it "sour soil".

A field recently had pulp and paper biosolids applied. The newly planted crop looked great, until the seedlings ran out of seed reserves and started utilizing soil nutrients. The crop then turned a neon shade of yellow. What happened?

Organic-N & Ammonium-N

When an organic amendment is applied to a field, it adds nutrients and organic matter to the soil. The organic matter contains about 60 percent organic carbon. The carbon:nitrogen (C:N) ratio shows the proportion of organic carbon to total nitrogen of a manure or organic material.

Nitrogen is a food source for the micro-organisms ("soil bugs") while they break down organic carbon. The nitrogen can come from the added organic material or it can come from the soil. During the process of carbon breakdown soil microbes die and decompose. The microbial nitrogen is then returned to the soil and becomes available to the plants. This adds to the organic nitrogen pool within the soil along with the added organic material. How long the carbon breakdown process takes depends on the ratio of carbon to nitrogen in the material and in the soil.

Pulp & Paper Biosolids C:N Ratios

Liquid hog manure has a C:N ratio near 10:1. The nitrogen in liquid hog manure will become quickly available when the soil microbes are active in the soil. Pulp and paper biosolids have a C:N ranging from 25:1 to 200:1, depending on how much nitrogen is added by the company producing the biosolids material. Pulp and paper biosolids without added N could have a C:N ratio of 200:1. The nitrogen contribution from this paper biosolids could take considerably more than one growing season to become available. To compensate for the high C:N ratio, some pulp and paper processors will add nitrogen to balance the carbon and reduce the ratio down to about 25:1. Each company provides a different product, so analysis of individual products is important.

High C:N Ratios

The carbon to nitrogen ratio of soil is about 10:1. When solid manure or other organic material has a C:N ratio of greater than 30:1, there is a higher risk that the soil bugs will "steal" nitrogen from the soil and tie it up. Therefore this nitrogen is unavailable to a crop while breaking down the carbon material. A crop with higher nitrogen requirements,

such as corn or wheat, will show nitrogen deficiencies in that situation. When a material has a C:N ratio less than 20:1, there is generally enough nitrogen in the organic

material to break down the carbon without causing a nitrogen deficiency in the crop. Table 1 lists approximate C:N ratio of various organic materials applied to soils.