The carbon to nitrogen ratio is a very important indicator that signifies the health of the coffee ecosystem. carbon-to-nitrogen ratios are an indicator for nitrogen limitation of plants and other organisms. In simple words, the carbon-to-nitrogen ratio (C/N ratio or C: N ratio) is a ratio of the mass of carbon to the mass of nitrogen in a substance. The carbon to nitrogen ratio has a significant effect on crop residue decomposition, particularly residue cover on the soil and crop nutrient cycling (predominantly nitrogen), it is important to understand these ratios when planning multiple crops, cover crops, and the use of raw material for composting in agricultural systems.
Indian coffee plantations belong to a select few in the entire world, where coffee is grown under the canopy of a three-tiered shade system. Secondly, the entire coffee is grown along with multiple crops. These two intrinsic factors literally translate into the setting up of biomass factories where tons of biomass in the form of leaf litter, wood shavings, and animal droppings get incorporated into the soil from time to time. Animal wastes (cattle/sheep/poultry, piggery) and coffee husk are also constantly incorporated into the soil and then get converted into available nutrients for plant growth and development.
Coffee Pulp / Husk
Most of the coffee farmers own pulper machines which aid in removing the mucilaginous outer cover of the fruit. This outer cover is often referred to as coffee pulp. If dry coffee is processed, then the outer layer is referred to as husk. Both pulp and husk have high amounts of nutrients. Husk contains 1.5% nitrogen, 0.5% phosphorus and 2.2% potash.
For every ton of clean coffee produced, one ton of dry matter is obtained either as cherry husk or fruit pulp. 7000 kg of fruits results in a net gain of 15 kg nitrogen, 3kg phosphorus, and 35kg potash. Pulp or husk has to be composted and only then applied to the field. Undecomposed or fresh husk may lead to the production of acids and thus bring down the ph of the soil.
Coffee Soils and Organic Matter
Firstly, organic matter consists of residues of plants and animals at various stages of decomposition and essentially mediated by soil microflora. The biological transformations taking place result in various products at different stages, ultimately leading to the formation of rich organic matter and humus inside the plantation. Dead and decaying animal and vegetative matter are periodically recycled into the coffee soils. Macro organisms like insect and worm castings also get incorporated after their respective life cycles. Soil carbon enters the soil as soil organic matter. Carbon makes up approximately 60% of the soil organic matter (SOM), with the remaining 40% of SOM containing other important elements such as calcium, hydrogen, oxygen, and nitrogen.
Microorganisms and transformations
Billions of microbial cells, namely fungal mycelium, viral capsules, bacterial and actinomycetes cells which have a very short generation time build up a strong reservoir of nutrients for microbial succession to act upon the organic matter. In addition to this planters have mastered the art of composting farm wastes, green manures, bio-slurry, and recycling it into the plantation thereby accelerating the entire process of organic matter production. The coffee plantation itself acts as a huge compost pit and a perfect zone for the formation of organic matter because of the heterogeneous mixtures available within the plantation. The soil itself is a microcosm. It harbors a wide variety of microorganisms that attack, decompose and assimilate every known type of substance. The changes taking place are numerous and complex.
Ideal C: N ratio
A carbon-to-nitrogen (C:N) ratio of 24:1. (24 parts carbon to 1 part nitrogen) is ideal for microbial decomposition. For optimum health, the microbe requires approximately 16 parts of carbon for energy and then 8 parts for maintenance. This is where the ratio of 24:1 comes from. Under these optimum conditions, soil microbes can help the release of nutrients like N, phosphorous, and zinc to crops. Meanwhile, this ratio influences the amount of soil-protecting residue cover that remains on the soil. The 24:1 ratio strikes a balance between the two. The ratio 24:1 is the proper carbon and nitrogen that a microorganism must have in order to sustain its health.
C: N Ratio in Organic Matter
The carbon-to-nitrogen ratio of organic matter means the amount of carbon relative to the amount of nitrogen present. There is always more carbon than nitrogen in organic matter.
When an organic substrate has a C: N ratio between 1 and 15, rapid mineralization and release of N occur, which is available for plant uptake. The lower the C: N ratio, the more rapidly nitrogen will be released into the soil for immediate crop use.
If the C: N ratio of the decomposing plant material is above about 30:1 the soil microbial population may take nitrogen in mineral form (e.g. nitrate). This mineral nitrogen is said to be immobilized. Microorganisms out-compete plants for NH4+ and NO3- during immobilization, and therefore plants can easily become nitrogen deficient.
A-C: N ratio greater than 35 results in microbial immobilization. A ratio of 20–30 results in an equilibrium state between mineralization and immobilization. Soil microorganisms have a C: N ratio of around 10. They must acquire enough carbon and some nitrogen from the soil to maintain that ratio in their cells. When composting, microbial activity utilizes a C/N ratio of 30-35:1 and a higher ratio will result in slower composting rates. However, this assumes that carbon is completely consumed, which is often not the case. Thus, for practical agricultural purposes, compost should have an initial C/N ratio of 20-30:1. The usual recommended range for C/N ratios at the start of the composting process is about 30/1, but this ideal may vary depending on the availability of carbon and nitrogen. As carbon gets converted to CO2 (and assuming minimal nitrogen losses) the C/N ratio decreases during the composting process, with the ratio of finished compost typically close to 10/1.
Coffee soils are subjected to compost application from time to time as a regular package of practice. As such a majority of the coffee planters use biodigesters or waste decomposers in the biomass for quick composting. The fastest way to produce fertile, compost is to maintain a C: N ratio somewhere around 25 to 30 parts carbon to 1 part nitrogen, or 25-30:1. If the C: N ratio is too high (excess carbon), decomposition slows down. If the C: N ratio is too low (excess nitrogen) the end product will be unfit for soil application.
High C: N ratios may be lowered by adding cattle manure, poultry manure, or sheep manures. Low C: N ratios may be raised by adding paper, dry leaves, or wood chips.
Estimated Carbon-to-Nitrogen Ratios
Soil Microbes (Average) 10:1
Ashes, wood C: N 25:1
Alfalfa C: N 12:1
Clover C: N 23:1
Coffee grounds C: N 20:1
Food Waste C: N 20:1
Weeds C: N 30:1
An understanding of the intrinsic composition of organic residues helps in deciphering the entire process of soil organic matter accumulation. Soil organic matter consists of 2 main groups of compounds
- A) Nitrogenous
B) No nitrogenous.
Nitrogenous compounds contain chiefly proteins and nitrogen bases. Whereas the no nitrogenous group contains carbohydrates, lignin’s, tannins, organic acids, ash, mineral matter, water, phosphates, sulfates, calcium, magnesium, sodium, potassium, iron, zinc, copper, manganese, boron, etc. The entire makeup of this organic complex consists of substances of widely different chemical compositions. This acts as a precursor in the breakdown of the complex both by macro and microorganisms.
The entire process is evolutionary in nature. As a first step, the animal and plant residues are subjected to microbial attack. In the second step, rapid multiplication of microorganisms results in the increase of enzymatic activity and builds up of biomass. Lastly, the production and accumulation of end products needed for plant growth and development take place. The simple constituents are assimilated directly by the microorganisms, part of the carbon is utilized for synthesizing the microbial tissue and the rest is oxidized and eliminated as carbon dioxide. This in turn is used for photosynthesis by plants and trees. However, the crucial parameters for soil organic matter decomposition are optimum temperature, Moisture, hydrogen ion concentration, and aeration.
An area of main concern over the years has been regarding the insoluble complex of soil organic matter. This complex consists of cellulose, hemicelluloses, lignin proteins, starch, gum, mucilage, fats, and pectic substances.
Future Area Of Research
The Soil Scientists at the Central Coffee Research Station should help the Coffee Planters understand in simple terms the significance of the Carbon, Nitrogen (C: N) ratio and its importance in the availability and recycling of nutrients. Carbon is the key that impacts and controls many variables or processes in the soil and plant. The amount of carbon in the soil impacts the development of soil structure, water storage, and nutrient recycling. This information is of prime importance to manage the organic matter content of the soil. Further, they should also equip the Planting community with the required knowledge to bring down the Carbon, Nitrogen ratio which is invariably wide in coffee soils.
Soil carbon has declined significantly in many coffee soils due to intensive agroforestry. Consequently, these soils may be less able to supply nutrients to meet plant demands.
The C: N ratio is important, due to the fact that it has a direct impact on residue decomposition and also nitrogen cycling in our soils. Soil organic matter is an excellent conditioner for coffee plant rejuvenation and to a great extent responsible for controlling the relationship between healthy soil and healthy plant. One must bear in mind that as cultivation progresses, the organic matter of the soil decreases. Hence, adequate steps should be taken to compliment the organic matter status of the soil by way of incorporating crop residues from time to time. Hence it is of paramount importance to maintain a healthy C: N ratio in the soil for better organic matter decomposition and release of nutrients from time to time for better productivity.
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