# How to Calculate the Amount of Compost Applied to the Field to Meet the Nitrogen (N) Requirement?

#### Dr.(Mrs)Menaka Fernando, Phd.

###### PUBLISHED ON June 11, 2021

The most commonly used compost has a C: N ratio of 10-15 and research has shown that plants absorb only 25% of the total N content. Therefore, not all the N plants in the compost we apply to the soil are conducive to growth. The following information is required to accurately calculate the amount of compost to be applied.

• N quantity of compost (N fraction per million compost by dry weight)

• Dry matter % of compost

For example, assuming a total N content of 18,785 ppm in the compost we have,.

Total N Quantity relative to Wet Weight = Total N Volume (Dry Weight) x Dry Material Volume (%)

= 18,785 ppm x 54/100

= 10,144 ppm

The amount of N per ton of compost should be multiplied by 0.002, the coefficient for calculating in pounds. Then you can get the number of N pounds per ton of compost. Accordingly,

= 10,144 ppm x 0.002

= 20 lbs N (per ton of compost)

As mentioned earlier, when the efficiency of N supply for plants in compost is 25%,

= 20 x 25/100

= 5 pounds N (per ton of compost) This can be better illustrated by the following example.

Assuming that the soil nutrient report recommends an N application rate of 140 pounds per acre for maize cultivation,

The actual amount of N that plants get per ton of compost = 5 pounds

if,

The amount of compost required to obtain 140 pounds N = 140/5

= 28 tons

That is 28 tons of compost to be applied to the field. To provide the required amount of N per acre of maize cultivation, 28 tons of compost with C: N ratio of 10-15 should be applied.

Applying this size in practice is a challenging situation. In sustainable agriculture, farmers add 5-10 tons of compost per acre to improve the soil and its conditions over time. This improves soil structure and nutrient retention capacity. Also, organic fertilizers like compost have the ability to add nutrients to the soil very slowly over a long period of time.

Instead of using compost as a whole, many farmers add atmospheric N legumes to the soil as cover crops or as green manure. In addition, liquid manure made from sand, which is a waste of fish, is widely used. However, compost production has been identified as the easiest and most efficient method of managing waste, which has become a global crisis, and farm-based preparation can reduce the cost of transportation.

###### Eng. Lanka Ramanayaka - CEO Design Knigdom Lk
BSc(Eng), PG Dip.(BSE), CEng., MIE(SL), MEC(SL)
Chartered Engineer

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