Compost Quality and Application Rate
A farmer makes compost from crop residues and animal manure. A laboratory test shows the compost has 1.5% nitrogen on a dry basis, a carbon-to-nitrogen ratio of 25:1, and a moisture content of 40%. The farmer plans to apply 5 tonnes of compost per hectare to a vegetable plot before planting. The soil is low in organic matter at 0.8%. The farmer has 12 tonnes of compost available and 3 hectares to cover. A neighbour warns that immature compost with a high carbon-to-nitrogen ratio can tie up nitrogen and harm crops. The compost has a faint earthy smell and reaches 55 degrees Celsius in the pile, cooling after turning.
1. Judge whether the compost is mature enough to apply safely. Use the carbon-to-nitrogen ratio, temperature, and smell in your reasoning.
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The compost appears reasonably mature. A carbon-to-nitrogen ratio of 25:1 is close to the ideal range for finished compost, which is typically below 30:1 and often around 15:1 to 20:1. At 25:1, nitrogen tie-up is possible but modest, especially compared with immature material at 40:1 or higher. The temperature of 55 degrees Celsius shows active microbial decomposition, and the fact that the pile cools after turning suggests the thermophilic phase is ending, which is a sign of maturation. The faint earthy smell is characteristic of stable compost; a sour or ammonia smell would indicate immature or anaerobic material. The neighbour's warning is valid in principle but does not clearly apply here. The judgement is that the compost is usable, but the farmer should allow it to mature further if time permits and should monitor crops for nitrogen deficiency after application.
2. Calculate whether the farmer has enough compost to apply 5 tonnes per hectare to 3 hectares. If not, propose a fair allocation and explain the trade-off.
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Requirement = 5 tonnes/ha × 3 ha = 15 tonnes. The farmer has only 12 tonnes, so there is a shortfall of 3 tonnes. A fair allocation is to apply 4 tonnes/ha across all 3 hectares, using 12 tonnes exactly. This gives every part of the plot some organic matter, which is important because the soil organic matter is low at 0.8% everywhere. The trade-off is that 4 tonnes/ha is below the planned rate, so the improvement in soil organic matter and nutrient supply will be smaller. Alternatively, the farmer could apply 5 tonnes/ha to 2.4 hectares and leave 0.6 hectares untreated, but this creates uneven fertility. The judgement is that spreading the available compost evenly is usually better for a uniformly poor soil, and the farmer should plan to produce more compost or add other organic matter next season.
3. The compost is 40% moisture. Calculate the dry matter applied per hectare at 4 tonnes/ha and explain why this matters for nitrogen supply.
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At 4 tonnes/ha of fresh compost, dry matter = 4 × (1 - 0.40) = 4 × 0.60 = 2.4 tonnes of dry matter per hectare. Nitrogen content is 1.5% of dry matter, so nitrogen applied = 2.4 × 0.015 = 0.036 tonnes = 36 kg N/ha. This matters because the nitrogen is not all immediately available. With a carbon-to-nitrogen ratio of 25:1, some nitrogen will be immobilised by microbes as they break down the carbon, so the effective nitrogen supply to the crop in the first season is lower than 36 kg/ha, perhaps half or less. The farmer should not rely on compost alone to meet the crop's nitrogen needs and may need a supplementary nitrogen fertiliser, especially early in the season. The judgement is that compost is valuable for organic matter and long-term fertility, but its immediate nitrogen contribution is limited by moisture content and carbon-to-nitrogen ratio.