Is the compost heap fit for a tomato nursery?
A 3.0 t heap of on-farm compost (cattle dung, wheat straw, vegetable waste) has been turned twice and is six weeks old. Analysis: total C 32%, total N 1.23% (C:N 26), NH4-N 1,800 mg/kg, NO3-N 40 mg/kg, EC (1:5) 4.2 dS/m, pH 8.6, moisture 55%, core temperature 34 °C and still above ambient, 40% of the mass coarser than 10 mm, and a faint ammonia smell. The grower wants to spread all of it at 15 t/ha on raised nursery beds for tomato, with seed to be sown in 10 days. The rest of his plan is for a chilli field to be transplanted in six weeks, where he intends to cut urea by 25% because 'compost is as good as urea'. He has done no germination test and does not want to wait.
1. Assess the maturity of this compost and decide whether it should go into the nursery.
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Every maturity indicator is outside the acceptable range.
- C:N 26 is above the 15-20 band expected of mature compost; at 26 the material will still immobilise nitrogen when incorporated.
- NH4-N 1,800 mg/kg against NO3-N 40 mg/kg gives an NH4:NO3 ratio of about 45. Mature compost should be below 0.5 - the ammonium has not been nitrified, which means the heap is still in its early, ammonifying phase.
- Core still at 34 °C and a faint ammonia smell confirm active decomposition, not curing.
- EC 4.2 dS/m (1:5) is high. A 15 t/ha dose in a shallow nursery bed would create a saline root zone; tomato germination and early seedlings are salt-sensitive.
- pH 8.6 will further reduce micronutrient availability in the nursery.
- 40% of the mass above 10 mm indicates incomplete screening and uneven material.
Decision: not mature, and certainly not for a nursery. Immature compost can carry volatile fatty acids, ammonia and soluble salts that reduce or delay germination. The heap goes to the chilli field after further curing, not to the raised beds.
2. What curing and testing protocol would you prescribe before any of this compost is used, and what would you accept?
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Curing: turn the heap again to aerate it, keep moisture at 50-60% (squeeze test), and shape it so the core does not dry out. Cure for a further 4-6 weeks until the core temperature stays within 5 °C of ambient for three consecutive days.
Testing before use:
- Re-analyse NH4-N, NO3-N, EC and C:N. Accept NH4:NO3 below 0.5, C:N 15-20, EC below about 2.5 dS/m in 1:5.
- Bioassay. Extract compost with water (1:5), soak filter paper or a 50:50 compost:sand mix, sow 10 cress or radish seeds in the extract and 10 in water as a control, and record germination and root length after 5 days. Accept only if germination and root length are at least 85-90% of the control.
- Screen to below 10 mm.
Placement: in the nursery, use only mature, screened compost at no more than about 25-30% of the growing mix, or substitute vermicompost or a peat-based mix. Save the bulk material for the chilli field, and apply it at least 3-4 weeks before transplanting so that mineralisation can stabilise.
3. Evaluate his plan to cut urea by 25% in the chilli crop because of the compost.
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The plan overestimates the compost and underestimates its immaturity.
Nutrient arithmetic: at 1.23% N, 10 t/ha of compost carries about 123 kg N/ha, but this is total N, not available N. For a partly mature manure, first-year N availability is typically 10-20%. At 15% availability that is only about 18 kg N/ha. A 25% cut in a 150 kg N/ha urea programme is about 37 kg N/ha - more than double the realistic credit, and it comes from a material with a C:N of 26 that may initially immobilise N.
Also consider: compost supplies useful P, K, S and micronutrients and improves soil physical condition, but it is not a substitute for fertiliser N; and the high EC of this heap means the application rate itself should be moderated.
Recommendation: apply 10 t/ha of properly cured compost, reduce urea by roughly 10-15 kg N/ha (about 25-30 kg urea), not 25%, and keep the first top-dressing on schedule. Add a small starter dose of N at transplanting to offset immobilisation, and settle the final adjustment on the basis of a soil test and crop appearance rather than a fixed percentage.