Integrated nutrient management
Combining fertilisers, manures, legumes, residues and biofertilisers to feed crops and soil, with the evidence from long-term trials and a nitrogen budget.
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Fertilisers transformed food production in the twentieth century, but fertilisers alone rarely keep soils productive for decades. Where only nitrogen is applied, other nutrients run down; where crop residues and manure are removed, organic matter declines; and where farmers cannot afford enough fertiliser, yields stay low. Integrated nutrient management (INM) combines all available nutrient sources, mineral and organic, so that crops get what they need and the soil keeps its fertility.
FAO promoted the idea as integrated plant nutrition systems from the 1990s, and it is central to nutrient management teaching and to exams in soil science and agronomy.
What INM means
INM is the maintenance or adjustment of soil fertility and plant nutrient supply to an optimum level for sustaining crop productivity, by making the best use of all nutrient sources in an integrated way. In practice it means:
- Use every source: soil reserves, fertilisers, animal manures, compost, crop residues, green manures and legumes, biofertilisers and, where safe, urban and industrial organic wastes.
- Count what each source supplies, so fertiliser fills only the gap.
- Balance the nutrients, including secondary and micronutrients, not just nitrogen.
- Maintain soil organic matter and soil life.
- Reduce losses to air and water through timing, placement and appropriate sources.
The components
Mineral fertilisers
Concentrated, fast-acting and precisely dosable. They supply most of the nutrients in high-yield systems and are essential for replacing phosphorus and potassium removed by crops, which organic sources on a farm mostly only recycle.
Organic manures
- Farmyard manure (FYM): typically about 0.5 per cent N, 0.2 per cent P2O5 and 0.5 per cent K2O on a fresh-weight basis, although it varies widely with feed, bedding and storage.
- Compost and vermicompost: more concentrated and stable than raw manure; vermicompost often around 1 to 2 per cent N on a dry basis.
- Poultry manure: richer, often 2 to 4 per cent N.
- Oilcakes and other concentrated organics.
Only part of the nitrogen in organic manures becomes available in the first season, often about 20 to 40 per cent for FYM; the rest is released over later years.
Green manures and legumes
Legumes in the rotation, as intercrops, cover crops or green manures ploughed in at flowering, add nitrogen from biological fixation. A good green manure crop of Sesbania, sunn hemp, vetch or clover can add tens of kilograms, and sometimes over 100 kg, of nitrogen per hectare. Green leaf manuring brings leaves and twigs from shrubs and trees grown elsewhere.
Crop residues
Straw and stalks returned to the soil recycle much of the potassium and some of the other nutrients the crop took up, and feed soil organic matter. Burning residues loses most of the nitrogen and sulphur and harms air quality.
Biofertilisers
Live microbial inputs: rhizobia for legumes, free-living nitrogen fixers, phosphate-solubilising microbes and mycorrhizal fungi. Their contribution is real but variable and often modest, and depends on the strain, soil and crop.
Why combine sources: evidence from long-term trials
The best evidence comes from experiments running for decades. On the Broadbalk experiment at Rothamsted, England, wheat has been grown since 1843 with different treatments. Plots receiving large annual doses of FYM and plots receiving adequate mineral NPK give similar yields, while the manured plots have much more soil organic matter. Unfertilised plots yield little.
Long-term fertiliser experiments in Asia and Africa show a consistent pattern:
- Nitrogen alone gives early gains that fade as phosphorus, potassium, sulphur or zinc become limiting and soils sometimes acidify.
- Balanced NPK sustains yields much better.
- NPK plus organic manure usually gives the highest and most stable yields, with better soil organic carbon, structure and biological activity.
Organic sources alone, at the rates farmers can realistically produce, often cannot supply enough nutrients for high yields, especially phosphorus. The combination does better than either alone.
Fertiliser after counting organic sources
A maize crop needs about 120 kg of available nitrogen per hectare. The soil supplies about 25 kg/ha. The field follows a legume that local trials credit with 30 kg/ha, and the farmer applies 10 t/ha of FYM containing 0.5 per cent N, of which about 30 per cent is available in the first season.
- N in the FYM = 10,000 kg × 0.005 = 50 kg total; available this season ≈ 50 × 0.30 = 15 kg.
- Fertiliser N needed = 120 − 25 − 30 − 15 = 50 kg/ha.
- As urea (46 per cent N): 50 ÷ 0.46 ≈ 109 kg/ha, compared with 95 ÷ 0.46 ≈ 207 kg/ha if the legume and manure were ignored.
The manure also supplies phosphorus and potassium, which should be counted against those fertilisers, and its remaining nitrogen helps later crops.
Managing INM well
- The 4R principles: the right source, at the right rate, at the right time, in the right place, applied to every nutrient source.
- Test soils and manures, because manure composition varies several-fold.
- Store manure well, under cover and compacted, to cut nitrogen and potassium losses.
- Time organic inputs so that mineralisation matches crop demand; incorporate manures quickly to reduce ammonia loss.
- Watch the C:N ratio: residues with a high C:N ratio, such as cereal straw, temporarily lock up soil nitrogen while they decompose, so a little extra nitrogen may be needed.
- Avoid contamination: sewage sludge and some industrial wastes can carry heavy metals, pathogens or contaminants and are regulated.
Key points
- INM uses fertilisers, manures, compost, residues, legumes and biofertilisers together, counting each source so fertiliser fills the gap.
- FYM contains roughly 0.5 per cent N, 0.2 per cent P2O5 and 0.5 per cent K2O; only part of its N is available in the first season.
- Long-term trials show that nitrogen alone fades, balanced NPK sustains yields, and NPK plus manure is usually best for yields and soil.
- Organic sources alone rarely supply enough phosphorus for high yields.
- Apply the 4Rs to all sources, and test soils and manures.
What the exams ask
- School-leaving and entrance papers: definition and components of INM, organic manures and their nutrient content, and green manuring.
- Degree and research-fellowship entrances: long-term fertiliser experiments, nutrient release from organic sources, C:N ratio, biofertilisers, and calculating fertiliser after organic credits.
- Professional examinations: designing INM plans for cropping systems, the 4R framework, and regulations for organic wastes.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is INM the same as organic farming?
No. Organic farming excludes synthetic fertilisers. INM uses them, but alongside organic sources and in amounts adjusted for what those sources supply.
Why not use only manure?
On most farms there is not enough manure to supply all the nutrients for high yields, and manure mostly recycles nutrients within the farm rather than replacing those exported in produce.
How do I know how much nitrogen manure supplies?
Test the manure or use local tables, then apply an availability factor for the first season. Local trials and soil nitrate tests help refine the estimate.
Sources
- Roy, R. N., Finck, A., Blair, G. J. and Tandon, H. L. S. Plant Nutrition for Food Security: A Guide for Integrated Nutrient Management. FAO Fertilizer and Plant Nutrition Bulletin 16, 2006.
- Johnston, A. E. and Poulton, P. R. "The importance of long-term experiments in agriculture: their management to ensure continued crop production and soil fertility; the Rothamsted experience." European Journal of Soil Science 69, 113–125 (2018).
- Vanlauwe, B. et al. "Integrated soil fertility management: operational definition and consequences for implementation and dissemination." Outlook on Agriculture 39, 17–24 (2010).
- International Plant Nutrition Institute. 4R Plant Nutrition Manual. IPNI, 2016.
- Havlin, J. L. et al. Soil Fertility and Fertilizers, 8th edition. Pearson, 2014.
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