Seminarios-suelos 7: "abonos orgánicos"

Summary

The seminar examines how organic fertilizers can support sustainable soil management and food security. Presenter Francisco Rodríguez Nieves emphasizes that organic fertilizers can serve three purposes:

1. Build or maintain soil organic matter, improving soil fertility.
2. Supply nutrients and other beneficial compounds to crops.
3. Put organic waste to productive use, reducing pollution and helping retain carbon in soil.

The central message is that an organic product is not automatically safe or effective. Its composition, maturity, salinity, and intended use should be assessed before application.

How organic fertilizers work in soil

After application, soil microorganisms guide two related processes:

• Mineralization: Microorganisms convert organic nutrients into forms plants can take up, such as ammonium. This supports crop nutrition.
• Humification (called “unification” in the subtitles): Microorganisms transform part of the organic material into more stable soil organic matter. This contributes to longer-term soil improvement.

A useful organic fertilizer should ideally support both processes: provide nutrients that benefit the current crop while also contributing to soil organic matter.

Soil organic matter: targets and practical limits

The presenter describes 3–4% organic matter as a desirable range, while noting that the appropriate level depends on the soil and that raising depleted soils to this level can be extremely difficult.

The seminar gives these approximate decision guidelines:

• Below about 1–1.2% organic matter: adding organic fertilizer is urgent.
• From 1.2% to 3%: adding it is necessary.
• Above 3%: it may still be useful, depending on the fertilizer’s other functions and the soil’s needs.

A worked example for a soil in Teposcolula, Oaxaca, estimated that increasing organic matter from 1.48% to 3.5% would require about 150 tonnes of organic fertilizer per hectare. The presenter considers that impractical in many cases because of cost, transport, labor, and availability. Suggested alternatives include spreading applications over several years, applying material in bands near crop roots, or calculating the amount needed to meet a crop’s nutrient requirement rather than trying to raise the entire field’s organic matter quickly.

Choosing and handling organic materials

• Maturity matters. More mature compost generally has greater potential to contribute to stable organic matter and is often less saline. Composting also helps stabilize materials and reduce risks from excessive salts or harmful emissions.
• Do not assume “organic” means safe. Some fresh or poorly matured materials can harm seeds or plants through high salinity, ammonia, or other compounds. The presenter described fresh bakery waste that burned plants when applied directly.
• Check the product’s properties, especially when purchasing or applying large quantities. The seminar discusses pH, electrical conductivity, cation-exchange capacity, nutrient content, and possible contaminants.
  • The pH range cited as desirable for compost is approximately 6–8.
  • High electrical conductivity can damage germination and plant growth; the cited standard recommends staying below 10, though the subtitles do not clearly preserve the units.
  • A cation-exchange capacity above 40 is cited as desirable, particularly for improving sandy soils’ ability to retain nutrient cations.
• Incorporate crop residues where appropriate. Rather than burning straw, the presenter recommends shredding and incorporating it so microbes can transform it. Adding nitrogen may help microbial breakdown of high-carbon residues. Leaving residues on the surface can also be useful, though the process is slower and serves somewhat different purposes.
• Use laboratory analysis and crop-specific calculations. Organic materials vary greatly, so recommendations should be based on the material’s actual composition and the crop’s needs.

The presentation also notes that straw, manures, green manures, and some crop residues can contribute to compost. Brassica residues such as broccoli may provide nitrogen and, according to the presenter, compounds that can help with soil sanitation. Sawdust used alone is generally discouraged because it contains little nitrogen and can slow the composting process, though it may be useful as animal bedding that absorbs manure.

Matching fertilizer rates to crop needs

Nitrogen is presented as the key nutrient for calculating organic-fertilizer rates, because many composts have relatively low nitrogen content. The presenter’s examples illustrate the calculation:

• For a crop expected to yield 5 tonnes of corn per hectare, the example assumes about 100 kg of nitrogen must be taken up by the plant. The soil’s contribution should be subtracted to determine the shortfall.
• A compost with 1.5% nitrogen and an assumed 30% first-year mineralization rate would require about 22 tonnes per hectare to supply 100 kg of usable nitrogen.
• Chicken manure with 3.5% nitrogen and an assumed 75% mineralization rate would require about 3.8 tonnes per hectare to supply the same amount.

These are illustrative figures, not universal rates. The actual amount depends on the crop’s yield target, soil nitrogen supply, and laboratory analysis of the organic material. The presenter also notes that compost can supply other nutrients and micronutrients, but its nutrient profile varies.

Organo-mineral fertilizer

Rodríguez Nieves favors combining organic and mineral fertilizers where appropriate:

• Mineral fertilizers can supply concentrated nutrients for high-yield crops.
• Organic materials can contribute organic matter, biologically active compounds, and other useful elements.

He cites corn-production examples in which combining organic and mineral sources performed better than either alone, and argues that the approach may be particularly useful in intensive systems—provided the cost and material availability make it viable.

Questions and answers

Among the points raised in the discussion:

• Cereal straw: Barley straw can generally serve the same composting role as other cereal straws; rice straw is noted as having more silicon.
• Treated wastewater: It may supply nutrients, but heavy metals should be analyzed before agricultural use.
• Leachates: They can be rich in potassium and biologically active compounds. Dilution should be adjusted to manage electrical conductivity. Foliar application may make these compounds more directly available to plants; soil or irrigation application can also be used.
• Phosphate rock: Its dissolution may be too slow in some soils to meet crop needs. The presenter mentions acids or phosphorus-solubilizing microorganisms as possible ways to support its availability.
• Soil affected by long-term potato production: The suggested starting point is to assess organic matter, pH, and disease pressure. The presenter proposes compost and a suitable microbial consortium to help restore soil biological balance.
• Sawdust and molasses: Sawdust alone is considered a poor composting material because of its very high carbon-to-nitrogen ratio; molasses and other organic wastes are preferably composted or matured before use when possible.

Speakers and sources featured

Seminar speakers and hosts

• Francisco Rodríguez Nieves — Main presenter on organic fertilizers; the subtitles render his surname inconsistently in places.
• José Ángel Montaño Morales — Guest representative from the Secretariat of Agriculture in Hidalgo; introduced the presenter and offered closing remarks.
• José Antonio Maldonado Estrella — Identified in the opening as director of the Department of Soils at the Autonomous University of Chapingo.
• Professor Arely — Moderator/host; surname is not clear in the subtitles.

Audience participants named in the Q&A

The subtitles identify questions or comments from Lidia de Marcos, Jesús Lara, Sergio de Jesús, Héctor Penagos, Carlos Guadalupe Canché, Francisco Ramírez Vázquez, Ignacio Morales, Isidro Quevedo, Carlos Cordero, Rogelio Medina, Javier Díaz, Guadalupe Chávez, and María del Consuelo. Some names or attributions are unclear in the auto-generated subtitles.

Research and institutional sources mentioned

• The Department of Soils and central university laboratory at the Autonomous University of Chapingo, including laboratory analyses and compost trials.
• A 2009–2013 laboratory review of many organic fertilizers submitted for analysis.
• U.S. corn-yield competitions cited as examples of farmers using organic materials alongside mineral fertilizers.
• Compost standards and analysis methods discussed by the presenter; some technical details and units are unclear in the subtitles.