Plants for Plants: What Is Organic Plant Feed?

Nitrogen, phosphorus and potassium are essential nutrients, but the NPK numbers on a bottle do not describe everything a plant-derived fertilizer contains or how its nutrients become available. Organic Plant Feed, or OPF, is used here to describe feeds made from plant-derived materials rather than one uniform formulation.

These materials can supply nutrients and organic carbon, with behaviour that depends on their composition, processing and the growing system. Understanding those differences is more useful than assuming that all organic feeds act slowly or that all mineral feeds behave identically.

Where plant-derived feeds come from

Plant-based fertilizers can be made from residues, fermented crops, sugar-industry by-products, cereals, legumes and other materials. Commercial examples include materials derived from molasses, maize, wheat and barley.

Different feedstocks contain different proportions of nutrients and organic compounds. Combining a potassium-rich material with a source of nitrogen or phosphorus can therefore change the final nutrient profile. Processing also matters, so the raw material alone does not establish the composition or performance of the finished feed.

What the NPK label does not describe

A soluble mineral fertilizer can be formulated to supply predictable nutrient concentrations. An NPK grade such as 10-10-10 describes nutrient content using the applicable labelling convention; the numbers alone do not establish chemical form, solubility or release rate.

Plant-derived feeds may contain calcium, magnesium, sulfur and trace elements alongside nitrogen, phosphorus and potassium. They may also supply organic carbon, amino acids and other feedstock-derived compounds. A broader mixture does not automatically provide better nutrition: amounts, chemical forms and availability still need to match plant demand.

EC measures only part of the nutrient pool

Electrical conductivity reflects dissolved ions in a solution or extract. Soluble mineral salts contribute directly when they dissolve, whereas organically bound nutrient pools may contribute differently before decomposition and mineralisation release ions.

Some plant-based feeds can therefore contain substantial total nutrients without producing a proportionately high immediate EC. This does not mean that organic feeds are necessarily low in salts. Molasses and vinasse, for example, can contain substantial dissolved mineral concentrations.

EC needs to be measured and interpreted for the sample and system, rather than inferred from the organic label. A low reading does not establish adequate nutrition, and a reading taken immediately after mixing does not fully describe later nutrient release or root-zone accumulation.

Mineralisation and nutrient release

Many nutrients in complex organic materials become available after microbial decomposition and mineralisation. Bacteria and fungi transform organically bound nutrients into mineral forms that roots can acquire. Organic feeds may also contain nutrients already in mineral form, and plants can take up some small organic compounds, so mineralisation is not a prerequisite for every component.

Release rates depend on temperature, moisture, oxygen, substrate, pH, the nature of the material and its carbon-to-nitrogen ratio. Some materials release nutrients relatively quickly; others do so more slowly. Microorganisms can also temporarily retain nutrients in their own biomass, making the timing of supply important when crop demand changes.

Carbon and the root-associated microbiome

Plant-derived feeds introduce organic carbon that can provide resources for microorganisms. Around roots, bacteria and fungi transform, immobilise and release nutrients while interacting with the plant and with other organisms.

This does not mean that adding carbon creates a healthy microbiome or improves every growing system. The response depends on the material, community and conditions. The feed becomes part of a carbon and nutrient cycling system rather than simply acting as a direct substitute for an equivalent amount of soluble salt. The relevant microbial habitats are described in The Plant Spheres Explained.

Plants regulate uptake under both organic and mineral nutrition

The grower expression that plants can choose what they need under organic nutrition is not a literal description. Roots regulate transport systems for nitrate, ammonium, phosphate, potassium and other nutrients in response to availability and plant nutritional status. This regulation also operates when mineral fertilizers are supplied.

Root exudates influence local chemistry and microorganisms, which can in turn change nutrient availability. These processes create a dynamic root environment, but they do not guarantee an appropriate supply or prevent excesses and deficiencies. A plant cannot acquire a nutrient that is absent or inaccessible, and regulation does not make it immune to unsuitable concentrations.

The distinction between supply and acquisition is explored in More Fertilizer, Less Uptake? The Roots Decide.

My observations in commercial growing

In commercial growing, I repeatedly observed slower development in fully organically fed plants than in plants receiving readily available mineral nutrition. Delayed nutrient release is one possible explanation when demand rises faster than mineralisation can supply nutrients, but the observation alone does not isolate the cause.

I also observed that plants not continually driven by a highly soluble feeding programme sometimes appeared more balanced and resilient. This remains an observation from practice, not a universal finding that organic nutrition increases resilience. Slower growth is not automatically healthier, and differences in nutrition, irrigation and other growing conditions can influence the comparison.

Plant extracts are not all fertilizers

Not every plant-derived input is used primarily to supply nutrients. Seaweed extracts may contribute minerals, but many investigated responses concern other components and biological activity rather than their NPK contribution alone. The evidence and its limits are discussed in Seaweed Extracts: What Does Science Actually Say?

For any formulation, distinguish nutrient supply from effects on microbial processes or plant physiology. These roles may overlap, but evidence for one should not be assumed to establish the others.

Combining organic and mineral sources

Organic and mineral nutrition do not need to be treated as opposing systems. Soluble mineral sources can provide immediate control of supply, while plant-derived materials may contribute nutrients over a different timescale and supply carbon for microbial processes.

A combination can be useful where it matches crop demand and the growing system, but it is not automatically superior to either approach alone. Total nutrient inputs, release rates, EC, irrigation delivery and root-zone conditions still need to be managed together. The crop-specific comparison is developed further in Organic or Mineral Nutrition: Which Is Better?

References

• Bergstrand, K.-J. et al. (2022). Organic fertilizers in greenhouse production systems – a review. Scientia Horticulturae.
• Möller, K. & Schultheiß, U. (2020). A typological concept to predict the nitrogen release from organic fertilizers in farming systems. Agronomy, 10, 1448.
• Chand, A. et al. (2026). Liquid organic fertilizers in soilless cultivation: a systematic review of production, performance, challenges, and future prospects. Frontiers in Sustainability.
• Jacoby, R. et al. (2025). Microbiome-facilitated plant nutrient acquisition. Cell Host & Microbe.
• Maillard, A. et al. (2020). Reappraisal of the central role of soil nutrient availability in nutrient management in light of recent advances in plant nutrition at crop and molecular levels. European Journal of Agronomy.