How Spectrum Influences PPM

PPM (parts per million) describes the concentration of nutrients in a growing solution. While growers typically adjust PPM based on crop stage and variety, light conditions also influence how effectively plants use those nutrients.

Light does not directly control nutrient levels, but it regulates plant activity, including photosynthesis, transpiration, root function, and nutrient transport. Through photoreceptors such as phytochrome and cryptochrome, plants detect different wavelengths and adjust processes related to growth, development, and nutrient uptake.

Change the light spectrum, and you change the signal the plant receives. That signal influences how the plant grows, transpires, and uses available nutrients. This is the physical connection between lighting and nutrient management.

In this article, we will explore how spectrum changes affect plant physiology and what that means for PPM management.

Connecting Light to Nutrient Uptake

To understand how lighting affects nutrient demand, we need to look at stomatal conductance and transpiration.

Stomata are microscopic pores on leaf surfaces that regulate gas exchange. When plants receive light, stomata open to allow carbon dioxide to enter for photosynthesis.

As stomata open, water vapor leaves the leaf through transpiration. This creates a pulling force that moves water from the root zone to the leaves. Since dissolved minerals travel with this water, transpiration becomes a key driver of nutrient movement throughout the plant.

In controlled environments such as greenhouses and vertical farms, PPFD influences this process. Higher light levels can increase photosynthetic activity and transpiration, which may increase the plant’s demand for water and nutrients.

However, more light does not always mean simply adding more fertilizer. Nutrient uptake also depends on root health, irrigation strategy, environmental conditions, and nutrient balance.

When a lighting system increases plant activity without a corresponding adjustment to the nutrient program, plants may experience nutrient imbalance or deficiency symptoms, even when PPM levels appear correct.

For this reason, lighting and nutrition should be managed together. The right spectrum and intensity help create the conditions for efficient nutrient uptake, while the correct PPM and EC strategy ensure nutrients are available when plants need them.

What the Research Shows

The clearest evidence comes from controlled LED studies on leafy greens, especially lettuce and pak choi grown in hydroponic systems. These trials are valuable because they isolate spectrum as a variable while keeping the nutrient solution consistent, making it easier to understand how light quality influences nutrient uptake.

Spectrum Can Influence Mineral Uptake

A study comparing different LED spectra with conventional high-pressure sodium (HPS) lighting in hydroponic lettuce found that a combination of far-red, deep-red, and blue wavelengths improved growth performance and nutrient uptake. The treatment showed higher uptake indices for key elements including potassium, calcium, and magnesium compared with HPS-grown plants. [1]

Another lettuce study evaluated different red-to-blue LED ratios and found that a higher red proportion (red: blue ratio of 3) produced the highest uptake of nitrogen, phosphorus, potassium, and magnesium, along with improved yield compared with lower red: blue ratios. [2]

However, the study also highlighted an important distinction: higher nutrient uptake does not always mean higher nutrient use efficiency. Spectrum affected different nutrients in different ways. For example, nitrogen and phosphorus efficiency remained relatively unchanged, while potassium, calcium, and magnesium responded differently depending on the lighting treatment.

This means spectrum can influence how much nutrient a plant absorbs, but the relationship is not a simple “more light or more red equals more nutrient demand” formula.

Root Development: One Key Pathway

Research on pak choi provides another explanation for how spectrum affects nutrient uptake. Plants grown under white light combined with red and blue wavelengths showed improved plant height, leaf area, biomass, and root development compared with white LED controls. [3]

The improved root structure was associated with greater uptake of nitrogen, phosphorus, potassium, calcium, zinc, and iron.

This suggests that spectrum influences nutrient uptake not only through leaf-level processes but also by affecting root growth. Healthier, more developed roots can increase the plant’s ability to access and absorb available nutrients.

Where the Results Are Less Straightforward

Spectrum does not affect every crop or nutrient in the same way.

A study on common beans showed that nutrient responses varied depending on the light spectrum and the plant tissue being measured. For example, potassium and iron concentrations changed differently under red, green, and white light treatments. [4]

This highlights an important point: a spectrum response observed in lettuce cannot automatically be applied to tomatoes, strawberries, cannabis, or other crops. Crop species, cultivar, growth stage, growing system, and environmental conditions all influence the final response.

The same applies to nutrient measurements. Nutrient concentration in roots does not always represent nutrient availability in leaves, flowers, or fruit, which are often the parts growers care about most.

Spectrum may also influence nutrient uptake through multiple pathways. Root development is one possible mechanism, but light can also affect plant signaling systems, including photoreceptor responses and nutrient transporter activity.

What This Research Does Not Tell Us

Current research clearly shows that spectrum influences plant physiology and nutrient uptake, but it does not provide a universal formula for adjusting PPM.

Most available studies focus on fast-growing leafy greens in controlled hydroponic environments. Commercial fruiting crops have longer growth cycles and different nutrient demands across vegetative, flowering, and fruiting stages.

Spectrum should be considered one part of a larger crop management system that includes:

• Crop genetics and growth stage
• PPFD and daily light integral (DLI)
• CO₂ levels and environmental conditions
• Irrigation strategy
• Root-zone health
• Nutrient formulation and EC management

What This Means in Practice

The key takeaway for commercial growers is simple: lighting and nutrition should not be managed as separate systems.

When spectrum changes, plant behavior can change, including photosynthetic activity, transpiration, root development, and nutrient demand. A lighting system with consistent spectral output helps growers maintain a more predictable nutrient program.

This is also why spectral consistency matters when evaluating LED fixtures. A fixture that varies from batch to batch is not only delivering inconsistent light quality; it may also introduce variability into crop response and nutrient management.

A stable spectrum is a critical tool that helps growers create a repeatable production environment.

As a trusted OEM and ODM partner, our R&D approach is to engineer spectrum as a crop management tool, not simply to produce high PPFD, but to deliver consistent spectral output that supports plant needs throughout the growing cycle.

For more details, contact us.

References

[1] Pinho, P., Jokinen, K., & Halonen, L. (2017).

The influence of the LED light spectrum on the growth and nutrient uptake of hydroponically grown lettuce.

Lighting Research & Technology, 49(7), 866–881.

https://doi.org/10.1177/1477153516642269

[2] Pennisi, G., Orsini, F., Blasioli, S., et al. (2019).

Resource use efficiency of indoor lettuce (Lactuca sativa L.) cultivation as affected by red: blue ratio provided by LED lighting.

Scientific Reports, 9, 14127.

https://doi.org/10.1038/s41598-019-50783-z

[3] Mahesh, R., Hasan, M., Singh, D. K., et al. (2025).

Influence of artificial light spectral quality for enhancing growth, nutrient uptake and resource use efficiency of pak choi cv. Choko in indoor agriculture.

[4] Li, Y. N., Liu, N., Ji, F., & He, D. X. (2022).

Optimal red: blue ratio of full spectrum LEDs for hydroponic pakchoi cultivation in plant factory.

International Journal of Agricultural and Biological Engineering, 15(3), 72–77.

https://doi.org/10.25165/j.ijabe.20221503.7362