Feeding to sufficiency, not excess

Nitrogen and potassium both do real work. Nitrogen builds the frame of the plant; potassium runs its plumbing and helps fill the flower. But both obey the same quiet rule, and it is the rule most feeds ignore: a plant needs a certain amount, and once that amount is met, more does not make a better crop. It makes bulk without quality, input the plant cannot use — and, with potassium, a shortage of the calcium and magnesium the plant genuinely needs. The controlled trials on flowering crops are consistent on both counts. Dr Forest is built around them.

Nitrogen: the yield-and-quality trade-off

Give a plant more nitrogen and it grows — bigger leaves, more stem, more biomass — up to a point. In controlled trials that point sat at about 160 mg/L of nitrogen in the nutrient solution: below it the plants were starved, showing visible deficiency and a heavy loss of growth and photosynthesis; at it, growth and yield were at their best (Saloner & Bernstein 2020). Push nitrogen past that and yield does not keep climbing — it holds around the plateau — but the crop keeps changing, and not in your favour.

The change is in quality. The compounds that give a flower its aroma and character — its terpenes and the resinous, active secondary compounds the plant makes — are richest when nitrogen is modest, and they thin out as nitrogen climbs. In the same body of work, flowers grown at the optimum nitrogen level carried roughly a third less of their principal acidic compounds than flowers grown lean, and the concentration fell further as nitrogen rose (Saloner & Bernstein 2021). The authors describe it plainly as an inverse relationship between yield and quality: as the plant is pushed to bulk up, the good is diluted. More nitrogen buys weight, and pays for it in quality.

Schematic. Drawn from the trends reported in Saloner & Bernstein 2020 and 2021, not a re-plot of their measured data. Each curve is scaled to its own maximum: relative yield climbs to the optimum nitrogen supply (about 160 mg/L in the nutrient solution) and then holds, while the concentration of the crop's aromatic and active compounds is highest at low nitrogen and falls as supply rises.

There is a second cost. Growth pushed hard on nitrogen is soft and green, and it ripens late and less cleanly — the plant is still in leaf-making mode when it should be finishing. That is why Dr Forest feeds nitrogen high early, while the frame is being built, then eases it right down for bloom and lets it fall to nothing for a clean, water-only finish.

Potassium: why more doesn't mean better

Potassium is the nutrient plants take up most freely — routinely far past what they need. This is called luxury consumption, and it has been understood in agronomy for the best part of a century (Bartholomew 1929; Marschner's Mineral Nutrition of Higher Plants). A plant will keep drawing potassium in as fast as you supply it, banking the surplus in its tissue whether or not it can do anything useful with it.

In controlled trials, potassium supply was raised across a wide range. Yield rose only until sufficiency was met — around 60 mg/L at flowering — and then flatly refused to climb any higher, however much more potassium was fed. Meanwhile the plant kept taking the extra up, so its use-efficiency fell away with every added unit. And the concentration of the crop's aromatic and active compounds did not improve with more potassium; it declined (Saloner & Bernstein 2022). Past sufficiency, extra potassium is money the plant banks and never pays back.

Worse, potassium does not sit quietly. It competes at the root with calcium and magnesium — the more potassium you push, the less calcium and magnesium the plant can take up. In the same trials, leaf calcium and magnesium fell steadily as potassium rose, and at the highest potassium level one variety was visibly harmed by the resulting shortage (Saloner & Bernstein 2022). Calcium and magnesium are not optional. Starving the plant of them to feed a surplus of potassium it never needed is the opposite of a balanced feed.

Schematic. Drawn from the trends reported in Saloner & Bernstein 2022, not a re-plot of the measured data. Potassium uptake keeps rising with supply while use-efficiency falls — luxury uptake; inflorescence yield plateaus once sufficiency is met (about 60 mg/L at flowering in that study); and leaf calcium and magnesium decline as potassium climbs — cation antagonism.

This is why Dr Forest holds a target calcium-to-magnesium-to-potassium balance rather than chasing high potassium "bloom" numbers. Potassium is lifted for bloom, because bloom genuinely calls for more of it — but only to what the plant will use, never into the luxury range where it starts crowding out the nutrients that matter.

What the plant needs, and what the feed delivers

Put the two together and the programme is simple: meet demand as it changes through the grow, and stop there. Nitrogen is fed high while the plant builds its frame, then eased through bloom and dropped for the finish — so it is never diluting the flower's quality while quality is being laid down. Potassium is lifted for bloom, in step with real demand, then eased for a clean finish — never spiked into the luxury range. Neither is fed to excess at any point.

Illustrative. The band is the shape of a flowering crop's demand; the gold line is what the feed delivers. Curves are relative within each nutrient, not compared between them. Nitrogen is fed high in veg and eased to nothing for a clean, water-only finish; potassium is lifted for bloom demand, then eased — met at sufficiency, never spiked.

Sources: Saloner & Bernstein 2020, Frontiers in Plant Science 11:572293 · Saloner & Bernstein 2021, Industrial Crops and Products 167:113516 · Saloner & Bernstein 2022, Agronomy 12:1242 · Saloner, Sacks & Bernstein 2019, Frontiers in Plant Science 10:1369 · Bernstein, Gorelick, Zerahia & Koch 2019, Frontiers in Plant Science 10:736 · Bartholomew 1929, Agronomy Journal 21:751 · Marschner's Mineral Nutrition of Higher Plants, 3rd ed., 2012.