Managing nutrient ratios

Photo: JackF, Adobe Stock

Providing the proper mineral nutrients to hydroponically grown food crops grown in controlled environments is a primary concern for any producer. But managing mineral nutrients isn’t something that can be accomplished on “cruise control” or with a “set it and forget it” mentality. Adjusting fertilizers throughout production, both the total quantity of nutrients provided and relative proportions or ratios of select nutrients, can improve crop quality and yield. This article will focus on the key points to managing nutrient solution ratios for both leafy green and fruiting vine crops.

Lettuce, specialty greens, culinary herbs and other leafy crops are some of the most popular food crops to grow hydroponically in controlled environments due to their quick production time and popularity with consumers. Leafy green production can be divided into two primary stages: Stage 1: Seeding and Seedling Development; and Stage 2: Production or Grow-out. The degree of adjustment is less than the changes required for vining crops.

For leafy crops, the goal of production is to maximize leaf production, including the number of leaves, as well as size and weight. The two nutrients that are most responsible for enhancing growth and development are nitrogen and phosphorous. Since the vegetative growth (i.e. the foliage) is the focus of production, the ratios of elements in the nutrient solution doesn’t need to be adjusted during production — growing leaves is always the goal.

Since the ratios of mineral nutrients in relation to one another does not need to change from Stage 1 to Stage 2 for leafy green production, this simplifies fertilizer practices. A two-tank approach is common in hydroponic leafy green production and works very well for leafy green fertilizer programs. However, since the quantity of nutrients changes, but not their proportions relative to one another, a one-bag or one-tank approach to fertilization can also be used, as fixed ratios of nutrients are suitable for finishing leafy crops. Though the concentration, or electrical conductivity, should increase from Stage 1 to Stage 2, the ratios of nitrogen and phosphorus can stay the same.

Starting during Stage 1, Seeding and Seedling Development, nutrient solutions start at the lowest nutrient concentration. The relative proportion of nitrogen to potassium is the highest, with a ratio of 1-1. Early in production, vegetative growth (i.e. leaves) is the primary concern as the plant is sizing up to move from the small substrate cube seeds that are started into the larger blocks they will be transplanted into. Just like with leafy greens, nitrogen plays an important role in enhancing vegetative growth.

As vining crops move into Stages 2 and 3, although the total amount of nitrogen still increases, the ratio of nitrogen to potassium should decrease due to greater increases in potassium concentrations. Potassium is the predominant cation in fruits, and potassium concentrations in the nutrient solution need to be increased to reflect the increased demand. By Stage 3, the ratio of nitrogen to potassium should be 1-1.8, with potassium concentrations at nearly two-times that of nitrogen.

In addition to increasing potassium relative to nitrogen as fruiting vine crops move from Stage 1 to Stage 3, calcium concentrations also increase, though not as dramatically as potassium. By Stage 3, the ratio of nitrogen to calcium should be approximately 1-1, balanced with each other. Insufficient calcium can be particularly damaging to fruiting vine crops in the Solanaceae (“nightshade”) family — tomato, pepper and eggplant. A calcium deficiency can lead to blossom end rot, which renders fruits unsaleable.

As with leafy crops, A and B tanks are widely used in hydroponic systems used for fruiting vine crops, which can provide a distinct advantage when adjusting nutrient ratios. A and B tanks are primarily used to keep calcium and sulfur separate; if they are mixed together in a concentrated solution, calcium sulfate or gypsum can precipitate and fall out of solution. In practice, one tank usually contains calcium nitrate, with a nitrogen-phosphorous-potassium analysis of 15-0-0. The other tank contains the remaining macro- and micronutrients and has a nitrogen-phosphorous-potassium ratio of approximately 5-12-26, though this varies with manufacturer. Having the bulk of nitrogen separated from potassium in different stock tanks, this allows nitrogen and potassium and, to a lesser degree, calcium to be adjusted more independently from one another for different stages of fruiting vine crop production.

Aside from increasing fertilizer concentrations as leafy greens and fruiting vine crops progress through production stages, consider how concentrations of individual mineral nutrients and their ratios with other key nutrients should be adjusted during production. Taking a more dynamic approach to managing mineral nutrient solutions can improve the productivity of food crops grown hydroponically in controlled environments.

Christopher (ccurrey@iastate.edu) is an associate professor in the Department of Horticulture at Iowa State University.