Fruiting crop requirements

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Sometimes one of the more well-known lines from George Orwell’s Animal Farm can be modified to describe how we can approach the 12 essential elements for plants: “All nutrients are equal, but some nutrients are more equal than others.” No fruiting vine crop can be grown without receiving all primary and secondary macronutrients and micronutrients. But the nutrient requirements of high-wire crops like tomato, pepper, eggplant and cucumber are different from many of their leafy crop counterparts.

Before we look at the specific role of some individual nutrients in fertilizing vine crops, one of the first steps to improve nutrition management for fruiting vine crops is to understand the different phases of nutritional requirements in their life cycle. There are three primary phases in the production life of high-wire vine crops: 1) seeding and seedling development; 2) transplanting and early growth; and 3) production, training and pruning. These three phases influence several aspects of a fertilization plan, including the overall concentration of macro- and micronutrients, as well as the concentrations of individual nutrients.

First, the overall concentration of mineral nutrient, as measured by electrical conductivity, increases as high-wire vines as they progress seedlings in propagation to fully mature plants being trained and harvested. This makes intuitive sense, as providing lots of nutrients. But it is not just more of everything in the same proportion — specific nutrient concentrations are more dynamic than others as solution strength changes.

Nitrogen is one of the most important nutrients for plants and is a central nutrient for vegetative growth and development. Insufficient nitrogen will cause the lower leaves on crops to turn yellow as it is remobilized. Nitrogen concentrations may start at around 50 ppm in propagation, then up to around 200 ppm during production, training and fruiting. Another nutrient that can cause deficiency symptoms to form on lower leaves is magnesium (Mg). A shortage of it can cause yellowing that can mature into a bronzing under severe conditions. This can be a problem if high calcium (Ca) concentrations are used, as it can outcompete Mg for uptake. In cucumber, potassium (K) deficiencies can manifest themselves readily on leaves, with a thin band of marginal chlorosis around the leaves.

While we want to produce healthy foliage, we also need to produce high-quality fruits. The two biggest nutrients associated with fruiting are Ca and K. If insufficient Ca is allocated to eggplant, peppers and tomatoes, the fruits can develop blossom end rot, a necrotic lesion on the bottom of fruits that renders them unmarketable. While Ca deficiencies can be the result of insufficient Ca applied to plants, it can also be deficient if excessive Mg is competing for uptake (the inverse problem as described earlier) or if environmental conditions are diminishing water (and Ca) uptake.

In addition to changing the nutrient solution as vine crops transition from propagation into full production, changes can be made to enhance fruit formation, yield, quality and nutrient solution or to steer crops. For fruiting vine crops — especially those Solanaceous crops, including tomato, pepper and eggplant — running a leaner and lower EC can steer plants towards a more reproductive or generative state, whereas increasing from moderate to more luxurious (but not toxic) EC concentrations can promote vegetative growth.

As your crops are growing, keep in mind how the nutrient solution can and should be changed to accommodate the unique requirements of fruiting vine crops to get the best yield and quality.

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