Plant Nutrition: To Supplement, or Not?

Today’s grower can choose from a wide variety of products designed to stimulate root growth, enhance vegetative vigor or generate greater fruit yields and higher-quality crops. My advice for growers is to proceed with caution by questioning whether such supplements are really needed, and finding out what the potential side effects are.

Experience shows the best plant growth and function is obtained when the essential plant nutrient elements are present initially at their optimal level in the rooting medium and subsequently maintained during the growth cycle, with supplementation based on established procedures for maintaining consistency surrounding plant roots.

Inconsistency can lead to poor plant performance and disappointing product yield and quality. The following assessments are based on the use of supplements containing a specific element or elements, or supplements that are applied as an organic or microbial amendment.

Composts

Composted materials may be added to an inorganic rooting medium as a means of increasing the organic matter content, or as an organic source for an essential plant nutrient element or elements. Composts vary widely in their organic composition and elemental content, depending on the source material, degree of decomposition, and the quantity and quality of added amendments, such as animal or poultry manures. Therefore, adding a compost to a rooting medium may also add unwanted elements, creating the potential for an elemental insufficiency or toxicity.

Depending on the degree of composting, an increase in microbial activity may not occur in the rooting medium with the addition of a composted material. If there is an increase in microbial activity, it can pose a competitive challenge to the growing plant. Depending on the amount of compost added, the physical properties of the rooting medium may be altered, such as making the rooting medium slow to drain, remaining wet after rainfall or applied irrigation, while keeping the rooting medium cool, thereby slowing plant growth. Soils with a high amount of organic matter tend to be more plastic in their physical consistency, requiring care when being tilled to avoid puddles in the soil.

Micro-organisms

Microorganism activity in the rooting medium occurs primarily around plant roots as the food required for microorganism growth is sloughed off when roots advance through the rooting medium. Adding a microbial supplement to a rooting medium may create competition for the essential elements required by plants, as micro-organisms are aggressive competitors. Micro-organism growth and survival also depends on the food supply, which may rapidly dissipate depending on temperatures and the level of easily oxidizable energy source substances.

The number and variety of microbial species present in a garden will depend on the plant species being grown. The continuous cropping of the same plant species will reduce the number of different microbial species, and possibly lead to overpopulation, which could become pathogenic to the growing plant. Micro-organisms that exist around a plant root may be species-specific, requiring the matching of an applied micro-organism with the plant species for a beneficial effect to occur.

Nitrogen

The two ionic forms of nitrogen that can be absorbed by roots and used by plants are the nitrate anion and the ammonium cation. In most rooting media, the ammonium-nitrogen form can, and usually is, converted to nitrate-nitrogen by microbial nitrification. The nitrate anion moves in a rooting medium with the movement of water, while the ammonium cation can be retained by being fixed to the rooting medium’s cation exchange capacity (CEC).

Ammonium-nitrogen can be toxic to plants when it is the major portion of the total nitrogen being absorbed by roots, thereby interfering with calcium nutrition that can result in vascular vessel decay as well as reducing fruit yield and quality. Blossom-end rot in fruits such as tomatoes can be induced by high ammonium-nitrogen nutrition.

Supplementing with a nitrogen fertilizer, whether organic or inorganic, can adversely affect plant growth, depending on the form of nitrogen used. Too much nitrogen can stimulate excessive plant growth, making it sensitive to moisture and temperature stresses, and making it more vulnerable to insect and disease infestation. Maintaining a consistent, sufficient supply of nitrogen with less than 25% in the ammonium-nitrogen form promotes steady, healthy plant growth.

Phosphorus

Phosphorus exists in solution as the orthophosphate anion, either as the monohydrogen phosphate form or the dihydrogen phosphate form, depending on pH. Both anion forms are highly reactive and will easily combine with iron, aluminum and calcium cations in the rooting medium. Phosphorus can interfere with the root absorption of both iron and zinc when at an elevated concentration in the rooting medium, and interfere with iron and zinc metabolism in the plant when existing at a high concentration in the plant. In most instances, phosphorus excess is more common than deficiency, so supplementation needs to be carefully considered to avoid creating an excess.

Potassium

Potassium exists in solution as a cation and is one of the three major essential plant nutrient element cations—the other two being calcium and magnesium. For sufficiency among these three cations, there is an optimal ratio requirement that varies with plant species. If this ratio is altered, an insufficiency can be created, the most vulnerable element being magnesium as it is the least competitive of the three major cations.

Supplementing a rooting medium with potassium can create a magnesium insufficiency, which in turn will reduce the rate of photosynthesis as well as affect energy-related processes as magnesium is a co-enzyme in most energy transfer systems.

Growers have never had as many options as they do today. To sort through all of the choices for supplementation, it is important to get a feeling for how plants work, and how different supplements effect plant growth. Always read the labels carefully, and never be afraid to ask questions at your local hydroponics retailer.