Most gardening resources list the mineral needs of plants. But do plants, like humans and animals, need vitamins? The answers may not be quite what you expect.
Vitamin B1 (Thiamine)
Vitamin B1 is often recommended for trees and other larger plants to prevent transplanting shock.
It is important to note that the early research done on the effects of vitamin B1 on plants involved mixing the vitamin with auxins, or natural plant growth regulators. Later studies that used vitamin B1 alone did not show the same results.
Healthy, vibrant soil contains many microbes that produce this vitamin, and plants are able to produce it themselves, as well. Therefore, as long as the soil community is thriving, there will likely be little benefit to supplementing plants with vitamin B1.
Vitamin C has now been proven to be essential for plant survival and drought tolerance. In one study, plants were genetically modified to inhibit their ability to produce vitamin C. None of them survived past the seedling stage, likely because vitamin C protects plants from UV radiation.
On the other hand, another study that involved adding vitamin C to the soil of parsley reduced its growth. Therefore, the likely conclusion appears to be that, while plants do need vitamin C, they normally make their own and do not need supplementation.
Tests involving vitamin D and plants have had mixed results to date. Vitamin D alone appears to have a beneficial effect on growth. However, when combined with other minerals, such as iron, the results can be harmful to plants.
Many gardeners have experimented with crushing multivitamins to use as a fertilizer for their plants, and most report excellent results. Faster plant growth and superior flavor are among the reported benefits.
Of course, it is important to note that a multivitamin is not a panacea. Plants may still be susceptible to insect attacks, in particular. Also, any benefits will be limited by the genetic potential of the plant, with some plants showing more dramatic improvement in flavor than others.
Based on the evidence it would appear that plants do indeed need vitamins to thrive.
However, keep in mind that, based on casual research conducted by enthusiastic gardeners, different types of plants appear to have different vitamin needs.
Furthermore, supplementation with vitamins should not be necessary in gardens with healthy soil. This is because one of the many roles of soil microorganisms is to produce these vitamins and make them available for plants to use. Healthy plants will also manufacture their own vitamins.
That said, plant roots do take up vitamins in the soil. Those vitamins can then be converted to nutrient-rich food that will benefit the humans that eat the produce.