Be Careful with Fertilizers for Tomatoes

Q. I was reading an older article from Linn Mills about foliar fertilizing tomatoes. Linn was saying to spray tomato plants with phosphorous. I read an organic article in which they use fish-seaweed extract in your opinion which is better to use?

High phosphorus fertilizers do not cause all vine and no fruit

A. Great advice from Linn! You want to give tomatoes a complete fertilizer at the time of planting. You can use organic sources or conventional fertilizers. It will not make much difference to the plant.

These can be applied to the soil or to the plant leaves if they are a liquid. After an initial

application of a fertilizer, phosphorus should be in abundance, not nitrogen, with further applications.

The problem with organic sources like compost added to the soil at the time of planting is that the fertilizers or nutrients they contain last so long. This is because the nitrogen in the compost is released slowly over a period of several months. This is not true of organic foliar sprays like seaweed extracts.

An overabundance of compost added to the soil will cause tomatoes to produce a lot of vine and delay flowering. This is not necessarily true of their close cousins, peppers and eggplant. If flowering is delayed in tomato and begins during the heat of the summer, it can result in little to no fruit production.

Example of a high phosphorus foliar fertilizer
to stimulate flowers and fruit

Inexpensive conventional fertilizers (farm fertilizers) release its nutrients quickly. One big shot is released during the first few irrigations and the amount of fertilizer available to the plant diminishes quickly after that.

These quick release or conventional fertilizers, at least the nitrogen component, lasts four to six weeks and 80% of it is gone. Quick release fertilizers, applied at planting time, are perfect for tomato growth and fruit production.

Foliar fertilizers, fertilizers applied as a liquid to the leaves and stems, act very similarly to quick release fertilizers applied to the soil. Liquid fertilizers applied to leaves last a very short time; 2 to 3 weeks.

Foliar fertilizers need to be applied more often than fertilizers applied to the soils. Foliar fertilizers should always contain a wetting agent to help that fertilizer gain entry inside the plant.

We want tomato plants to gain size quickly for about four weeks before they set fruit. If nitrogen is released in large amounts after that time, it will likely cause a delay in flowering and put on leaf and stem growth instead.

Example of a 1:1:1 fertilizer

If you are foliar feeding, you should have at least two different types of foliar fertilizers, maybe even three. The two which are most important are a high nitrogen content (highest first number) fertilizer and the second is a foliar fertilizer with high phosphorus content (highest second number).

If you get a third one then make it a fertilizer in a 1:1:1 ratio of nitrogen, phosphorus and potassium. Whenever possible make sure the third number, potassium, is also high in all three

fertilizers or make sure it is applied to the soil and available to plants.