Tomato Fertilization

Asked June 24, 2020, 9:13 PM EDT

In last week's Maryland Grows blog, Jon T stated it is common to add high N fert after the first fruit appears. I am curious why this would be. I know there is a tension between N and P which, with any plant, can result in excess foliage diminishing flowers/fruits. Intuitively it would seem that N might be preferred earlier to achieve plant size, but then once flowering/fruit occurs, P would be preferred to further production. Would like to understand the physiological dynamics that indicate N best at that juncture. Thanks.

Anne Arundel County Maryland

3 Responses

Hi- of the three primary macronutrients (N, P, K) nitrogen is required by plants in the greatest amount throughout the lifecycle. The ratio of N:P inside plant tissue is about 4:1.

The bigger nutrient inbalance issue for commercial growers occurs with N and K. In some cases this is a result of low K levels in the soil and in some cases it's been caused by the use of ammonuim N fertilizers (positively charged ammonium and potassium ions competing for space on clay and organic matter particles, and uptake by plant roots).

My question was what precipitates a need for high N fert at the moment of fruit formation, or is it just a coincidence that it typically happens to be when fert applied at planting has been depleted?

Hi- unless the organic matter level is very high in the soil the plants will not get sufficient nitrogen (release is dependent on air, water, temperature, and microbes) during the extended fruiting period- a time when additional nitrogen is needed.

Many experienced gardeners do not sidedress with additional N at fruiting and are happy with the harvest. Could they produce a higher yield by applying supplemental N? Possibly, yes.