MadSci Network: Agricultural Sciences

Re: How does density and nitrogen availability affect plants?

Date: Fri Apr 17 11:42:13 1998
Area of science: Agricultural Sciences
ID: 892471522.Ag

MadSci Network: Agricultural Sciences


Your timing with this question is perfect!! We just finished up a nitrogen experiment in a Horticulture lab I teach here at Penn State.

First off, I have to ask a question. What exactly do you mean by density? For the time being, I am going to assume you mean number of plants in a given area.

The number of plants in a given area will determine amount of competition for the available resources. Obviously, there is only so much nitrogen, potassium, phosphorus, and other essential elements in the soil. The more plants that share that given block of soil, the less nutrients each plant will be able to pick up. In addition, competition for water and light (shading) should also be considered.

From an agricultural perspective, the objective with planting densities is to maximize the yield. More plants will produce more crop only up to a certain point. Above that, the plants compete for resources causing a reduction in plant size and health, thus reducing yield.
Nitrogen is a very important element to the survival of the plant. It is a major component of, among other things, amino acids, numerous proteins, and chlorophyll. Without nitrogen, plants are unable to carry out some of the most basic functions they need to survive.

A plant that does not have enough nitrogen exhibits the following symptoms:
- Restricted growth
- Generally yellow in color (chlorotic) due to a lack of chlorophyll, especially in lower leaves.
- Stems, petioles, and lower leaf surfaces can be purple in color.
- Increased root mass in comparison to shoot mass (high root to shoot ratio)

Misapplying fertilizer can lead to too much nitrogen, which causes:
- Dark green color
- Abundant foliage and a restricted root system (low root to shoot ratio)
- Flowering and seed production can be retarded, delayed, or prevented all together.

Plants have mechanisms to help them survive when they encounter the rather common condition of nitrogen deficiency. The plant allocates its energy to making roots so that it can search more soil at the cost of reducing foliage growth. On the other hand, plants have no way to stop taking up nitrogen and no place to store it. The response is the produce less roots so it takes up less nitrogen and produce more leaves so it has some place to use the excess nitrogen.

Like planting density, more nitrogen can only increase the yield up to a certain point. There is a point called the "economic threshold". What this means is that applying more nitrogen will increase the yield, but will not produce enough crop to offset the cost of more nitrogen. Due to the various fertilizer needs of different crops, different soils, and economic situations of farmers, this threshold is varies widely.

Texas A&M has put together a good sight for identifying nutrient problems and diseases on cucurbits (squash, cucumbers, pumpkins, and the like).

Thanks for the question!

Eric J. Biddinger
Grad Student - Department of Horticulture
Penn State University

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