Nitrogen: The Builder

Nitrogen is one of the main nutrients controlling the productive capacity of a crop. It is incorporated into amino acids, proteins, enzymes, nucleic acids and chlorophyll, which means that almost every process associated with growth depends on an adequate nitrogen supply.

That makes nitrogen extraordinarily powerful. It also makes nitrogen easy to misuse.

A crop supplied with too little nitrogen lacks the raw material to build productive leaf area and photosynthetic machinery. A crop supplied with more nitrogen than it can efficiently assimilate may continue producing lush vegetative growth without converting that nitrogen efficiently into functional proteins and high-quality tissue.

The objective is therefore not to maximise nitrogen concentration. It is to maximise nitrogen conversion.

Nitrogen's personality and its relationships

Think of nitrogen as an ambitious builder. It is highly productive, growth-oriented and always ready to expand the project.

Its strength is speed. Give nitrogen favourable conditions and the plant can rapidly produce leaves, shoots and new biomass.

Its weakness is that it cannot complete the project alone.

Sulfur is nitrogen's finishing specialist. Nitrogen can provide the raw material for amino acids, but sulfur is required to produce sulfur-containing amino acids such as cysteine and methionine. If sulfur supply falls behind nitrogen supply, nitrate and other non-protein forms of nitrogen can accumulate while protein synthesis becomes less efficient.

Molybdenum is the gatekeeper when nitrogen enters the plant as nitrate. The nitrate-reductase enzyme requires molybdenum. Without sufficient Mo, the first step in nitrate reduction becomes restricted.

Iron works one step further downstream because nitrite reduction also depends on iron-containing enzyme systems.

Magnesium is part of the technical team. It is central to chlorophyll and is required for numerous enzyme, ATP and protein-synthesis processes. A plant cannot make efficient use of nitrogen if its photosynthetic machinery is not generating sufficient energy and carbon skeletons.

Phosphorus supplies much of the energy infrastructure. Nitrogen assimilation is energy demanding, and adequate P nutrition supports ATP-dependent metabolism.

Potassium runs the logistics network. K contributes to nitrate transport, ionic balance, photosynthesis and movement of assimilates. Adequate K is therefore important for efficient nitrogen use.

The builder becomes problematic when nitrogen outruns this entire support team. More nitrogen does not solve a sulfur, molybdenum, magnesium or energy problem. It can simply produce a larger backlog of nitrogen waiting to be converted.

The agronomy behind these relationships

Plants acquire much of their inorganic nitrogen as nitrate and ammonium. Nitrate must first be reduced to nitrite by nitrate reductase and then to ammonium before the nitrogen can be incorporated into amino acids. Molybdenum is required for nitrate reductase, while nitrite reductase contains iron-dependent components. Assimilation subsequently connects nitrogen metabolism very closely with carbon, sulfur and energy metabolism.

Nitrogen and sulfur metabolism are particularly closely coordinated. Sulfur deficiency can decrease nitrate assimilation and lead to accumulation of non-protein nitrogen forms.

Nitrogen and phosphorus frequently show a positive interaction because vigorous growth requires both nutrients, while nitrogen supply can stimulate root growth and phosphorus acquisition.

Potassium is similarly important to nitrogen efficiency because of its contribution to nitrate transport, membrane potential, phloem function and carbohydrate movement.

The form of nitrogen matters as well. Excessive ammonium can compete with other cations and contribute to lower potassium, calcium or magnesium uptake. Nitrate and ammonium can also have opposite effects on rhizosphere pH.

What should a grower look for in plant sap?

Do not interpret total nitrogen in isolation.

Look at nitrate-N, ammonium-N and total N together, and compare young and old leaves using the crop-specific target ranges of the laboratory.

High nitrate combined with disappointing growth should prompt a different question from simply, “Does the plant need more nitrogen?”

The better questions are: Is the nitrate being converted efficiently? Is sulfur adequate? Is molybdenum sufficient? Are magnesium and iron supporting photosynthesis and nitrate metabolism? Is potassium supporting transport? Is the crop producing enough photosynthetic energy to assimilate the nitrogen already present?

Plant sap analysis becomes particularly useful when it changes the nitrogen question from “How much N is there?” to “How well is this plant using the N it already has?”