Three macronutrients are essential for all plant growth. Nitrogen, potassium and phosphorus must all be present for plants to grow and develop, along with other minor nutrients such as iron. Nitrogen is often added to soil in the form of nitrate, which is commonly found in inorganic fertiliser compounds.
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Nitrates are commonly added to inorganic fertilisers in the form of ammonium nitrate or calcium nitrate. To be usable by plants, these nitrates must be converted into nitrogen. Nitrates are brought into the plant through the roots, where they are then absorbed into different areas of the plant. Nearly all plant processes from photosynthesis to seed production require the presence of nitrogen, and scientists have isolated the plant gene that regulates the intake of nitrates into the plant when there is an abundance available. Once the plant absorbs the nitrate, it is transported through a complex network to the parts of the plant that need it the most. During the growing season, converted nitrates are taken to leaves and fruits while at the end of the season it is used primarily in the production of seeds.
Nitrates and Photosynthesis
Nitrogen -- and in turn nitrates -- is a major component in the structure of chlorophyll. The central molecule of chlorophyll is magnesium, which is then in turn connected to four molecules of nitrogen that have undergone the transformation from nitrate. Chlorophyll -- which also gives plants their green colour -- captures light energy from the sun. With the aid of a combination of sugars also absorbed from the soil, chlorophyll turns nitrogen, sulphur and other nutrients absorbed from the soil into organic molecules that are then delivered to areas throughout the plant. At the final destination, these molecules are used for energy to fuel the growth of leaves, stems and flowers.
Plant seeds contain nutrients necessary for germination, and healthy seeds contain enough of each of the macronutrients to support the plant until roots and internal structures have formed and can begin taking in nutrients from soil and processing them. When there is an ample amount of nitrates present in the soil at the time of seed development, valuable nitrogen is packaged into the seeds. Studies by the U.S. Department of Agriculture have shown that there is a significant difference in the ability of a seed to germinate based on whether or not the parent plant is supplied with nitrogen at the time the seed develops. Those with nitrates available germinate faster and outpace other seeds produced by plants that did not have nitrates available.
Where there are not enough nitrates present in the soil, all parts of plant growth are impacted. Mature plants are much smaller than healthier specimens, often with shorter stems, smaller leaves and small flowers. New growth still develops on a plant that is lacking nitrogen, but this new growth is smaller yet green, and saps valuable nutrients away from older leaves. The plant will not have enough nitrogen to keep old leaves healthy while growing new ones, so the result is often a plant that has yellow and wilting mature leaves with new growth at the tips or centre.
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- University of Missouri Extension; Nitrate in Soils and Plants; J.R. Brown, et al.; October 1993
- Colorado State University Extension; Nitrogen Sources and Transformations; K.A. Barbarick; January 2006
- NASA: Soil Science Education: How Does Your Garden Grow
- Ohio State University Horticulture: Photosynthesis
- USDA; "Nitrogen Effects on Seed Germination and Growth"; Thomas A. Monaco, et al., November 2003