Environmental Issue Associated with Phosphorus in NB

WHAT IS SOIL PHOSPHORUS?

Phosphorus (P), like nitrogen and potassium, is an essential nutrient for plant growth. It is a critical plant nutrient because of its low concentration in soils ( 600 ppm total P) and its low solubility (average of 0.05 mg P L^-1 in soil solution). Soil P exists in inorganic and organic forms (Fig. 1). Inorganic forms are associated with amorphous and crystalline aluminium and iron compounds in acid soils and calcium compounds in alkaline soils. Organic P forms are associated with organic materials in soil.

Continuous P exchanges take place between various P forms. Plants absorb phosphorus from the soil solution. Soil solution P concentration decreases with the increase of plant P uptake, P fixation on soil minerals, and micro-organism activities. That decrease triggers P desorption from mineral P forms. In addition, the mineralization of plant residue P under microbial activity increases available soil phosphorus (both labile and dissolved forms). All soil P forms also change when P fertilizers or manures are applied. All the P forms may also be lost through soil erosion and leaching processes.

WHAT DOES PHOSPHORUS DO?

The most essential role of phosphorus in plants is to store energy and provide it again plant cells when they needed it. This is done through complex organic molecules commonly referred to as ADP (adenosine diphosphate) and ATP (adenosine triphosphate). These two compounds are formed and regenerated in the presence of sufficient phosphorus. Phosphorus is also an important structural component of nucleic acids (DNA or deoxyribonucleic Acid, and RNA or Ribonucleic acid), coenzymes, phosphoproteins, phospholipids, and sugar phosphates.

Adequate P supply is important at early stages of the plant growth and at the development of reproductive plant organs. Phosphorus is also needed for adequate root development and crop maturity. Large amounts of phosphorus are found in seeds and fruits. Adequate P supply improves the quality of certain fruits, forage and vegetables. It also increases plant resistance to diseases, winter damage, and unfavourable growing conditions. Phosphorus is mobile in plants. When P deficiencies occur, P moves from old to young, more active tissues. A purple discoloration of leaves or leaf edges is a common symptom of P deficiencies.

HOW DOES PHOSPHORUS BECOME AN ENVIRONMENTAL ISSUE?

Phosphorus nutrient is not toxic to plants. Crop P requirements are generally much higher than actual crop P uptake. A small fraction of available phosphorus in the soil is only taken up by plants. The amount of P that remains in the soil, as opposed to the amount taken up for plant growth, is known as residual soil P. That residual P contributes the build-up of soil phosphorus, particularly the build-up of available phosphorus in the tillable soil layer.

The increase of available soil phosphorus associated with continuous additions of P fertilizer results, in the long term, in excessive phosphorus levels. The soil therefore becomes saturated in phosphorus. The soil P saturation percentage is estimated through the ratio (%) of soil available phosphorus to soil exchangeable aluminium, both elements being extracted and measured from the same Mehlich III extracting solution.

Information collected from the database of the Agricultural Laboratory, Fredericton, New-Brunswick, from September 1999 to July 2001, indicates that 60% of New-Brunswick agricultural soils test more than 78 ppm phosphorus. For most crops, notably potatoes, forages, and cereals, above 78 ppm P the tillable soil is considered very rich in available phosphorus.

High P levels in the 0-15 cm surface soil layer are associated with high soil P saturation percentages. Under erosion conditions, potential risks are high for dissolved and sediment-attached phosphorus to get into watercourses. When phosphorus enters a watercourse, it contributes watercourse enrichment in phosphorus. This enrichment is also known as water eutrophication. Water eutrophication results in an increased growth and development of aquatic plants. More plants grow, produce, and die. It is the decomposition of those dead organic materials that then causes the depletion of dissolved oxygen and put the development of beneficial, aerobic aquatic life at risk.