The Effects of Leaching on Plants

Plant Nutrients

• Primary nutrients essential for plant growth are nitrogen, phosphorus, and potassium. Secondary nutrients are sulfur, calcium and magnesium. In addition, there are the micronutrients plants need in small amounts: They are zinc, iron, copper, manganese, boron, molybdenum and chlorine. Leaching of nutrients is affected by a number of factors, including whether the ions of dissolved nutrients are negatively or positively charged, whether the nutrients are easily dissolved in water, what type of soil is present in terms of moisture content, pH, particle size, presence of organic matter and clay, and what kinds of plants are growing in the area.

Nitrogen

• Many different types of nitrogen compounds are found in soil, including amino acids, urea, ammonia, ammonium, nitrite, nitrate and free atmospheric nitrogen. Of these, nitrate is of most importance in leaching, as it dissolves readily in water and moves freely through the soil. Nitrates can come from plant decomposition products, from manure, or from applied fertilizers. Because nitrates move quickly through the soil and can become depleted in the root zone from heavy rainfall in a relatively short time, soils usually need regular reapplications of nitrate fertilizers for good plant growth.

Phosphorus

• The forms of phosphorus found in soil are mostly insoluble in water. Plants need phosphate ions, which are not common in soil. Phosphate ions are taken up by roots and can be leached, so application of phosphate fertilizers are second only to nitrates in amount of fertilizer needed by cropland.

Other Nutrients

• Few of the other plant nutrients are affected by leaching. Potassium, calcium, magnesium and sulfur can be leached from sandy soils like the coastal plain soils of North Carolina during excessive rainfall periods. Potassium is needed for good fruit formation and good yields. Signs of calcium deficiency are deformed new leaves and short roots. Plants lacking magnesium have yellow areas separated by dark green veins on leaves. Sulfur-deficient plants begin to turn yellow with stunted growth.

Leaching Benefits

• In some areas, excessive soil salinity in the root zone causes problems with plant growth. Arid areas of Colorado, for instance, have areas of excess salinity from natural weathering of minerals or from salinity remaining from ancient sea beds. In arid areas of the American Southwest, repeated irrigation with highly mineralized water can lead to salinity buildup, especially on soils with poor drainage. Leaching these soils with more water than the plant requires helps to reclaim the soil by dissolving the saline compounds and moving them beyond the root zone.