Nutrients in topsoil are essential to plant growth; topsoil is the uppermost layer of soil, containing high nutrient levels that are not available in the lower levels of soil referred to as subsoil. Healthy topsoil includes the presence of the nutrients phosphorus, potassium, iron, and nitrogen, according to the Utah State University Cooperative Extension.
Phosphorus (P) is a nutrient in topsoil that is necessary for healthy plant growth. This nutrient plays a key role in photosynthesis (the conversion of sunlight into sugar that acts as food for the plant). Also, phosphorus encourages the following when absorbed by plants: normal maturing of the plant, rapid growth rate and successful blooming and growth of roots. Phosphorus is a macronutrient; macronutrients are those nutrients in plants needed in large quantities for proper development, as explained by the North Carolina Department of Agriculture and Consumer Services.
Potassium (K) is a nutrient found in topsoil that is absorbed in very large quantities in comparison to other essential nutrients, making it a macronutrient. Potassium benefits the growth of plants through its following functions: promotion of protein construction, assistance with successful photosynthesis, production of high quality fruit and encouragement of plant vitality that results in lower occurrences of plant disease, according to the North Carolina Department of Agriculture and Consumer Services.
The nutrient iron (Fe) found in topsoil is essential to healthy plant growth because it acts as a building block, according to the North Carolina Department of Agriculture and Consumer Services. Iron is needed for the plant's creation of chlorophyll, a green substance in plants that absorbs energy from the sun; the stored energy becomes converted into the plant's food source during photosynthesis. Since it is needed in only very small amounts, iron is referred to as a micronutrient.
Nitrogen (N) is an essential nutrient in topsoil for many functions of a plant. This macronutrient makes up a part of every cell of a plant. When a plant converts energy, it utilizes proteins, enzymes and metabolism; nitrogen, present in proteins and enzymes, is essential in each aspect of a plant's energy conversion, according to the North Carolina Department of Agriculture and Consumer Services. Additionally, nitrogen is responsible for the following: acts as an essential component to chlorophyll and encourages rapid growth and higher production of seeds and fruit.