Soil Fertility

Soil Fertility: An Important Key to a Productive Pasture
  • Mike Baden, Resource Specialist
One of the most important keys to a productive pasture is good soil fertility. Nutrients are used by plants to perform a number of functions necessary to complete their life cycles. Certain nutrients called essential nutrients are required by the plant; the plant cannot carry out normal growth without each and every one of them being present in adequate amounts. These essential nutrients can be further divided into 2 groups: the macronutrients and the micronutrients. Nutrients are placed into 1 group or another depending on the relative amounts in which they are required by the plant.

Macronutrients & Micronutrients
Macronutrients are required by plants in greater amounts than micronutrients due to their functions. Macronutrients are used in building tissues and proteins within a plant, which make up the bulk of it. Micronutrients are commonly associated with chemical reactions, which are essential for growth and development, but are not large systems.

Macronutrients can be further divided into groups based upon the amounts used by plants. Primary macronutrients include:
  • Carbon
  • Hydrogen
  • Nitrogen
  • Oxygen
  • Phosphorus
  • Potassium
Secondary macronutrients consist of Calcium, Magnesium, and Sulfur. Micronutrients consist of Boron, Chloride, Copper, Iron, Manganese, Molybdenum, and Zinc.

Carbon, Hydrogen, and Oxygen are supplied to plants from the air and water and are considered non-mineral nutrients. The rest of these essential elements must be supplied by the soil and are taken up by the plant in mineral form. These elements are either present in the soil or are added by fertilization.

Nutrients are constantly being removed from the soil. They can be lost by plant uptake, leaching through the soil profile, and erosion. In some cases, many of these elements may already be present but are tied up by soil particles, making them unavailable to plants.

Balance Nutrients
A fertilization program needs to identify which nutrients are lacking and supply these in amounts, which meet, but do not exceed the amounts required by plants. A soil test is one of the most accurate and practical ways to determine the fertility state of your soil. It is important to reach a balance as best possible by adding enough fertilizer to meet the needs of the plant without over-applying. Over application may lead to environmental degradation as well as being a waste of time, money, and resources.


There are a wide variety of fertilizers. They differ in classification (i.e. organic or inorganic), content (what nutrients are present and in what amounts), quality, price, and formulation (i.e. liquid, or dry).

Fertilizer packages will have three numbers on them. These numbers tell you the relative amounts on Nitrogen, Phosphoric Acid (the fertilizer form of Phosphorus), and Potash (the fertilizer form of Potassium). For example, if a bag of fertilizer has the numbers 10-20-20 on it, the bag contains 10% Nitrogen, 20% Phosphoric Acid (P2O5), and 20% Potash (K2O).

The required content of your fertilizer and your application rates will vary according to what type of soil you have, what nutrients are already present in your soil, what crop you are growing, and what purpose the fertilizer is to serve (i.e. fertilize an established pasture or start a new seeding). Different soil types have different qualities; which nutrients are naturally present is one of those qualities. Some soil types may be naturally high in one element but low in another. A soil test can help to determine the fertility state of your soils.

Mineral Nutrients

Generally, the mineral nutrients which are used the most (i.e. macronutrients) will need to be added most frequently. Also, some nutrients are more mobile in the soil than others. This results in a loss by leaching and reiterates the importance of proper application rates, as these types of nutrients may affect groundwater if they are over-applied.

Other nutrients such as Cobalt, and Vanadium may also be applied to your soil. They are called beneficial nutrients because they are not essential in plant development although they may contribute to higher productivity or have other desirable results. Also, some Nitrogen-fixing bacteria that live in association with legumes may require a nutrient such as cobalt to function properly.

It should be stressed that a fertile soil is not necessarily a productive soil. Soil productivity depends on a number of factors, fertility being just one. Others include: soil moisture, pH, sunlight, soil physical properties, slope, etc. However, fertility is one of the easier and more practical factors for us to manage and should be considered an important part of land improvement.

If you have any questions about soil fertility, fertilizers or soil testing please call the District at 253-845-9770.
Essential Plant Mineral Nutrients
Nutrient Primary Forms Absorbed by Plant
Plant Functions
General Deficiency Symptoms
Primary Cations Anions

Nitrogen (N) NH4+ NO3- Protein synthesis and structure, chlorophyll synthesis and structure, vitamin component yellowing of leaves (chlorosis)-starting with older leaves
Phosphorus (P)
H2PO4- Photosynthesis, metabolic processes, plant growth, fruit and seed development, transfer of hereditary traits Slow growth, stunting, distorted leaf shape, purplish foliage color, chlorosis of older leaves
Potassium (K) K+
Photosynthesis, respiratory regulation, protein synthesis, control of ionic balance, disease resistance, fruit formation, winter hardiness, drought tolerance "Burnt" leaf margins, slow growth, small, shriveled seeds and fruits, susceptibility to disease secondary
Secondary Cations Anions

Calcium (Ca) Ca2+
Plant strength, cell formation, enzyme activator, root and leaf development Stunted root growth, death at growing points, foliage darker than normal, weak stems, premature blossom shedding
Magnesium (Mg) Mg2+
Photosynthesis-central to chlorophyll molecule, enzyme activator Chlorosis between veins of older leaves, leaf curling along margins, yellowing of margins
Sulfur (S)
SO42- Enzyme and vitamin formation, protein structure, seed production, chlorophyll formation Uniform yellowing starting in young leaves, thin and weak stems, retarded growth

Cations Anions

Boron (B)
H2BO3- Necessary for pollination, seed and cell wall formation, sugar transport, protein formation Stunted plants, young tissue affected 1st
Chloride (Cl)
Cl- Metabolic reactions, enzyme activator, cation transport, disease resistance Wilted leaves, bronze in color, dead spots (necrosis)
Copper (Cu) Cu2+
Chlorophyll formation, metabolic catalyst Young leaves affected, darker than usual, misshaped leaves with necrotic spots
Iron (Fe) Fe2+
Carries oxygen, chlorophyll formation, enzyme activator Leaves pale green with dark green veins, starts in young leaves
Manganese (Mn) Mn2+
Enzyme systems, activates various metabolic reactions, aids in chlorophyll synthesis Chlorosis between veins of younger leaves, may have necrotic spots
Molybdenum (Mo)
MoO42- Converts nitrate to ammonium in plant, N-fixation in legumes, convert inorganic Phosphorus to organic forms General yellowing and stunting
Zinc (Zn) Zn2+
Plant growth, metabolic reactions, chlorophyll and carbohydrate production Buds have a white color, young leaves may develop yellow bands
Cobalt (Co)* Co+
Required by nodulating bacteria in Nitrogen-fixing plants
*Not considered an essential element for higher plants, but is required by nitrogen fixing bacteria of legumes
Deficiency symptoms are intended as general guides, not absolutes