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 two groups: the macronutrients and the micronutrients. Nutrients are placed into one group or another depending on the relative amounts in which they are required by the plant.

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.

Macronutients can be further divided into groups based upon the amounts used by plants. Primary macronutrients include Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, and 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.

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. Overapplication 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, and 20% Potash.

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.

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 which 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 (425) 277-5581.

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