probably because plants uptake nutrient IONS, not salts,LOL
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Fertility and Fertilization
Many people confuse plant nutrition with plant fertilization.
Plant nutrition refers to the need for basic chemical elements for plant growth.
The term
fertilization refers to the application of plant nutrients to supplement the nutrients naturally occurring in the soil. Nutrients may be applied as commercial manufactured fertilizers, organic fertilizers and/or other soil amendments. Organic fertilizers and soil amendments are typically low in plant-available nutrient content. For additional information about fertilizers and soil amendments, refer to the
CMG GardenNotes #232,
Understanding Fertilizers and #241,
Soil Amendments.
Adequate soil fertility is only one of the many soil-related growth factors. Fertilizers will increase desirable plant growth only if the plant is deficient in the nutrient applied and other growth factors are not also significantly limiting plant growth. Fertilization will not compensate for poor soil preparation, the lack of water, weed competition and other non-nutrient growth limiting factors! Fertilization will not enhance desired growth if the nutrients applied are not deficient.
From the nutritional perspective, a plant cannot tell if applied nutrients come from a manufactured fertilizer or a natural source. Plants use nutrients in ionic forms. Soil microorganisms must break down organic soil amendments, organic fertilizers and many manufactured fertilizers before the nutrients become usable by plants.
From a nutritional perspective, the primary difference between manufactured and organic soil amendments/organic fertilizers is the speed at which nutrients become available for plant use. For manufactured fertilizer, their release is typically, but not always, a few days to weeks. Some are specially formulated as “controlled release”, “slow release” or “time release” products that release over a period of months. With natural-organic fertilizer, nutrients typically become available over a period of months or years. However, there are exceptions to this general rule. The high salt content of some manufactured fertilizers and some organic soil amendments could slow the activity of beneficial soil microorganisms.
Benefits of organic fertilizers and soil amendments include improvements in soil tilth (suitability of the soil to support plant growth). This should not be confused with “fertilization”, a distinctly different soil management objective. Organic soil amendments are typically low in nutrient content.
Remember that fertility is only part of the soil’s role in supporting plant growth. The organic content of the soil also directly affects plant growth due to its influence on soil tilth and the activity of beneficial soil microorganisms. Relying solely on manufactured fertilizers is not recommended as this does not support good soil tilth.
Plant Nutrients
Plants need 17 elements for normal growth. Carbon, hydrogen, and oxygen come from the air and water. Soil is the principle source of other nutrients.
Primary nutrients (nitrogen, phosphorus, and potassium) are used in relatively large amounts by plants, and often are supplemented as fertilizers.
Secondary nutrients (calcium, magnesium, and sulfur) are also used in large amounts but are typically readily available and in adequate supply.
Micronutrients or trace elements are needed only in small amounts. These include iron, zinc, molybdenum, manganese, boron, copper, cobalt, and chlorine. [Table 1]
Roots take up nutrients primarily as
ions dissolved in the soil’s water. The ions may be positively charged (
cations) or negatively charged (
anions). The nutrient ion soup in the soil’s water is in a constant state of flux as the variety of ions dissolve in and precipitate out of solution.
Clay particles and organic matter in the soil are negatively charged, attracting the positively charged cations (like ammonium, NH4+, and potassium, K+) and making the cations resistant to leaching. Negatively charged anions (like nitrate, N03-) are prone to leaching and can become a water pollution problem. Both ammonium and nitrate are important plant nitrogen sources and are commonly found in salt forms in fertilizers.
The
Cation Exchange Capacity, CEC,is a measurement of the soil’s capacity to hold cation nutrients. More precisely, it is a measurement of the capacity of the negatively charged clay and organic matter to attract and hold positively charged cations. CEC is useful in comparing the potential for different soils to hold and supply nutrients for plant growth.
http://www.ext.colostate.edu/mg/gardennotes/231.html