Soil Nutrient management
Essential Plant Nutrients
Plants absorb a wide range of nutrients from their environment, typically as simple compounds. For example, most plants obtain their carbon as gaseous carbon dioxide (CO2) from the atmosphere, their nitrogen as the ions (charged molecules) nitrate (NO3¯) or ammonium (NH4⁺), their phosphorus as phosphate (PO43¯), and their calcium and magnesium as simple ions (Ca2⁺ and Mg2⁺). If any of these nutrients are not available, plants cannot complete their life cycles and accomplish normal physiological functions. Growth and yield of crops are reduced by their deficiencies. Not only are these nutrients essential, but must also be available in sufficient amounts to support optimum growth and production. Mineral nutrients are divided into macronutrients, which are elements that plants require in large amounts—nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur, and micronutrients, which are needed only in small amounts—boron, copper, iron, manganese, molybdenum, and zinc.
Movement of Nutrients from Soil to Plants
Generally, plants absorb essential nutrients from the soil in soluble, inorganic forms. Nutrients in organic form must be converted to inorganic forms prior to plant uptake. Exceptions to this generality include some metal elements that can be absorbed as organic complexes. In order for ions to be absorbed by plant roots, they must come into contact with the root surface.
Root Interception
In general, white wines exhibit more acidity than red wines. Acidity gives wine its crispness on the palate. A dry wine needs acceptable levels of acid to provide liveliness and balance; sweet wine needs acidity so it does not seem cloying. Too much acidity will make the wine seem harsh or bitter; too little and the wine will seem flabby and dull. Lighter-style red wines may have high acidity, while heavier-bodied red wines tend to have low acidity. Acidity is not only important in protection from microbial spoilage but also in producing a clean fresh taste and favoring color stability, and is crucial to proper aging of wine.
Mass Flow
Growing plants are continually taking up water from the soil profile, a process driven by transpiration (loss of water from the plant via stomata on the leaves). Dissolved in the soil water are soluble nutrients. These nutrients are transported along with the water to the root surface.
Diffusion
As the concentration of nutrients around the root system drops, nutrients from higher concentrated areas move—or diffuse—toward low concentration areas and toward the roots. They only move a small distance, though. Potassium and phosphorus mostly move through diffusion. And, since phosphorous and potassium aren’t extremely mobile, it is important to have a high concentration of those nutrients throughout the soil, and to apply those nutrients as close to the root zone as possible.
The Concept of the Limiting Factor
The law of the minimum states that plant production can be no greater than that level allowed by the growth factor present in the lowest amount relative to the optimum amount for that factor. This growth factor, be it temperature, nitrogen, sulfur, or water supply, will limit the amount of growth that can occur and is therefore called the limiting factor.
Conditions Affecting Nutrient Availability
Nutrient availability can be impacted by soil chemical and physical properties, including parent material and naturally occurring minerals; amount of organic matter; depth to bedrock, sand, or gravel; and permeability, water holding capacity, and drainage. In addition, environmental conditions and crop characteristics have an important impact on nutrient availability. It is not unusual for plants in the field or portions of the field to show nutrient deficiencies during periods of the growing season, even where an adequate nutrient management plan is followed.
Antagonistic and Synergistic Effects of Soil Nutrients
Two or more nutrients may interact in plants. Nutrient interactions are generally measured in terms of growth response and change in concentration of nutrients. When an increase in crop yields by the addition of two nutrients is more than adding only one, the interaction is positive (synergistic). If adding the two nutrients together produces fewer yields than the individual ones, the interaction is negative (antagonistic). When there is no change, there is no interaction.
Nutrient Management for Organic Crops
Nutrient management in organic systems is more complex. Organic inputs cannot easily be added to the soil to provide the exact balance of nutrients needed by the plant for at least three reasons. First, many organic inputs (such as cover crops, crop residues, weeds, and compost) are added to the soil for reasons other than fertility management, yet they contribute to the pool of nutrients in the soil.
Crops Preferences for Soil pH
Crops differ in their optimal soil pH requirement. Some crops may thrive best in very acid or alkaline soils, but most agriculturally important crops do well on soils of neutral pH (6.5–7.5). Crops such as corn, sorghum, wheat, and cool-season grasses tolerate a wider pH range, including moderate acidity as well as neutral pH conditions. Soil pH preference of some crops is given in Table 10.3.
Climate Change and What it Means to for Plant Nutrition
Evidence suggests that climatic conditions are changing and will continue to do so, resulting in higher average temperatures and rates of evaporation, rainfall with greater seasonal variability, and more frequent extreme weather events. With respect to plant nutrition, changes in temperature and evaporation are likely to affect the availability of soil water to the plants as well as the internal mechanisms of water movement through the plant. Heavier and more frequent rainfall events could increase the incidence of water logging, leaching, and erosion causing unfavorable conditions for nutrient uptake as well as a loss of nutrients from the soil.
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