Soils Reader

Mineral Nutrition for Plants

Soils as a Media for Plant Growth

© 2007 Donald G. McGahan (aka soilman) All Rights Reserved

Role of Soil in Plant Growth

1. Support - a place for roots to anchor the plant
2. Water source
3. Aeration - provides O₂ for root respiration
4. Nutrient source

Essential Nutrients (essential element) - those elements necessary for the plant to complete its life cycle: survive, grow, and reproduce. Determined by growing plants in hydroponic solutions and regulating which nutrients are given to plants.

Plants in general need 17 essential elements (out of a total of 103 known elements); 14 are derived from the soil. Some other plant species also need sodium (Na) and silicon (Si). In addition to these elements, animals require selenium (Se) and iodine (I) in small amounts.

Processes by which plants acquire and redistribute nutrients

1. Uptake - How do plants take up nutrients?
   1. Foliar uptake - leaves and other green tissues can absorb nutrient ions directly through their stomata.
   2. Root uptake
      1. The primary uptake mechanism for plant nutrients other than carbon, hydrogen and oxygen.
      2. Root uptake can be facilitated by mycorrhizae fungi.
2. Translocation
   1. After uptake, nutrients are free to move throughout the plant through the xylem tissue and can go to the point of most demand.
3. Remobilization
   1. As tissues age, some elements are removed from the older tissue and transported to new tissue.
   2. Deciduous trees remove many nutrients from their leaves before senescence.
   3. Only some nutrients are capable of being remobilized.

Plant nutrient deficiency is when the concentration of an essential nutrient is low enough to severely limit yield. Some deficiency symptoms are visual. When distinct visual deficiency are symptomatic the need is typically severe. Many elements have nutrient deficiency symptoms that look similar. Additional information is often necessary.

Symptoms of nutrient deficiency will appear first in older leaves if the deficient nutrient is mobile and in the younger leaves if the deficient nutrient is non-mobile.

Sufficient is the nutrient concentration range in which added nutrients will not increase yield. Luxury consumption is nutrient absorption by the plant that does not influence yield.

If available plant nutrients are extremely low in concentration then visual deficiency symptoms will be obvious. The point between obvious visual deficiency symptoms and the sufficiency range is termed hidden hunger. Additional fertilizer in this zone will result in an increase in the yield.

Plants will often take up more of a particular nutrient than required for maximum yield; this is termed luxury consumption. If nutrient concentrations become too high, they may cause a toxicity to the plant.

Visual Deficiency → Hidden Hunger → Sufficiency Range (Luxury Consumption) → Toxicity

Sources of nutrients in the soil ecosystem:

1. Weathering of rocks and minerals
2. Organic matter (via mineralization)
3. Sorbed cations/anions (especially important for phosphorus)
4. Exchangeable cations and anions
5. Soil Solution
6. Atmospheric input (wet and dry deposition)
7. Nitrogen fixation by microbes and lightning
8. Vegetation (leaching from foliage and remobilization)

Nutrient availability to plants from the five major soil pools follows:

Soil solution > exchangeable ions > sorbed >> organic matter >> rocks/minerals [soil-solution-exchangeable-ions-sorbed-organic-matter-rocksminerals]

These pools can be supplemented by addition of fertilizers. Why are fertilizers so effective?

How do essential nutrients get to the plant root?

1. Mass flow - nutrients flow with the water as it moves toward the root
2. Diffusion - depends on the concentration gradient (driving force) and the ion’s diffusion coefficient (ease of movement)
3. Root extension - roots grow into a new soil volume tapping it’s nutrients

Composition of the geosphere, soil, plants and humans

Essential elements for plants and the common plant available soil solution form(s).

Micronutrients (Trace elements) for plants and the common plant available soil solution form(s).

Nutrient Deficiencies

Nutrient deficiencies are major constraints to commercial plant production, along with drought, disease, and pests. Nutrient deficiencies can be corrected by additions of fertilizer or compost.

What quantities of nutrients are required by crops?

Economic considerations

• Law of diminishing returns
• Fertilizer level for maximum yield is greater than fertilizer level for maximum profit
• Developing countries: more total yield by adding a little fertilizer to a larger acreage than adding a lot to a smaller acreage.

Environmental considerations

• The more fertilizer you apply, the greater the chance for nutrient leaching and negative environmental consequences. Addition of fertilizer beyond the point of sufficiency will cause a potential pollution problem. Examples: nitrate NO₃¯ in the groundwater and P causing eutrophication in lakes.
• Slow release fertilizers (e.g., Osmocote) and nitrification inhibitors (N-serve) help keep nutrients in a non-mobile state.

Because plant yields are regulated by the most limiting nutrient, fertilization with one nutrient may induce the deficiency of another nutrient. If a plant’s growth is limited by nitrogen, fertilization with nitrogen will increase growth until the plant becomes limited by another nutrient. Thus, nitrogen fertilization may lead to a phosphorus deficiency.

Deficiency Symptoms

1. Chlorosis - a yellowing of plant material caused by the lack of chlorophyll.
   • General chlorosis most often due to lack of nitrogen or sulfur. In older leaves, it may be due to the loss of mobile elements such as phosphorus or magnesium.
   • Interveinal chlorosis - yellowing between veins, with the veins forming a green pattern. Deficiency symptom for iron, manganese, or zinc.
2. Purpling - accumulation of anthocyanin pigments. A common sign of phosphorus deficiency.
3. Local necrosis (death of tissue) - patches or margins on leaves that have died. A common sign of potassium deficiency or excess salts.
4. Stunting - overall stunting (reduced growth) is a sign of all nutrient deficiencies.
   • “Little leaf” – zinc deficiency in which youngest leaves do not grow as large as normal, mature leaves
   • “Pygmy forest” – lack of nutrients leads to very stunted tree growth (2 to 3 m at 80 years old)

Predicting Nutrient Needs and Deficiencies

• Plant tissue analysis
• Soil analysis

Buying fertilizer

• most commercial fertilizers contain nitrogen, phosphorus and potassium (N-P-K)
  • Example: 20–20–10; reported in terms of N%, P₂O₅% = 43.7% P; K₂O% = 83% K
    • 100 pounds of this fertilizer would contain 20 lbs. N, 8.7 lbs P and 8.3 lbs K