Plant Nutrients and Fertilizers

This lecture deals with an addition of soil nutrients to enhance crop productivity. Given the problems of maintaining fertility with increased tillage and the resultant depletion of organic matter (O.M.), the addition of plant nutrients to the system becomes a part of crop management.

 

	To become aware of the impact of fertilizer addition to crop productivity.
	To know the basic, primary and secondary micronutrients essential for crop and plant growth and development.
	To introduce the primary materials used in cropping systems to deliver nutrients to the field.
	To become aware of different fertilizer combinations and methods of application of these materials
	To become aware of different fertilizer combinations and methods of application of these materials

    

Fertilizer use has led to the ascendance of U.S. agriculture in the world. After World War II, the munition of plants producing Nitrogen were converted to plants that could manufacture fertilizer Nitrogen. Cheap, available Nitrogen was responsible for the substantial increase of corn production.  Fertilizers accounted for about half of the increase in crop yields in the U.S. between 1941 and 1955.

The primary fertilizer elements are called "primary" because they are used in the greatest amount by the plant. These elements are Nitrogen (N), Phosphorus (P), and Potassium (K). A typical corn plant producing 150 bu/a would need 150 lb.. of N, 55 lb.of P, and 65 lb. of K. Nitrogen is especially important in the photosynthetic reaction and the formation of amino acids in DNA/RNA and the cell divisions and growth. Phosphorus is essential in respiration, providing the "P" in the ATP cycle, and in photosynthesis by providing the P energy reaction" for the NADPH₂ cycle of photosynthesis. Potassium is essential to the smooth running of plant enzymes into the plant.

These are several common carriers of these elements, and their use is ultimately determined by their cost and the concentration of the essential elements. Application of these materials is often a compromise between economics and plant intake efficiency. Management of fertilizers is a major part of a modern agricultural system, and is an important consideration in a sustainable agricultural system. As fertilizer use becomes more and more expensive, and application methods become critical for both economically sound and environmentally responsible agriculture.
 
In modern agricultural systems, increased emphasis is being made to properly obtain soil samples to estimate fertilizer weeds.  Detailed soil mapping, and state of the art application equipment is being used to prescription feed crop fields.  These systems address crop yield, economic, and environmental concerns.  Uniformity, in many developing countries, the fertilizer technology is very expensive, primitive and, in some places, is nonexistent.  Here lies one of the real differences in agriculture between the developed and developing worlds.

  

• 12 are frequently deficient in agriculture soils
•  N-P-K known as fertilizer elements
• Ca and Mg constituents of lime
•  S another major element

1. Three figures written - 10-20-10 refer to percentage of N, P₂O₅ and K₂O.
2. Acidity and basically of fertilizers

•  Fertilizers substantially increase soil acidity
•  The increase exchangeable and toxic Al
•  All fertilizers containing ammonium compounds have acidic residual effects on soils
•  Acidic fertilizers are generally considered desirable in irrigated alkaline calcareous soils

•  Cost/unit of element can be estimated by dividing percent of element into cost/ton

1.  Nitrogen

• Nitrogen is the most extensively deficient and most widely applied plant nutrient
• Its availability in soils influenced by:
•  Microbial activity
•  Moisture
•  Temperature
• Nitrate is subject to leaching
• Nitrogen can be denitrified by soil microorganisms
• Can be lost to atmosphere
• Nitrogen fertilizers
•  Formed primarily by catalytic process combining pure N and hydrogen gas to form ammonia
• With high energy cost - nitrogen fertilizers have risen rapidly
•  Nitrogen carries:

Anhydrous Ammonia - 82% N - most concentrated gas under pressure - converted to liquid form.  Least expensive N fertilizer not widely used in developing countries, handling high pressure tanks a problem.

Urea - 46% N - relatively low cost, easily vocalized - should be incorporated, over 1/4 of world's N supply and increasing.

Ammonium Nitrate - 34% N - half in ammonium form, half in nitrate form - higher price, 1/4 of world's N supply, decreasing.

Ammonium Phosphates - example 18-46-0 usually thought of as phosphate fertilizer.

Ammonium Sulfate - 21% N - declining in usage - from 30% in 1955 to less than 10% now - strong acidifying effect - low N content/marketing cost high.

Nitric Phosphates - currently used in Europe.   CONCEPT The more concentrated the Nitrogen in the fertilizer carrier, the cheaper the cost of transportation.

•  High yields of crops most often effected by deficiencies in P (one of most common limitations).
•  Phosphorus fertilizers value based on its relative solubility.
•  Phosphate carriers:

Ordinary Superphosphate - (16-22% P₂O₅) -treating ground rock phosphate with sulfuric acid.  Declining in use because of low content/shipping cost.

Phosphoric Acid - (55% P₂O₅, 24% P) can be evaporated to a 72% P₂O₅) product. Rock phosphate with sulfuric acid - used in liquid fertilizers - suited for application in irrigation water.

Concentrated Superphosphate - (44-52% P₂O₅, 19-23% P) one of the most extensively used P fertilizers - rock phosphate with phosphoric acid - is 95-98% water soluble.

Ammonium Phosphates - produced by reactions between ammonia and phosphoric acid high content of plant nutrients/shipping and handling costs relatively low.

•  Phosphorus is usually absorbed by plants as H₂PO₄ and HPO₄₂ - ions
•  In acid mineral soils, aluminum and iron are absorbed on clay and react quickly with soluble phosphate to produce compounds that decrease in solubility
•  Ultisols and oxisols generally have high P-fixing capabilities
•  Soils between pH 6.0-8.5 are usually most favorable for P nitration of plants

•  Is absorbed by plants in higher quantities than most mineral elements
•  K is absorbed as the K+ ion
•  K deficiencies often occur because of the large and rapid uptake as farmers create conditions for high yields
•  Potassium carriers:

Potassium Chloride - (50-60% K).

Potassium Sulfate - (45-50% K).   Source of S where element is deficient
 
• In strongly acid soils having Al as dominant exchangeable element, potassium not firmly held and it leaches
• Liming leads to replacement of Al by Ca and K is better retained

• Ca and Mg essential plant nutrients
• Both originate in soil from weathering of rocks and minerals
• Mg deficiencies are fairly extensive
• Acid soils
•  A high content of exchangeable Al can cause toxic levels of Al in soil solution
•  Acid soils do not retain K, Ca, and Mg well and they leach out
•  Phosphate ions react with Al and Fe and their effectiveness is reduces
 
• Lime treatments
•  Treatment of acid soils with lime is multiple purposes:
• reduce acidity
•  reduce exchangeable Al, Fe, and Mn
• furnish Ca and Mg
• improve physical properties of soil
• improve P availability
• improve capability of soil to retain and release K, Zn, and Mo
 
•  Materials for soil liming
• finely ground limestone
• ground dolomitic limestone
• burnt lime
• calcium oxide
• calcium hydroxide
 
•  Neutralizing capacities of lime materials expressed in calcium carbonate equivalents
• pure calcite value = 100
• calcium hydroxide = 136
 
•  Lime materials evaluated on basis of
• neutralizing value
• fineness of grinding
• content of Mg
 
• For oxidized and leached soils such as oxisols and ultisols - pH of 5.0-6.0 usually sought
• In many tropical humid regions with highly weathered and leached soils/local lime sources not available - shipping cost prohibitive
• In general - pH above 5.0 not necessary in tropics
    CONCEPT If you can not afford to do anything else, LIME!  Adjustment of pH improves natural systems ability to supply nutrients.

• Required by plants in small quantities ranging from a few hundred grams to a few kilogram/A
• Small quantities can be fixed in fertilizer or applied as foliar sprays

1.  Soil tests

•  Soil analysis has become most extensively used method for diagnosing fertilizer needs.
•  Must secure a representative sample

• Fresh tissue test in field
• Tissue analysis in the laboratory (more accurate)

1.  Common practice to mix materials so 2-3 or more elements in fertilizer.

2.  Bulk blending common in major farming areas.

•  Bulk handling - save $8 - 12/ton

4.  In U. S. and Europe, N applied as a separate material (at least portion).

• This is because of high level of N use
• Application at special times

1.  Usually a compromise between ideal and practical.

2.  In course soil - split applications reduce leaching.
 
3.  In loams and clays - apply before planting.

4.  Best to have a good nitrogen fixing legume crop followed by crop with high N requirements.

5. N usually applied with each crop.

6. P and K frequently made on selected crops in the cropping sequence.

Method of Application

1. Most efficient utilization of fertilizer N is when applied during periods of high plant demand - e.g. tasseling.
 
2.  Best to make nutrients position available to plant roots in early growth stages.
 
3. Banding and size dressing advantageous where soils low in fertility and only limited quantities of fertilizer are used.
 
4. Broadcast applications used - where soils are maintained in high states of fertility.
 
5. Anhydrous ammonia and nitrogen solutions usually injected.

6. Broadcast applications on perennial sods or legume crops.

7. Broadcast where no-till is practiced.

• Plowing every 4-5 years helpful to distribute fertilizer

 

	Be able to list primary nutrients and descrive why they are primary.
	How do primary elements differ from secondary and micro elements?  Which are more important?
	How would you chose a fertilizer for your crop and how would you determine the proper application?
	Describe what systems the primary elements impact most within crop plants.
	Be able to contrast fertilizer usage in the developed and developing worlds.  What problems do you forsee if this trend continues?