Soils 205- General Soils

Lecture 30- Nutrient Management

I.  Goals of nutrient management

        1)  Cost-effective production of high quality plants.  Management will depend on what is being produced:

            Traditional agriculture:  historically interested in short term increases in yield

            Forestry:  lumber, recreational areas, or wildlife habitats; production is over a longer time period, so management is not as intensive.

            Horticulture:  quality of plant

        2)  Efficient use and conservation of nutrient resources.  Two keys to achieving this goal are:

                a.  renewal or reuse of resources

                b.  balancing inputs and outputs.  Nutrient imbalances can be seen on the watershed, regional, or national scale.

         3)  maintenance or enhancement of soil quality.  Go beyond supplying nutrients needed for this year's crop by increasing the nutrient supplying and cycling capacity of the soil and improving soil physical properties.

        4)  Protection of the environment beyond the soil.  Achieved through the development and adoption of best management practices (BMPs).

            Examples:

                a.  buffer strips- establishment of dense vegetation along water bodies (riparian zones).  The vegetation slows down water and sediment drops out.  Extra nutrients are taken up by plants.

                b.  cover crops- crops that are grown and then turned into the soil.  Protects water quality by:

                    -increasing infiltration rates (decreasing runoff)

                    -when runoff does occur, the biomass decreases the velocity of the water and sediment is deposited before reaching a       

                    waterway.

                    cover crops also improve soil quality (goal #3) through the addition of n and OM

            c.  selective timber harvest- When whole blocks of trees are removed at once, the disturbance and increase in soil temperature  

                speeds up organic matter decomposition and nutrient release.  If there are not actively growing roots to take up the nutrients, 

                they will be lost from the soil.  Streams draining clear cut areas are often high in N and base cations.  Selective removal leaves

                active roots to take up nutrients and the litter layer is left in tact to protect the soil from raindrop impaction.

II.  Nutrient resources-  Nutrient sources applied to the soil can be either organic or inorganic.

        a.  Inorganic resources

            i.  fertilizer grade

            ii.  Inorganic N fertilizers.  These fertilizers are manufactured through the Haber Processes.

 This process takes place under high temperatures and pressure in the presence of a catalyst.  The ammonia that is produced may be used to manufacture other forms of N fertilizers or may be used directly.

           Examples of N fertilizers:

            Ammonia NH₃(g)  Ammonia must be injected below the surface.  It should be applied to moist soils and should not be used in soils with pH > 8.5.  It is relatively expensive, but has a high analysis (82% N).

            Other N fertilizers:

         iii.  N-fertilizing

        -application should be matched to the needs of the plant.  Timing is critical.  How do we achieve this?

                 1)  use of slow release fertilizers   

                    -manures, sewer sludges, and composts (2-4% N)

                    -sulfur-coated urea- the sulfur coating on these pellets protects the N from microorganisms.  Eventually, there is an opening

                    and the N leaks out.

                 2)  split applications of N- apply 1/2 of N in fall during planting and 1/2 in spring.

                 3)  nitrification inhibitors     

           iv. P fertilizers- The problem with P fertilizers is that they are relatively insoluble and the P that is released is quickly "fixed" by soil colloids.  To avoid these problems, rock posphate is reacted sometimes reacted with sulfuric acid. 

            P and N fertilizers     

         v.  Ca and Mg fertilizers- usually added as limestone or dolomite

         vi.  sulfur- typical amendments are gypsum (CaSO₄) or elemental

        vii.  potassium fertilizers- orthoclase, biotite and muscovite provide K in many soils from weathering processes.  In intensively weathered or coarse-textured acid soils, fertilizer K is often applied as KCl and K₂SO₄.

    b.  Fertilizer application methods

        Questions to ask:

            1.  Is the land bare or vegetated?

            2.  What crop is being grown?

            3.  What stage of growth has it achieved?

            4.  What is available and affordable?

            i.  application methods- solids

                1.  Broadcast- fertilizer is spread evenly over a field.  Relatively easy application, should be tilled in soon afterwards.

                2.  banding- deposits the fertilizer in strips right next to the roots.

                 3.  perforation- used for tree crops  

        ii.  application methods- liquids

                1.  sprayed or injected into soils

                2.  added to irrigation water- especially in drip irrigation

                3.  spray it on the leaves- this is good for micronutrients

       iii.  application methods- gases

                -must be injected into the soil or dissolved in irrigation water

   c.  Micronutrient fertilizers- added as liquids or chelates

    d.  organic fertilizers:  May provide N,P,S and micronutrients.  They are slow release fertilizers and have relatively low analysis which increases their cost, but OM has benefits beyond fertilization.

II.  Soil testing:  Soil sampling is a critical step in soil testing and making fertilizer recommendations.  A very small soil sample (1-5grams) is often used to represent the millions of kilograms of soil in a field.  The soil sample analyzed must be truly representative of the conditions in the field.

Two sampling/fertilization schemes are used:

    1.  single rate fertilization

    2.  variable rate fertilization

    a.  soil testing and spatial variability