Nutrient management is the careful monitoring and amending of soil fertility to meet crops needs with emphasis on improving agricultural profitability and minimizing impacts on water quality. While other BMPs’ effectiveness and implementation depend upon specific site characteristics (drainage, soil, slope, etc.), N management is universal and has potential regardless of landscape is nitrogen management.

 

 

Presently, nutrient management as it relates to the Neuse Rules focuses on nitrogen.  The cornerstones of nutrient management are:

·          Rate-  total N amount

·          Source-  form of N

·          Timing-  applications of nutrients related to crop needs

·          Placement-  distribution of fertilizer on or in the soil

 

 

Although each component of nutrient management is important, the rate of N may be of greatest importance since it most directly impacts residual N.  The N needs of a crop and ultimately the N rate are related to expected yield.  Field selection and as a consequence, the soil type, impact yield the most, primarily due to physical characteristics of the soil that ultimately determine moisture availability to crops.    

 

Nutrient management combines crops’ needs for N together with realistic yield expectations (RYEs) based on predominant soil type to estimate N rates.  Growers can establish RYEs through yield records by averaging the three best yields of a five-year period.  Alternatively, RYE estimates by crop for various soil types are compiled with experience of soil scientists and agronomists http://www.soil.ncsu.edu/nmp/RYE_Alpha.PDF .  Soil type by field is acquired by using Natural Resource and Conservation Service county soil surveys.  Since slope and erosion potentially influence productivity, RYEs are adjusted by a factor depending on the site.  

 

N Rate Determination.  The crop needs for N are based on the efficiency of a specific crop in converting N applied into yield.  Soil characteristics affect this efficiency and therefore, ranges of  N factors (NF) are established for individual crops (link to first ta ble below).  Since there are hundreds of soils with varying characteristics and therefore management practices, soils are categorized into soil management groups (SMG) http://www.soil.ncsu.edu/nmp/SMG_Final_2000.pdf.  Although NF are assigned to SMG, efficiency also depends upon grower management and therefore flexibility in final selection.  As an example using corn, a NF of 1.0 to 1.25 lbs. N per bushel of grain produced is used.  A higher NF may be used on sandy soils where more potential for leaching occurs.   

 

The total rate of N fertilizer is calculated by multiplying the RYE by the NF.  As an example using corn grown on a Norfolk soil with a 0-2% slope (NoA), the RYE is 115 bushels per acre.  Since this soil is reasonably productive with only moderate leaching potential, a NF of 1.14 lbs. N per bushel of grain is chosen and the N fertilization rate as follows: 

 

115 bushels per acre (RYE) x 1.14 lbs N per bushel (NF) x 1.0 (Slope /Erosion Factor)  = 131 lbs N per acre.

 

A simple N worksheet (link to to table below-  2nd table)  is a convenient way to group information for N management planning.  It is important to realize that recommended N rates on soil test reports are not RYE rates or are they derived quantitatively from actual soil testing provided by the Agronomic Division of the NC Department of Agriculture and Consumer Services (NCDA&CS).   The rates recommended on soil test reports are general rates based on NC data compiled over many years.  Since the amount of residual soil N from a previous year is unpredictable in humid climates such as in NC, use of soil tests for N fertilizer recommendations is not reliable.    

 

Table 1.  Nitrogen factors for various crops.

CROP
NITROGEN FACTOR

Corn (grain)

1.0 – 1.25 lbs. N per bushel

Cotton (lint)

0.06 – 0.12 lbs. N per lbs. lint

Wheat (grain)

1.7 – 2.4 lbs. N per bushel

Barley (grain)

1.4 – 1.6 lbs. N per bushel

Rye (grain)

1.7 – 2.4 lbs. N per bushel

Oats (grain)

1.0 – 1.3 lbs. N per bushel

Small grain (hay)

50 to 60 lbs. N per dry ton

 

 

 

Table 2.  Example of a nitrogen worksheet used in nutrient management planning.

FSA

Tract

 

Field

 

Acres

 

Soila

 

Slope

 

SMG

 

Crop / Rotation

 

RYEb

 

N Factor

Total N

Ratec

Residual

N

Adjust N

Rate

101

1

8.3

Norfolk

A

25

corn

115

1.14

131

0

NA

101

1

8.3

Norfolk

A

25

wheat / soybean

60

2.09

125

10

NA

101

1

8.3

Norfolk

A

25

tobacco

3300

-

50-80

0

40-70

101

1

8.3

Norfolk

A

25

soybean

40

-

0

0

NA

a Predomiant soil type in field.

b  Adjustments for slope are made as follows:  Multiplication factor used with RYE:  3-4% slope-0.98, 5-6% slope-0.96, 7-8% slope-0.94, 9-10% slope-0.92, 11-15% slope-0.89, slope-16-90%, 0.87.  If soils are mapped as eroded, factors are as follows:  slope 0-2%-0.92, 3-4% slope-0.89, 5-6% slope-0.87, 7-8% slope-0.85, 9-10% slope-0.80, 11-15% slope-0.77, slope-16-90%, 0.75.

cRates for tobacco are based on soil texture and topsoil depth or depth to clay:  5” depth = 50 lbs. N per acre, 10” depth = 60 lbs. N per acre, 15” depth = 70 lbs. per acre, 20”+ = 80 lbs. per acre.  Darker soils will require less N than light colored, sandy soils.