Chapter 10: Soil Fertility Management

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Chapter 10 Soil Fertility Management
Homework 2, 9 Also add this question Why is
nitrogen fertilization more problematic than for
most fertilizers? Include in your answer a short
discussion of why ionic forms of N in the soil
(NH4 and NO3-) remain elevated for only a short
time following N fertilization Due 19 Nov
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• Goals for fertilization
• Increase crop yield
• Reduce costs per unit yield
• Increase plant quality (not always a factor -
  fast growing wood is lower quality than
  slow-growing wood)
• Reduce disease (can go the other way by making
  tissues protein-rich and juicy for bugs)
• Prevent environmental pollution (new since the
  60's - never used to be considered)

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Nutrient removal by plants Typical crop
uptake values in Table 10-1 (archaic units -
lb/acre - multiply by 1.12 to get kg/ha)
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Typical values for commercial forests that would
be fertilized

• Note that
• ?Much of the N needed for new tissue growth is
  obtained by translocation from old tissues
• ?Nearly all the rest is taken up from the soil,
  but recycled
• The amount that actually stays in the tree each
  year (wood increment) is very small)

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• Soil vs plant analysis
• Leibigs Law of the Minimum growth is limited by
  the essential nutrient present in the lowest
  relative amount.
• Thus, the plant is the ultimate judge. However,
• In annual crops, plant analysis may be too late
  (already grown)
• In forests and range, plant analysis is not too
  late (growth goes on for years)
• Plant analysis is generally more sensitive than
  soil analysis

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• Soil sampling
• No amount of care in prep and analysis can
  overcome poor or inappropriate soil sampling
• Soils vary continuously with depth you cannot
  sample all the variability.
• Know your horizons and sample accordingly when
  possible
• We often dig a quantitative pit and get horizon
  depths and then sample with augers thereafter
• Often sampling plow layer in ag soils this will
  NOT work in wildland soils.

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• Soil sampling
• When repeating sampling
• Be sure to keep depth content constant
• Try to sample at same moisture content
  (especially when smectite clays are present)
• Best to measure bulk density, also, and allow for
  soil expansion
• Do not ignore rock fragments!! Ag people often
  can you cannot.

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Horizontal variation grid vs random sampling
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Horizontal variation sample by landscape strata
that make sense (land use, soil series, slope,
aspect, current vegetation, etc.) (Fig 10-3)
Figure 10-3
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Sampling in the horizontal plane Islands of
fertility present special problems

• Stratify by island/interspace?
• Where are boundaries?
• Can they be replicated?
• Where to sample within the island?

Boundary?
Sample point?
Is it better to sample by grid point or randomly
and just note where the sample falls?
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Sampling in the horizontal plane Islands of
fertility present special problems
Is it better to sample by grid point or randomly
and just note where the sample falls?

• Advantages
• Scaling up to an aerial basis is valid
• No bias by sampler
• Information about islands is still preserved

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Vertical variation need to know horizon depths
and keep sampling constant by depth
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It is important to sample proportionally with
depth
Unbiased sample with trowel Remove first scoop,
take second layer
Biased sample (trowel)
Proportional Sample (core)
Depth
Carbon Concentration
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Very Coarse Fragments Heaving Lifting
Which soil has more C?
Soil A Soil B Db 1.2 g cm-3 Db 1.0
cm-3 2 - 50 mm 8 gt2-50mm 2 gt50mm
45 gt50mm 2 C 4.0 C 2.4 C
112,800 kg ha-1 C 115,200 kg ha-1
50 cm
Occular estimates prevail for very coarse
fragments
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Soil Tests pH, acidity Electrode in 11 or
12 soilwater ratio with 0.01M CaCl2. Some
people use distilled water this generally give
higher pH why? Al, H displacement. Also
review lime requirement. Soluble salts
saturated paste extract 11 or 12 Nitrogen not
a good test. Total N (already described), CN
ratio, extractable ammonium and nitrate, N
mineralization, resins.None cheap or very good.
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• Soil Tests
• Phosphorus
• The book says this
• Bray 1 0.025 M HCl 0.03 M NH4F (for acidic
  soils)
• Mehlich 1 0.05 M HCl 0.025 M H2SO4 (for acidic
  soils)
• Olsens bicarbonate 0.5 M NaHCO3 at pH 8.5 (for
  neutral and alkaline soils)
• Assumes all goes to H2CO3 in acidic soils
• We do not find this bicarbonate is our
  preferred method in Sierran acid soils
• Mehlich 3 0.2 M acetic acid 0.25 M ammonium
  nitrate 0.015 M NH4F 0.013 M HNO3 0.001 M
  EDTA

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Soil Tests Potassium, Calcium and Magnesium
Exchange with ammonium chloride or acetate (CEC).
No one I know uses the bicarbonate DPTA extract
mentioned. Total digests are usually not useful
except for research purposes. Sulfur SO42- is
the preferred way, by water, phosphate, LiCl.
Total S not usually useful except for research,
but with new CHNS analyzers, it is now easy to
get.
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Soil Tests Boron Hot water extract. Some
people use cold water and works just as well.
Zn, Fe, Mn, Cu Many trials on this using 0.1 M
HCl, Coca-Cola (carbonic acid sugar), chelates
like DPTA Mo, Ni Totals, resins, chelates Soil
tests are changing resins are coming into play
now and must be checked against older methods.
Also, total analysis may become easier now, as
for example CHNS analysis.
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• Plant analysis
• Not favored by ag people because it is "too late"
• Greatly favored by forest people because it is
  more sensitive - plant is final arbiter, after
  all
• Total Plant analysis
• Routinely used in ag only
• Calculated in forestry and shrub systems (cant
  grind up a whole tree)
• Specific tissue analysis at specific times is
  sometimes used in assessing nutrition (Table
  10-6)
• Oven-dry (65C) analysis (otherwise water content
  varies)
• Total foliar analysis - threshold or critical
  values (Table 10-7)

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• Plant analysis
• Vector analysis to assess growth response (weight
  concentration) - we will do this in 497/697
• DRIS (diagnosis recommendation integrated system)
• Critical nutrient range (Fig 10-6)
• Visual deficiency symptoms Table 10-8
• Mobile nutrients like N, S, P, Mg, K symptoms
  appear on older tissues because of translocation
• Immobile nutrients like Cu, Mn, Ca, Fe symptoms
  appear in newer tissues.

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Figure 10-6
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Table 10-10
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• Fertilizer recommendations
• Based on plant or soil analysis, growth being too
  low, or calculations.
• Still a very imperfect art, especially for
  forests.
• We will spend more time on this in 497.

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• Fertilizer calculations
• Note that the three numbers on the bag are N, P
  as P2O5, and K as K2O or maybe KCl sometimes
• See the calculations on p. 335-336 for future
  reference
• I will not test you on this, but be aware of it
  to avoid making mistakes if you are involved in
  fertilization
• Note that fertilizers do not contain P2O5 or K2O
• This is an artifact of very old methods of
  analysis where these nutrients were measured by
  combustion and ended up as oxides which needed
  to be weighed.
• The example on page 335-336 calculates P2O5
  applications why? Plants will only use the P in
  the P2O5

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Fertilizer P and K contents are calculated as
P2O5 and K20! P2O5 is only 44 P (P2O5 weighs 2 x
31 5 x 16 144 P 62/142) and K2O is only
83 K (K2O weights39.12 16 94.2 and contains
only 78.2/94.2 K!)
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Common fertilizers
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• Techniques of fertilizer application
• Starter with the seed, low amounts
• Broadcast spread evenly over the land
• Lowest efficiency
• Not near roots, may feed weeds
• P "fixation
• Why is it used?
• Often the only practical way - pastures, etc.
• Build up stocks in low-fert soils
• Easy and cheap (main reason most of the time)
• Best way to add fert to a crop that has started
  growth

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• Techniques of fertilizer application
• Deep banding
• 10-25 cm deep, often anhydrous ammonium
• Puts fert where roots have access to it
• High cost
• Split Application (multiple applications)
• Adding fert two or more times
• Especially useful for N, where available levels
  drop to background within 4-12 months
• May backfire if most uptake is early and levels
  are low in split at that time
• Timing is critical

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Figure 10-11
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Fertilization with P, K, Mg, and other nutrients
can (but may not) keep soil available levels
elevated for a very long time whereas this never
happens with N thus, repeated N fertilization or
slow- release N fertilizer is sometimes used
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Repeated N fertilization or slow- release N
fertilizer is sometimes used however, if this
releases N when plant uptake is not occurring, it
can go to waste and pollute groundwater with
nitrate
Plant uptake
Remember all N supplied in excess of biological
(plant and microbial) demand will nearly always
convert to nitrate and leach away!
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Nitrogen Cycling in soils Biologically controlled
Atmospheric Deposition or fertilization
N2 fixation
N2, N2O
Plant N
Litterfall Root turnover
Uptake
Denitrification
Uptake
Mineralization
Organic N
NH4
NO3-
Nitrification
Immobilization
Leaching
Immobilization
Clay-fixed NH4
Includes mostly microbial (biotic) but also some
abiotic immobilization
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• Techniques of fertilizer application
• Side dressing as described. After crop is
  growing
• Point Injector fert using a rod to make a hole,
  put fertilizer deep near plant.
• Dollop or tree tablet principle.
• Good for P in P fixing soils.
• Fertigation fertilizer is added to irrigation
  water.
• Not the same as foliar application.
• Obviously require irrigation equipment not
  normal for forests or range soils
• Can be very efficient
• Expensive

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• Techniques of fertilizer application
• Foliar application
• Aimed at foliar uptake
• Seldom used to get all macros into plants often
  used for micros which immobilize in soils
• May require wetting and/or sticking agents
• Quick response

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• Fertilizer Efficiency
• Defined as percentage of added fertilizer that is
  actually used by the plants
• Generally 30-70 for N, 5-30 for P, 50-80 for K in
  crops, according to the book
• (sounds high)
• Generally 5-40 for N, P, and K in trees,
  counting only what is in trees at any one time
• However, trees recycle nutrients, and forest
  floor contents can be re-used
• Difficult to assess this in percentages

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• Fertilizer Efficiency
• Why is fertilizer efficiency so low?
• Things we cannot help
• Microbial immobilization
• "Fixation" of P
• Things we can help
• Wrong nutrients
• Bad timing
• Incorrect amounts
• Too low feeds microbes, which are most efficient
  competitors
• Too high in the case of N causes nitrate leaching
  losses
• This is an area that needs much more research!