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Chapter 10: Soil Fertility Management


Vector analysis to assess growth response (weight concentration) - we will do this in 497/697 ... Still a very imperfect art, especially for forests. We will ... – PowerPoint PPT presentation

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Title: Chapter 10: Soil Fertility Management

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
  • 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)

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)

  • 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

  • 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
  • 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.

  • 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.

Horizontal variation grid vs random sampling
Horizontal variation sample by landscape strata
that make sense (land use, soil series, slope,
aspect, current vegetation, etc.) (Fig 10-3)
Figure 10-3
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?

Sample point?
Is it better to sample by grid point or randomly
and just note where the sample falls?
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

Vertical variation need to know horizon depths
and keep sampling constant by depth
It is important to sample proportionally with
Unbiased sample with trowel Remove first scoop,
take second layer
Biased sample (trowel)
Proportional Sample (core)
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
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.
  • Soil Tests
  • Phosphorus
  • The book says this
  • Bray 1 0.025 M HCl 0.03 M NH4F (for acidic
  • Mehlich 1 0.05 M HCl 0.025 M H2SO4 (for acidic
  • 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

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
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.
  • 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
  • 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
  • Oven-dry (65C) analysis (otherwise water content
  • 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.

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

  • 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
  • I will not test you on this, but be aware of it
    to avoid making mistakes if you are involved in
  • 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

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!)
Common fertilizers
  • 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

  • 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

Figure 10-11
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
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
Plant uptake
Remember all N supplied in excess of biological
(plant and microbial) demand will nearly always
convert to nitrate and leach away!
Nitrogen Cycling in soils Biologically controlled
Atmospheric Deposition or fertilization
N2 fixation
N2, N2O
Plant N
Litterfall Root turnover
Organic N
Clay-fixed NH4
Includes mostly microbial (biotic) but also some
abiotic immobilization
  • Techniques of fertilizer application
  • Side dressing as described. After crop is
  • 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
  • Not the same as foliar application.
  • Obviously require irrigation equipment not
    normal for forests or range soils
  • Can be very efficient
  • Expensive

  • 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

  • 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

  • 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
  • Too high in the case of N causes nitrate leaching
  • This is an area that needs much more research!