Soil Fertility for Feeding the World

Global population is increasing
Land use issues (forests, habitats, etc for food production)
Increased demand for more with less input
Farming on poor soils
Improved management practices- production per acre
Technological advances- fertilizer, pest management, seeds, etc.

A fertile soil does not equal a productive soil

Plants require 16 elements for growth and reproduction
Carbon, Hydrogen, and oxygen are acquired through air and water
13 are from soil
Primary= N, P, K
Secondary= Ca, Mg, S
Micro= Zn, Mn, B, Cu, Cl, Fe, Mo

Most NEB soils contain large amounts of all the elements, but only a small % of these total amounts are available to the plant

Factors that affect availability:
Form and chemical properties of the element
Soil pH
Interactions with soil colloids
Microbial acitivity
Soil physical conditions:
Aeration
Compaction
Temperature
Moisture

Mineral Components of Soil: 1. Silica= Si 2. Quartz= SiO2 3. Hornblende 4. Feldspar 5. Pyrite * Secondary= Montmorillinite, Kaolinite, Illite
Structure
Net negative charge absorbs positive particles
Interactions
Soil Minerals
Soil Organic Matter
Soil H2O
Soil Air

Broadcast application produces same yields as banded applications
Mobile nutrient
Wet soil? Band?
Optimum moisture (structure aggregation) pH? Highly fertile soil
Band applications produce higher yields than broadcast regardless of rate
Immobile nutrients
Drier soil
Low fertility
Wet and cool conditions
Soil with high P fixing capacity

Soil Fertility Evaluation
Asses nutrient status of soil-crop system
Diagnose suspected nutrient imbalances
Monitor effects of management
Provide basis for fertilizer recommendations
Objectives of Evaluation
Improving fertilizer use efficiency
Decreasing impacts on water quality
Tools for Evaluating soil fertility
Visual analysis for deficiency symptoms
Plant testing
Soil testing
Precision management and sensors
Visual Evaluation
Allows in season ID of problems
Reveals severe conditions only
Too late to correct
Qualitative: no recommendations

Plant Analysis
Xylem= water carrying vessels (all nutrients pass through)
Phloem= sugar carrying vessels
Mobile nutrient deficiencies tend to occur in older leaves- plant sacrifices old for new tissue
Immobile nutrient deficiencies= symptoms on shoot/root tips, fruits
Plant mobile nutrients= N, P, K, Cl, Mg
Plant immobile nutrients= Cu, Mn, Zn, Fe, Mo, S, B, Ca

Tissue testing (sap test): Analysis of a specific soluble fraction of a nutrient, often performed on fresh tissue…mostly in situ
N, K status in plants (potato, tomato, vine, canola)

Fresh samples
Quick results but frequent
Time
Leaf age
Plant part
Number of leaves (no less than 20)
Needs thorough calibration
Plant testing: sampling
Time of sampling
Plant part to sample
Number of plants or plant parts
Procedure:
Minimize exposure to contaminants
Dry if plant analysis
Keep cold if tissue testing
Plant Testing Steps
Plant sampling
Plant analysis
Correlation and calibration: research defined relationships between nutrient conc. And relative yields or quality
Plant sampling analysis Correlation/calibration Interpretations
Critical Values (CV)- concentration at which there is a 5-10% yield reduction (90-95% of max yield)
Sufficiency Ranges- hidden hunger can be ID’d
Upper limit can be ID’d at which the element may be in excess
Factors to Consider
Soil conditions can affect values: soil pH, drainage
Plant growth stage
Parts to sample plus sample size
Antagonistic effect
High K on Mg
High P on Zn
High S on Mo 1. False, qualitative 2. True 3. False, use deficiency ranges 4. True 5. True 6. False, you cannot see hidden hunger, so cant reduce 7. True 8. True

To obtain a value that will help to predict the amount of nutrients (fertilizer) needed to supplement the nutrient supplying capacity of the soil such that max economic yield is achieved without excess
Predict the probability of obtaining a profitable response to lime and fertilizer
The probability of a response to nutrient additions on low testing soils is greater than high testing soils
On low testing soils, a response to applied nutrients may not always be obtained
Test needs to be correlated and calibrated
Provide a basis for recommendations for amount of lime and fertilizer to apply
Relationships obtained through lab, greenhouse, and field studies
Soil Sampling Correlation Calibration Interpretations (recommend.s)
Correlation
The process of determining the relationship between plant nutrient uptake or yield and the amount of nutrient extracted by a particular soil test method
Amount of nutrient extracted from soil by a laboratory test and nutrient uptake by plants in the greenhouse or field and/or crop yield
Calibration
Determination of the correct amount of fertilizer nutrient to apply in relation to soil test value
Soil Test Interpretation
The process of developing nutrient app recommendations from soil test values and other soil, crop, economic, environmental, and climatic information
Development of soil test based fertility evaluation:
Correlation
Extractants used are related to their ability to mimic reactions that control nutrient supply and availability
Provide an index of nutrient availability in a given soil
Soil test measures a portion of a nutrient from a pool
Dissolving action of acid, anion or cation replacement to free up the P, lock or tie the anions or cations
Greenhouse trials- relate nutrient extracted with plant uptake
Philosophies
Deficiency correction
Maintenance
Nutrient removal determination and replacement
Correlation + Calibration = Interpretation

Build Maintenance= fertilizing the soil
Build= (Soil test goal- Soil test level) x 18 / number of years

Steps to Effective Use of Soil Testing
Collect a representative soil sample from the field (soil sample)
Determine the quantity of plant available nutrient in the soil sample (soil test)
Interpret results (soil test correlation and calibration)
Estimate the quantity of nutrient required by the crop (nutrient recommendation)
Step 1: Taking a soil sample
Put in paper bag because plastic heats up soil too much
Uniform field area

P tie P
Adsorption: attachment of P to soil clay
Precipitation: formation of solid mineral compounds
Immobilization by soil organisms
P reacts strongly with soil material- adsorbs or precipitates
Limits bioavailability
Limits transport through soil
Movement occurs via erosion
Organic P accounts for 30-50% of the total P in soil
Organic P is important for supplying P to plants vs mineral P
P-mineralization as microorganisms break down OM
P-mineralization rapid- warm plus well-aerated soils
Mineralized and immobilized like nitrogen
Net mineralization at C:P ratio 200:1 or less
Organic compounds in soil increase P availability
Coating of Fe/Al oxides by humus form protective cover and reduce P tie up
*Finer textured soils make P less available. More clay, more surfaces for P to bind to that surface
Factors in soil P availability
Soil pH
Soil OM
Soil texture
Mychorrhizae

Meeting Crop P Requirement
Soil solution P concentration ranges from .01-.1 ppm P with an average of .05 ppm
Most plants require .003 to .3 ppm P in solution
P in soil solution < total soil P
P in stable phase= larger fraction of total soil P (org & inorg)
Diffusion is the primary mechanism transporting P to the root surface
Based on soil solution (P) and rate of uptake
High plant uptake= zone around root that is depleted of available P
Mass flow contributes to P transport when solution P is high
Root Interception with symbiotic mychorrhiza interactions
Root systems that have higher ratios of surface area to volume more effectively explore a larger volume of soil- more finer and branched roots
Phosphorus Deficiency
Stunted and thin stemmed
Very dark foliage with purplish edges
Tillering greatly reduced
Mobile within plant and can translocate to new shoots
Daily, a rapidly growing crop may take up equiv. of 2.2 lb P2O5/A
Most un-manured soils contain little readily available P to meet the large demand of crops
Optimizing P availability
Banding
Dribble
Starter
*
What are the forms of potassium used by plants?
K+
What are 2 major functions of potassium?
Stomatal opening and closing
Enzyme activations
What are the 4 soil components of potassium?
Solution
Exchangeable
Fixed
Mineral (unavailable)
Mineral K form includes?
Micas
Feldspar
For mineral K to become available…?

The exchangeable postassium is the K held by the CEC and is available
Most soil tests attempt to measure exchangeable potassium
90-98% of K is in mineral form
1-10% is fixed
0.1 to 2% is in exchangeable form

Soil= 5 meq x 6% x (39 mg/meq)= 11.7 mg/100g= 0.0117 g/100g / 100 lbs x 2,000,000 lb = 234 lbs/afs
Uptake of alfalfa= 5 tons x 2000lbs x 3%= 300 lbs

Factors in K availability:
Mineral types
Soil texture (CEC)
Soil moisture
Soil aeration and O2 levels (structure)
Soil temp
Soil pH
Mineral Types
Feldspar, mica, hornblende, quartz
K content varies based on parent materials
Minerals weather slowly, releasing K
OM does not influence K as clay content & type
Soil Texture
CEC
Low pH= less exchangeable K+
Soil Aeration
Root respiration & K+ uptake dependent on O2
Soil compaction detrimental
Excess, anaerobic conditions detrimental
Influences diffusion rates
Potassium release(wet) and fixation (drying) on soil particles
Shrink/swelling of clay complexes; wetting & drying; freeze/thaw
Freezing/thawing cycles will release fixed K
Soil sampling in spring may be different than samples taken in fall
K+ uptake by plants increases with increase in moisture
Soil Temp
Optimal= 60-80 F
As temp goes up exchangeable K+ increases
As temp goes up plant root activity increases * Soil pH
Plant specific for optimal K+ uptake
Acid soils- soluble Al++ replaces and occupies CEC
Liming increase K availability through displacement of exch K+ by Ca++ and K+ into soil solution
Too much lime affects pH and reduces K on CEC

K+ Deficiency in Plants
K can translocate in plants
Older (lower) leaves show deficiency first
Visible yellowing on leaf margin of lower leaves
Weak stems- lodging
Poorly filled pods, ear tips

1. An acidic soil will have more hydrogen and lower soil pH 2. Calculate the H concentration of the following:
SOIL pH H+ Concentration
5.0 10^-5
6.0 10^-6
7.0 10^-7
8.0 10^-8
3. Characteristics of Soil Acidity
Active acidity (pH)
Refers only to H+ in the soil solution
Exchangeable acidity
Includes exchangeable Al3+
Includes exchangeable H
Dominant fraction of the total acidity
Represented by a % acid saturation
Relative cation saturations have an important effect on the cation compostition of the soil solution and in turn on plant growth
4. Effect of soil acidity
Herbicide effectiveness reduced when pH is less than or equal to 7
Effects of Al and H on growth
Injury to roots
Decreased uptake of Ca, Mg, K
Excessive levels of Al (ph less than 5)
Al inhibits DNA replication
Changes membrane permeability
Inhibits enzymes catalysis