SoilChapter 10

Soil Fertility ManagementPages 311 – 355

“Fertilizers, liming soils and irrigation are easily manipulated factors for increasing plant growth.”

pg. 311

When large amounts of plant nutrients are harvested from soils in the form of crops . . .

Fertilizers are commonly used to replenish lost nutrients in most all agricultural situations

Low cost fertilizers can yield good yield results

Excess use of fertilizers can yield pollution problems in ground or surface water

Soil fertility problems are typically easy to solve

GoalsThe goal is for minimum fertilizer

inputs - reduce costs- maximum yields - increase profits- reduce pollution- increase health and aesthetics of plants

Limiting factors to fertilization

Goal to spend less than 20% of all production costs to increase yields up to 50%

But fertilizer input may not be profitable when:

Limiting factors to fertilization

Water is a limiting factorGrowth hindered by insects, soil pH,

plant diseases, extreme temperatures

Increased yields has less market values than fertilizer costs

Replenishing nutrientsNitrogen can be replenished by

recycling crop wasteOr by growing legume crops and

recycling them

Replenishing nutrientsSoil minerals lost through crop

harvest are typically replenished by the addition of those minerals as mineral fertilizers

Replenishing nutrientsGood soil fertility improves yieldsPlant diseases can be linked to low

soil fertilityGood soil fertility can be linked to

favorable aesthetics of landscapes

PollutionPoor fertility management practices

can yield pollution problemsNitrate and phosphate fertilizers

misapplies can produce contamination of water

PollutionNitrate fertilizers Percolate through the soil Contaminate ground water Making it undrinkable

PollutionPhosphate fertilizers Runoff into ground water Causes algal blooms Kills aquatic wildlife populations

Are there benefits . . . To the use of fertilizers?The more vegetation we grow –

crops, trees, shrubs and grass we grow . . .

There are benefitsMore CO2 is removed from the

atmosphereMore plant cover over the soil surface

means:Less erosion of soil moving downslopeLess dust in the atmosphereLess runoff into bodies of water

There are benefitsThe more crops grown on stable

landscapesThe less pressure to grow crops on

fragile landscapes

Land managementWas typically done on a “large scale”Entire fields were treated alike“Medium scale” management

subdivided fields into smaller section“Site-specific” management or

“precision” management divides fields up into many smaller sections

“Site-specific” management

Can be based on the smallest sections of land and their individual requirements

“Site-specific” management

Continuous observations are made of: plant growth . . . Vigor . . .weed growth . . .water requirements Can be used to determine management

practices for each section

Soil samplingSoil samples should be

representative of the area in which samples are being taken

Composite samples at a given depth often provide the most uniform results

Soil samplingSamples can still be misleading

depending on relief of the area and previous grading or leveling

Topsoil depths may vary greatly and over different subsoils

Sampling depths . . .Vary depending on the tests to be

performed

Sampling frequenciesFor landscaping . . . Sampling should be done at the

beginning of any new, or redo, landscape project

Also a good idea to sample when starting new maintenance jobs

Sampling frequenciesFor agriculture . . .Annually with new land Every 2 to 3 years once established

Sampling . . .Inspect the areas to be sampled for

differences in soil characteristicsLook for obvious changes in texture,

structure, color, depth of topsoil, etc.

Sampling . . .Samples should be composites of

similar soilsIn some cases composite samples

can be separated from front to back yards

Samples may also be separated by the intended plantings

Sampling . . . The best advice is to “follow the

suggestions given by the laboratory that will be doing the tests.”

Inform the testing facility as to what types of plant materials will be grown

If there is a planting list – provide that to the testing facility

Soil testingIf we expect to obtain the maximum

growth response from added elemental nutrients . . .

All other essential nutrients must be in adequate amounts . . .

Amounts that will not injure plants

Soil testingSoil testing can be expected to

provide:Soil pHLevels of soluble saltsAvailable phosphorus (P)Available potassium (K)

Testing soil acidityBest done using pH electrode testing

equipmentpH test strips and pH color kits are

often inaccurate Tests are made using a saturated

soil solution of 1:1 or 1:2 soil:water suspension

Testing for nitrogen Due to its volatile nature Nitrogen is difficult to test forRecommendations are often based

on goals . . . Ex. 1 lb of nitrogen per 1,000 sq.ft.

for turf

Testing for phosphorus & potassium

Used to determine crop responses to fertilizers

Many sols are showing depleted levels of phosphorus and potassium

PhosphorusMuch of the phosphorus in soils is

unavailable to plantsIt must be placed directly into the

root zone

PotassiumPotassium tends to be more

availableSmall amounts are soluble It is also an exchangeable cation

Plant analysisMeans of measuring nutrient uptake

by plantsNot viable for new plantings – plants

must be established firstWorks well in conjunction with soil

testingCan help determine existing problems

Factors affecting nutrient uptake

Plant species – growth rate, etcSoil temperature – season, weather,

etc.Soil moistureSoil aeration, compactionRoot damage, pest infestationsInteractions among nutrients

Plant analysisSamples should probably be washed

and air dried for 24 hours prior to shipping

Samples should not be shipped in plastic bags

Ultimately, samples should be taken and processed according to the testing facility

Nutrient deficienciesNutrients (elements) are considered

deficient . . . When they are below a threshold

level

Threshold levelThe maximum level of a substance

(or condition) . . . That can be tolerated without ill

effects

Threshold level changesPlant species, plant parts, age of plantPlant health and root health, vigor,

disease Climate, time of year, season, weather

conditionsSoil pH

Nutrient deficienciesAre often recognized by outward

signsChlorosis and . . .Necrosis

ChlorosisReduction of the formation of green

plant pigment chlorophyllPlants become chlorotic Foliage appears unnaturally yellow

or whiteTypically associated with nutrient

deficiencies

NecrosisFormation of dead (or necrotic) plant

tissue – leaves, stems, roots, etc.Can be caused by deficiencies, salts,

disease, insects, various growth stresses like water

Visual symptomsAbnormal growth and size can be

indicatorsThe problem is narrowing down the

cause or causes

Nutrient movementNutrients travel up to the foliage . . .Through the xylem tissue . . .The water-carrying vessels

Nutrient movementThe phloem are the vessels that

conduct sugars manufactured during photosynthesis

Not all nutrients are able to transfer through the phloem equally

Plant nutrientsNutrients divided into two main

categoriesMobil nutrients

nutrients that can travel freely through the phloem

Immobile nutrientsthose that can’t. . . .

Mobile nutrientsDeficiencies appear in older foliageThe plant sacrifices older foliage to

protect new growth

Mobile nutrientsSymptoms appear on older tissue

Nitrogen (N)Potassium (K)Chlorine (Cl)Phosphorus (P)Magnesium (Mg)Sulfur (S) (in some plants)

Immobile nutrientsDeficiencies of immobile nutrients often

show in new growth – new shoot tips, root tips and phloem-fed fruit

Ex. calcium deficiencies and tomatoes, pears, apples

Calcium is highly immobile – soil applied calcium cannot be moved by the plant

Immobile nutrientsSymptoms appear on younger tissue

Copper (Cu)Manganese (Mn)Zinc (Zn)Iron (Fe)Molybdenum (Mo)Sulfur (S) (in some plants)

Highly immobile nutrientsSymptoms appear growing tips and

fruitBoron (B)Calcium (Ca)

Factors affecting mobilityCharge on the ionIon’s tendency to form insoluble

precipitates Soil textureAdsorption to soil surfacesWater movement in soilConcentration of other ions

Factors affecting mobilityPhosphates precipitate or are

adsorbed to soil solidsPhosphates are the least mobile of

the anionsPhosphates only move 1 to 2 cm

from placementExcept in sandy or other porous soils

Factors affecting mobilityK+ and NH4

+ move slowly because of their attraction to cation exchange sites

Nitrate and sulfate are more mobile moving with water

Phosphorus and potassium should be applied as close to the root zone as possible

Definition of a fertilizerAny material containing one or more

of the essential nutrients that are added to the soil or applied to plant foliage for the purpose of supplementing the plant nutrient supply can be called a fertilizer.

Fertilizer analysis (grade)Fertilizer labels display the fertilizer’s

‘grade’ Minimum guaranteed percentages of

nitrogen (N), phosphorus (P) and potassium (K)

Secondary nutrients and micronutrients

Weight and manufacturer

Fertilizer analysis (grade)Provides information about . . . Percentage of total nitrogen (N)Percentage of phosphate (P2O5)Percentage of potassium oxide (K2O)

Fertilizer analysis (grade)The numbers associated with the N-

P-K (ex. 20-20-20 ) on a fertilizer label are referred to as the fertilizer analysis or grade

The numbers indicate a percentage amount of those macro-nutrients in the product

Fertilizer analysisEx. - For a “triple 20” fertilizer, or a

fertilizer with a 20-20-20 analysis on a fertilizer label

The product contains: 20% nitrogen (N)20% phosphate (P2O5)20% potassium oxide (K2O)

The Label

The Label

Fertilizer effect on pHAcid forming fertilizers include:Most nitrogen fertilizersAll ammonium materials Many organic nitrogen fertilizers

Fertilizer effect on pHNeutral fertilizers include:Most all potassium fertilizersSuperphosphate Triple superphosphate

Fertilizer effect on pHBase forming fertilizers include:Potassium nitrate

Acidification of soilsCan be caused by nitrogen fertilizersAmmonium cations (NH4

+) are oxidized by bacteria to form nitrate anions (NO3

-)

NH4+ + 2O2 NO3

- + 2H+ + H2O

bacteria

ammonium

oxygen nitrate hydrogen water

Basic application methodsStarter fertilizerApplied near the seed at plantingTypically phosphates and potassium

because of low mobility

Basic application methodsBroadcast applicationsApplied in uniform applications to

the soil surfaceFertilizers either left on the surface

or incorporated into the soil

Basic application methodsSide-dressingOften broadcast around the bases of

plantsNitrogen can be lost through

volatilizationFertilizer should be ‘scratched’ into

the soil or covered with mulch

Basic application methodsFertigation Fertilizers applies by injecting into

irrigation waterFertilizers applied in large quantities

of irrigation waterThis is not a ‘foliar’ application

Basic application methodsFoliar applicationsFertilizers applies in solution to

wetted foliageNutrients move into the plant

through the stomata on leaves and through parts of the epidermis

Basic application methodsFoliar applicationsBest for small concentrations of

nutrient applicationsHigh concentrations can cause salt

burnWell suited for micronutrientsLess materials are required that with

soil applications

Basic application methodsFoliar applicationsSuited for iron chelates because of

the tendency to become immobile in soil

Quick uptake when other conditions do not permit nutrient uptake – cold soil temps, root rot, nematode damage . . .

Basic application methodsFoliar applications Quick responseMay be short-livedOften require subsequent applicationsBest applied in calm winds, relative

humidity greater than 75%, temps less than 85°F