soil fertility

Much of the changes in grain production can be attributed to the use of fertilizers such as

N, P

the 3 strategies for managing nutrients

response:

• deficiency

• sufficiency

• immediate profit

maintenance

• removal

• replenishment

• long-term stability

build-up

• long-term gains or improvement

perspectives of precision agriculture on why and how handle nutrients

response - profit oriented, so nutrient in sites or fields with high response

risk aversion - nutrient in the fields or areas with probable high producitvity

maintenance - replace removals by balancing inputs and outputs over time

building up - increase nutrient in areas with long-term potential

environmental - reduce environmental impacts in specific areas, fields or seasons

law of the minimum (AKA Liebig’s law)

plant growth will be constrained by the nutrient whose uptake is lowest with respect to that needed by the plant

law of diminishing returns (Mitscherlich’s law)

the two steps of cell growth

cell division (mitosis), cell expansion (turgor from water pressure)

Nitrogen

• most frequently deficient

• mobile

• chlorosis in older leaves

phosphorus

• next most frequently deficient

• energy storage and transfer

potassium

• cation in enzyme activation, water relationship, energy relations

• resistance to disease

calcium

• cell membranes, cell elongation and division

• deficiency impairs emergence and unfolding of new leaves in corn

• immobile

magnesium

• chlorophyl, ribosomes, phosphorylation

• deficiency —> interveinal chlorosis in older leaves

• mobile

sulfur

• immobile

typical concentrations of essential macronutrient elements in mineral soils and plants

N - 1.5%

P: 0.2%

K: 1%

Ca: 0.5%

the essential elements

C hopkins cafe managed by mine cousin Monical

the essential elements

C HOPKNS CaFe Mg B Mn CuZn MoNiCl

carbon

hydrogen

oxygen

nitrogen

phosphorus

potassium

calcium

magnesium

sulfure

cooper

zic

iron

magnanese

boron

molybdenum

nickel

chlorine

boron

• cell deveopment, pollination, protein synthesis

• most frequent micronutrient deficiency

• immobile

copper

• polymer synthesis, electron transfer

• deficiency —> yellowing, stunting, curling of younger leaves

• marginal necrosis

zinc

• growth regulatory sythesis

• deficiency: interveinal chlorosis, necrosis in older leaves, narrowing of leaves,

iron

• uptake reduced by high pH

• electron transfer, chlrorophyll synthesis, nitrogenase

• deficiency —> interveinal chlorosis in younger leaves

• toxicity due to excess accumulation is possible under low pH

manganese

generates H+, e-, O2 in Ps; e- transfer

deficiency: speckled interveinal chlorosis

toxicity due to excess accumulation possible under low pH

nickel

component of urease enzyme

lack of Ni —> urea accumulation (toxic)

molybdenum

nitrate reductase

nitrogenase

chloride

osmotic and cation neutralization

Ps

pH effects on availability of several micronutrients in soils

neutral to basic: Fe, Mg, Zn, Cu, B decrease in solubility

molybdenum increases in solubility at higher pH

beneficial/funcitonal elements

Na - partial sub for K as osmotic regulator

cobalt (co) - required for symbiotic N fixation in legumes and synthesis of B12 in ruminants

Vanadium (V) - partial sub for Mo in N2 fixation

silicon (Si) - grass tissue.

Se - not essential

toxic elements in plants

Aluminum (Al) - common toxicity

• occurs below pH 5.5, where CEC can have a base saturation lower than ~50%

• legumes susceptible

• liming - precipitate soluble Al (reduce toxicity). increases P bioavailability

  • shifts exchangeable ions

Fluor

How liming (e,g for Al toxicity) works

1. solubilization and disociation

2. neutralization of acids

• flocculation (soil clay in stable granule)

• aggregation, soil structure

• microbial activity

sources of acidity in soils in addition to AL3+

• organic matter decomposiiton

• root exudation

• synthetic fert

• acidity in rainwater

bioavailability

when a compound is accessible to an organism for uptake or absorption across its cellular membrane

nutrient bio-availability for plants depends on:

1) the release of nutrients from the solid phase in the soil to the solution phase

2) the movement of nutrients through the soil solution phase to the roots

3) the absorption of the nutrients by the plant-root mycorrhizal system

nutrient availability (bioavailability)

• quantity

• mobility, spatial availability

  • mass flow, diffusion

  • root growth, surface area, mycorrhizae

• root-induced changes in rhizosphere

2 steps of plant nutrient acquisition

1) root-nutrient contact

• root interception

• mass flow

• diffusion

2) nutrient uptake into the plant

• passive & active

root interception

roots encounter ions as roots grow through the soil. This can be less than 1% of the soil volume

mass flow

ions transported to the root in the convective flow of water. This convective flow of water through the soil is caused by plants transpiring water though the leaves via the stomata

diffusion

ion movement along a concentration gradient from points of higher concentration to points of low concentration (and hence closer to roots)

calculating mass flow

amount of water taken up by plant x concentration of nutrient in solution

factors driving mass flow

• H2O uptake rate

• climate

• time of day

• plant vigor and health

• nutrient concentration in soil solution

• buffer capacity

comparing mass flow and root interception

which one can contribute more?

• mass flow

what drives active uptake?

substrate concentration and temperature

immobile plant elements

Ca

S

Fe

B

Cu

Mg

can mobile or immobile nutrietns be taken up from a larger soil volume?

mobile

passive vs active transport in cell membranes

passive: uniport channels (pores, no binding) and carriers (bind)

active ion transport: pumps that use ATP to transport ions against a concentration gradient and generate electrochemical potential

carrier proteins: 3 types of ports

uniport, antiport, symport

aquaporin

water channel in a membrane

CKC

a measure of how many cations can be retained on soil particle surfaces

buffering capacity (BC)

change in Q/change in I

ratio between changes of nutrient concentrations in soluble (I for intensity) vs solid state (Q for quantity) during nutrient additions or removals

• mostly affected by solid states that exchange rapidly with soluble state (i.e adosrbed), but also by precipitated and organic states

• increases with CEC, clay and SOM

• the type of clay (2:1 » 1:1) is a key driver

• stabilizes soluble nutrient concentrations reduces leaching without reducing availability for uptake

• buffer capacity can be inferred from the slope of the adsorption curve. Steeper curves —> greater buffering

pH effects on bio-availability of nutrients

• acid pH: some micronutrients (Fe, Mg, Zn, Cu, B) increase in solubility

• alkaline: Mb inccreases in solubility

pH effect on macronutrient availability?

primary mechanisms of soil root nutrient contact

N: mass flow/diffusion

P: diffusion

K: diffsion

Ca: root interception

Mg: root interception/mass flow

sulfur: mass flow

micronutrients: root interception/mass flow

diffusion: Fick’s law

J = -D A dC/dx

J = diffusive flux

D = diffusion coefficent

A = area for diffusion

dC/dx = concentration gradient

soil salinity vs sodicity

soil salinity: excess salt impacting veg establishment and development

• EC measured. >4 DS m-1

soil sodicity: too much Sodium (Na) relative to Ca and Mg

• Ca2+, Mg2+, Na+ are measured in the solution or exchangeable phase

pH:

saline: <8.5

saline-sodic: <8.5

sodic: >8.5

all saline and sodic soils have pH >7

two ways to estimate sodicity

• soil solution data, sodium adsorption rate (SAR(

• exchangeable data, exchangeable sodium rate

where the salts come from

• evaporation move and concentrate salts from deeper layers into the surface. This raise of dissolved salts in the soil profile is in part caused by upward capillary flow and the lack of deep drainage

• using salty water for irrigation

• recurrent fertilizer additions, incl. synthetic fert, manure

• PM

• location: oceans

challenges for plant growth salt-affected soils

• osmosis - out of balance

• water & nutrient uptake compromised

• solute & water move outside of the root cells (dehydration)

• root tissue collapsed (plasmolysis)

excess Cl- or Na+ is toxic to most plants

examples of plant species and their tolerance to soil salinity

very tolerant to salinity: native plant species

tolerant to salinity: barley, wheat, corn, rice

intolerant to salinity: beans, alfalfa, potato, carrot, onion

how to manage salt-affected soils

• over-irrigate (can also remove nutrients)

• grow native plants/salt adapted plants

• inoculating plant seeds with endophytic microbes that exclude Na+

• additions of gypsum (displace Na)

• elemental S if there is a abundance of Ca. S —> acidity —> releases Ca