SA Grain July 2017

Julie 2017

FOCUS

Fertiliser

Special

CA COMPONENT AND

ECOSYSTEMS SERVICES

N (KG/HA)

P (KG/HA)

K (KG/HA)

Above-ground biomass/crop residues (through

microbial C transformation)

168

(40% available for next

crop = 70)

(20% available for next

crop in first year = 5)

249

(SA soils have sufficient K)

Below-ground biomass of roots

(20)

(2,2)

Soil organic matter (2,5%; 20 kg N released per

1% soil organic matter)

Nutrient cycling through microbes (e.g. with high

colonisation of arbuscular mycorrhizal fungi)

Nutrients available to next crop (kg/ha)

140

321

Nutrient costs (R/kg)

Nutrient value (R/ha)

2 380

1 120

4 815

TABLE 1: NUTRIENTS POTENTIALLY AVAILABLE THROUGH CA AND VARIOUS SOIL ECOSYSTEM PROCESSES.

N fixed by legumes and available for next crop (10% legumes in mix) forms part of above-ground biomass

Nutrient cycling by cover crop roots forms part of root biomass value

P and K could also be added; values will increase with higher soil organic matter levels

This value has a great potential to increase in future as microbial diversity and activity rise

Conservation agriculture – Part 1

Biodiversity is ultimately the key to the

success of any agricultural system. Lack

of biodiversity severely limits the potential

of any cropping system and disease and

pest problems are increased. A diverse and

fully functioning soil food web provides

for nutrient, energy, and water cycling that

allows a soil to express its full potential.

Grow living roots throughout

the year

There are many sources of food in the soil

that feed the soil food web, but there is no

better food than the liquid carbon exuded

by living roots (

Photo 3

Soil organisms feed on liquid carbon from

living plant roots first. Next, they feed

on dead plant roots, followed by above-

ground crop residues, such as straw, chaff,

husks, stalks, flowers and leaves. Lastly,

they feed on other organisms lower in the

soil food web.

Healthy soil is dependent upon how well

the soil food web is fed. The provision of

plenty of easily accessible food (liquid car-

bon) helps soil microbial communities to

colonise and recycle nutrients for plants to

grow. The functioning of the soil ecosystem

is therefore determined by the presence,

diversity and photosynthetic rate of active-

ly growing green plants and roots.

Cover crop mixtures produce root exudat-

es with varying composition and effects,

and have different zones of nutrient uptake,

because they differ in amount, depth, and

patterns of root branching.

Permanent organic soil cover

Soil should always be covered by growing

plants and/or their residues and soil should

rarely be visible from above. A mulch keep

the soil cool and moist which provides

favourable habitat for many organisms that

begin residue decomposition by shredding

residues into smaller pieces (

Photo 4

Important soil ecosystem

services and functions

underlying integrated soil

fertility management

Carbon transformations

The decomposition of organic materials

into simpler molecules is one of the most

important ecosystem services performed

by soil organisms. Decomposition is also

defined as the mineralisation of carbon;

90% is carried out by micro-organisms such

as bacteria and fungi greatly facilitated by

soil meso and macro fauna that fragment

residues and disperse microbial propagules.

Nutrient cycling

The cycling of nutrients is a critical ecosys-

tem function that has positive direct impacts

(through plant-microbial symbiotic relation-

ships) on crop yield due to increases in plant

available nutrients, especially nitrogen (N)

through biological nitrogen fixation by soil

bacteria (e.g. Rhizobium) and phosphorus

(P) through arbuscular mycorrhizal fungi.

As said above, it literally means that these

micro-organisms release nutrients to the

roots in exchange for carbon to feed on, ei-

ther from root exudates, or from plant/root

organic material. Increasing populations of

bacteria and fungi provide more food for

protozoa (feeding on bacteria) and nema-

todes (feeding on bacteria, fungi, protozoa,

other nematodes and roots) and their waste

(manure) is directly available to plants as

nutrients.

On a global scale, biological nitrogen fixa-

tion accounts for around 65% of the nitro-

gen used by crops and pastures. There is

scope for considerable increase. The sup-

ply of nitrogen is inexhaustible, as nitrogen

comprises almost 80% of the earth’s at-

mosphere.

While estimates of symbiotic biological

nitrogen fixation can be as high as

400 kg N/ha/year, average biological nitro-

gen fixation is about ten-fold lower. Grow-

ing legume rotational and cover crops

adds biologically fixed N.

Most soils in South Africa contain low

amounts of soluble phosphorus due to the

parent material and/or P being fixed in acid

soils. However, if levels of arbuscular my-

corrhizal fungi colonisation are high, there

will be no need to add large quantities of

inorganic P, or in some cases, none at all.

The additional plant diversity and growth

period obtained with cover crops promote

root proliferation and activity, stimulate a

greater variety of soil micro-organisms and

enhance carbon and nutrient cycling. The

soil surface is covered for a longer period

of time during the year, so nutrient losses

from runoff and erosion are reduced.