SA Grain July 2017

Julie 2017

soil fertility management

Part 1: Theoretical principles and practices

n South Africa, crop production sys-

tems based on intensive and continuous

soil tillage have led to excessively high

soil degradation rates with a reduction

in natural soil fertility in areas under grain

production. It also results in the consis-

tent recommendation of the use of huge

quantities of chemical fertilisers that are

biologically unnecessary, economically ex-

travagant and ecologically damaging.

Tillage results in the oxidation and destruc-

tion of carbon in the soil by increasing the

soil oxygen levels, thereby promoting bac-

teria populations to expand and consume

active carbon in the soil. Soil organic car-

bon, or soil organic matter, is the key ele-

ment that drives soil health, which in turn

is the primary factor having an impact on

sustainable crop production.

If sound farming practices are sustained

over time, soil health improvement could

significantly escalate, influenced by posi-

tive changes in a wide spectrum of soil pa-

rameters, including soil fertility, which then

result in improved productivity and profit-

ability of farming systems.

There is general agreement among key

stakeholders in South Africa, that soil health

and sustainable crop-livestock production

will only be achieved through the adoption

and implementation of conservation agri-

culture (CA) principles and practices.

CA is seen as an ideal system for sustain-

able and climate-smart agricultural inten-

sification and regeneration, through which

producers can attain higher levels of pro-

ductivity and profitability, while improving

soil health and the environment.

One of the good agricultural practices as-

sociated with CA is integrated soil fertility

management, which essentially depends on

locally adapted CA principles and practices

to build-up soil health, allowing producers

to reduce the use of fertilisers, while sus-

taining good and stable yields and increas-

ing profitability.

This understanding is important if we wish

to sustain productivity at the lowest pos-

sible costs, both economic and ecological.

Without being able to go into details, this

article aims to provide a few principles, ad-

vantages and examples of integrated soil

fertility management.

Integrated soil fertility

management

This concept of integrated soil fertility

management emphasises the maximisa-

tion of nutrient use efficiency, the enhanced

access of soil nutrients to plant roots, the

response of soil as a living ecosystem and

the role of sound locally adapted soil man-

agement practices enhancing ecosystem

functions and services leading to improved

soil fertility.

The concept acknowledges that neither

practices based solely on mineral fertilisers

nor solely on soil ecosystems services are

sufficient for sustainable crop production,

especially during the transition years after

starting with CA on degraded soils.

It also requires well-adapted, disease- and

pest-resistant germplasm, as well as other

good agricultural practices. The critical soil

ecosystem processes involved are trans-

formations of carbon, cycling of nutrients,

maintenance of the structure and fabric

of the soil and biological regulation of soil

populations.

Ways to increase nutrient

use efficiency

Nutrient use efficiency, which may be de-

fined as the yield obtained per unit of avail-

able nutrients in the soil (supplied by the

soil and fertilisers), could be improved as

follows:

Adjustment of fertiliser application rates

based on (natural) soil fertility levels tak-

ing account of soil organic carbon level,

organically bonded nutrients, nutrient

cycling and/or previous cropping prac-

tices, especially legumes and their resi-

due biomass.

Apply fertiliser at the right time and

place, using the right source.

Plant crops at the right planting density

having enough plants to ensure optimal

and efficient nutrient access and yield.

In CA higher planting densities (around

30% higher than the norm) or at least

above 22 000 plants per hectare ensure

effective use of soil nutrients and wa-

ter in the whole soil profile and surface

area, while reducing temperature at soil

surface level.

CA principles and prac-

tices enhancing integrated

soil fertility management

Many producers world-wide have achieved

large improvements in soil health in a rela-

tively short time. What are these producers

doing differently?

Minimum soil disturbance

Physical soil disturbance, such as tillage

with a plough, disk, or chisel plough, that

results in bare or compacted soil is destruc-

tive and disruptive to soil microbes and

creates a hostile, instead of hospitable,

place for them to live and work (

Photo 1

The soil may also be disturbed chemically

or biologically through the misuse of inputs,

such as fertilisers and pesticides. This dis-

rupts the symbiotic relationship between

micro-organisms and crop roots. By stra-

tegically reducing chemical inputs, we can

take advantage of these soil ecosystem ser-

vices to allow plants to freely access essen-

tial nutrients.

Diversify with crops

and animals

Sugars made by plants, through the miracle

of photosynthesis, are released from their

roots into the soil as liquid carbon and trad-

ed to soil microbes for nutrients to support

plant growth. This soil ecosystem service is

a vital element of healthy soils and can be

enhanced through the inclusion of as many

different plants and animals as practical.

Livestock utilising cover crop mixtures, for

example, contribute to this diversity.

With ultra-high-density grazing utilising

30% to 50% of available material, livestock

can stimulate root development and re-

cycle 80% of nutrients in the form of dung

(

Photo 2

). Biodiversity directly leads to-

wards a diverse array of soil microbes from

a range of functional groups, which again

improves the soil’s ability to support nutri-

ent dense, high vitality crops, pastures, fruit

and vegetables.

FOCUS

Fertiliser

Special

DR HENDRIK SMITH,

conservation agriculture facilitator, Grain SA and

GERRIE TRYTSMAN,

ARC-DPI