SA Grain July 2017

TABLE 4: CRITICAL PLANT NUTRIENT LEVELS IN MAIZE LEAVES OPPOSITE AND BELOW THE EAR AT TASSELLING.

% OF DRY MATTER

PPM OF DRY MATTER

2,9

0,25

1,9

0,15

0,4

0,15

Source: Hoeft & Peck, 1991

With less than 1% soil organic matter, the soil can be seen as high-

ly degraded due to a long period of continuous tillage practices.

The establishment of a multi-specie cover crop system is seen

and applied as the start of a process to build up the degraded soil,

which could take up to seven years or more, depending on the situ-

ation and the quality of CA application, the soil type and the climate.

However, it is anticipated that this type of cover crop system, ro-

tated every second year by a cash crop producing high volumes

of residues (such as maize), could fairly quickly restore soil ecosys-

tem functions and decrease the rehabilitation or transformation pe-

riod to three or four years.

In a high temperature, low rainfall environment such as Ottosdal,

the high volumes of cover crop residue would almost immediately

have had a positive effect on the soil water content with much high-

er infiltration rates and much less water loss through evaporation.

During the following few years, the presence of a diversity of plant

roots in the soil will most probably have a positive impact on soil

microbial diversity and activity, including mycorrhizas, which are

highly dependent on a host and living roots.

These effects are currently being measured in on-going research in

Grain SA’s on-farm CA trials across the country. The establishment

of perennial pastures is another possible solution, as part of the

crop diversity within integrated crop-livestock systems, to facilitate

the restoration process.

From a degraded soil to commercial

maize production

In order to put this field back into maize production (as was the aim),

the following fertiliser application rates (side dress) were used to

establish the maize crop aiming for a 5 t/ha yield in the 2016/2017

season:

33 kg of N/ha – this amount of N will cover a yield target of only

2 t/ha; the remaining N required will be provided through nu-

trient cycling and C decomposition of the cover crop mix’s dry

matter and roots. Depending on the production of the cover

crop, a saving of 70 kg/ha to 80 kg/ha of N can quite easily be

attained during the first year (

see Table 1, Part 1 on page 22

18 kg of P/ha – this amount of P will cover a yield target of

5 t/ha; from the Haney soil analysis it is clear that C content

and micro-bial biomass activity is not yet sufficiently restored

to recycle and/or release sufficient soil P to support plant P re-

quirements for a yield target of 5 t/ha. It is expected that more

P will be released from the soil in the next couple of years

through biological processes and colonisation of mycorrhizal

fungi. It has been shown in the past that only 20% P fertiliser

is taken up during the first year after application, while soil mi-

crobes provide plants with the amounts of nutrients required.

12 kg of K/ha – since there is sufficient amounts of K in the soil,

this application was just to establish strong vigorous seed-

lings; additional sulphur and zinc were also applied.

No further inputs as far as soil fertility management goes, were

deemed necessary. This was due to a great supply of nutrients in

the cover crop residues that will be made available as the cover crop

biomass decomposes through microbial activity.

Photo 1

displays the soil surface before cover crops were planted.

Low levels of cover with a soil surface crust and erosion can be

seen. The photo was taken on 27 January last year, just before the

cover crops were planted. At the stage when the cover crop was

fully developed,

Photo 2

was taken on 14 April last year. The crop

yielded an average biomass production of 12 ton dry matter/ha.

Photo 3

, taken on 2 September last year, shows the cover crop resi-

dues (left standing) killed by the winter frost. A decision was made

not to flatten it because of the positive effect the standing resi-

dues would have had on wind and water erosion. The cooler soil

under the residue cover will also benefit the water cycle due to the

lower evaporation from the soil surface.

Photo 4

was taken just before the maize was planted on 12 Decem-

ber last year. Note that the easy decomposable leaves containing

the most nutrients were already decomposed by the micro-organ-

ism. Only the woody plant material containing the less digestible

tannins and lignin fragments in the residues was left. It is in decay

or decomposition that this organic matter becomes useful as it be-

comes the fuel for ‘bacterial fires’ in the soil, which operates as a

factory producing plant nutrients.

Photo 5

shows the maize crop at tasselling and silking stage with

no signs of any nutrient deficiencies. The lower older leaves re-

main green. By April this year the predicted maize yield on this field

was 7,5 t/ha, indicating by all standards to a successful regenera-

tion (restoration) of a degraded soil into full maize production using

the principles of CA and integrated soil fertility management.

A good tool to monitor soil fertility or the uptake of plant nutrients

is leaf analysis (at this growth stage). Plant nutrient levels should

match the values shown in

Table 4

. Leaf analyses taken at the start

of the reproductive phase will show shortcomings. Correction

might not be possible, but valuable knowledge for future fertility

management will be gained.

Conclusion

This case study has demonstrated that CA facilitates the success-

ful application of integrated soil fertility management, the recovery

of critical soil ecosystem functions and the restoration of degrad-

ed soils. This process requires from producers a quality planning,

implementation monitoring (e.g. soil and leaf analyses) and adapta-

tion of CA practices such as fertiliser application, liming for an op-

timum pH between 6 and 7 (for maximum microbial activity), crop

diversity and more specifically, multi-specie cover crop systems.

It also requires an understanding of soil health and a long-term vi-

sion on soil restoration or regeneration, especially under dry and

sandy soil conditions.

References

Janzen, HH. 2002.

The soil carbon dilemma: Shall we hoard it or use it?

Soil Biol-

ogy & Biochemistry. Agriculture and Agri-Food Canada, P.O. Box 3000, Lethbridge,

Alta, Canada TIJ 4BI.

Hoeft, RG and Peck, TR. 1991.

Soil testing and fertility. In:

Illinois Agronomy Hand-

book, Circular 1 311, University of Illinois, Urbana-Champaign, IL, USA.

Conservation agriculture – Part 2

July 2017

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Fertiliser

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