Tillage plays a primary role in loosening and aerating aerates the top level of soil to facilitate planting the crop, says Mr Pitt.
“But it is much more than that, because it also it makes it easier to mix harvest residue, such as organic matter, and other nutrients into the soil, as well as killing weeds and other competitive plants.
“Moreover, when soils are wet, it helps them to dry out, and when it is done before cold winters, it facilitates the break-down of exposed soil through the natural process of frosting and defrosting.” He draws attention to the types of tillage: primary and secondary.
“The first soil tillage after the last harvest is generally deeper, more pervasive in location, and less sophisticated. It is often used with fertilizer application, such as mouldboard plows, disc plows, chisel plows, subsoilers.
“Secondary tillage, which is the last soil tillage prior to planting, is generally shallower, more selective in location, often calls for specialised equipment for soil preparation; soil smoothing; in addition to weed control through the growing season through the use of cultivators; harrows; hoes; tillers.
“The difference is often one of degree, and some implements can be set up to combine the two.” Mr Pitt goes on to emphasise that tillage technology is always developing, particularly to mitigate some of the negatives overaggressive tillage can induce , such as reducing the optimal levels of moisture; nutrients; organic matter; soil aggregates, or enabling soil erosion; chemical runoff, and the creation of hard pan soil levels below the level of the tillage.
“This development can be described as a struggle between ‘conventional’ and ‘conservation’ tillage,” he explains.
“Conventional, or perhaps better described as traditional tillage typically leaves less than 15 per cent crop residue or 560 kg/ha of small grain residue, and generally encompasses the primary and secondary functions outlined above.
“Conservation tillage, which in the USA is more prevalent than the traditional type, has itself evolved into several subsets.
“All the subsets typically leave at least twice as much residue as the traditional methods.
“The major benefits are savings in fuel and labour; much less soil erosion, and often better pest control.
“The major disadvantages are delayed planting owing to lower soil temperatures; the cost of specialized equipment; heavy dependence upon chemicals, and often periodic establishment of severe hard pans.”
The major subsets of conservation tillage are:
- No-till: an absence of any tillage other than the planting operation.
- Strip-till: narrow strips of deep tillage following the seed rows which are 20-25cm (8-10 inches) wide. This is also called ‘zone-till’.
- Ridge-till: similar to strip till, except the soil levels are raised for the 20-25cm tillage width.
- Mulch-till – full width tillage, with greater emphasis on retaining crop residue within the soil and on the surface.
- Rotational tillage – conventional tillage every 2-3 years.
Mr Pitt says: “All the above subsets are trying to realize the perceived gains from conservation tillage and minimize any disadvantages.
“There are also techniques within the subsets that attempt the same thing – faster speeds will minimize residue; increased disc angle will bury residue deeper; increased concavity will increase plowing action; increased use of herbicides will control weeds; specialized crop varieties that tolerate packed soil, and so on.”
The impact of climate on tillage
Mr Pitt goes on to remark that studies in North Dakota, an area of hot dry summers and cold dry winters, concluded that the key ingredient for sustainable farming was a resilient soil resource.
The studies examined the interaction of tillage practice; crop sequence and cropping intensity over two long term cycles nine years apart on soil quality – its physical, chemical, and biological qualities.
“In particular, the top 7.5cm of soil was examined for soil and particulate organic carbon; potentially mineralizable nitrogen; microbial biomass carbon; aggregate stability, and water infiltration rates.
“The conclusions indicated that improved soil quality and agricultural sustainability can be best achieved by adopting intensive cropping practices – heavy use of chemicals – with conservation tillage.”
Similar results were achieved in a separate study in Prince Edward Island – a particularly humid, cool environment in the Canadian Maritimes, he adds.
Fall vs spring tillage
“The two main determinants in any tillage decision are soil conditions and farm management.
“The soil condition is in turn a function of drainage, top soil depth, slope, texture and organic matter, and determine the effect tillage can have on soil quality, productivity, and water quality.
“Farm management runs the scope of residue management, crop rotation, the capabilities of any equipment being used and its correct use, drainage systems employed, fertilizer programs, soil testing, insect and disease control, crop hybrid selection, and other decisions made and not made. “Assuming the above two determinants are favourable, then the
next issues to be confronted are soil moisture and temperature, as both can have significant impact on soil fracturing, the depth to till, clod size, and any resultant soil compaction.
“Fall tillage is usually the better option as soil moisture is usually below field capacity.”
However, he cautions that an exception to this would be a harvest season with excessive rainfall.
“Excessive soil moisture results in any field traffic causing unwanted soil compaction, as well as poor soil fracturing and the creation of large sized clods.
“Late harvesting crops such as corn (maize) can result in fall temperatures below freezing, when ice in the soil will inhibit soil fracturing.“This of course can also happen in the spring as a result of longer than usual winters.
“If tillage is left until the spring, then any cold weather delays may shorten the growing season, resulting in lower yields.”
Wet harvest weather and the consequential impact on available time for fall tillage can dictate the need for tillage priorities, he notes.“Soils in need of more aeration or deep tillage to break up compaction are better suited for fall tillage.
On the other hand, soils just requiring shallow tillage can wait until spring because this enables the longer coverage to reduce potential soil erosion; those soils subject to greater soil erosion, such as soil type (lighter soils); slope; rainfall – all benefit from longer coverage.
Soil crusting (capping) from too much rain in the spring will reduce water infiltration and make it harder for seedlings to emerge, adds Mr Pitt, noting that some jurisdictions have specific timing requirements for manure application such as fall only.
Another important area where spring tillage can help is when herbicides have failed to control weeds, as it can bury them.
Fall tillage after harvest implies primary tillage.
“The usual choices are mouldboard plows; disc plows, or chisel plows – either spikes or sweeps.
“If greater depth is required, then the McKay-Empire-Wiese group of tillage companies has the market leading range of subsoilers.
“Subsoiling to remove the effects of compaction and hard pans has its own pros and cons as it relates to timing.
“First of all the need for subsoiling has to be established – the farmer can use a compaction probe or penetrometer to determine soil resistance – anything over 350 lbs PSI is likely to restrict root growth.
“An alternative approach is to dig a series of trenches about 60cm deep and use a pocket knife to probe for resistance at various levels.
“Although soil testing is best done when the soil is uniformly wet, subsoiling works best in dry soil conditions, otherwise there is a risk of creating another hard pan at the subsoiler depth.
“USA studies have shown that subsoiling generally pays off on a two-year rotation, providing yield gains of about 200kg/ha for corn and 1 67kg/ha for soybeans can be expected.”