The switch to no-till causes many soil changes. The type and extent of changes depend on soil type, climate and farming history. Farmers who have no-tilled for some years usually notice more soil moisture, better seedbed tilth, more organic matter and earthworms, less soil erosion and improved trafficability. These improvements are due to changes in soil physical, chemical and biological conditions which occur with successive years of low disturbance seeding into standing crop residues. On the northern plains, mature zero tilled soils often resemble soils in their natural state. In tillage-based systems, cultivation is used to create a seedbed every year. Zero till creates a permanent seedbed - one that must be maintained rather than re-built each year.

ORGANIC MATTER (OM)

Zero tillage normally increases organic matter, especially if summer fallow has been eliminated. Organic matter (OM) consists of living and dead plant roots, microorganisms, insects and earthworms. The active fraction of OM, which is either alive or rapidly decomposing, greatly increases with no-till practices. In some soils OM does not increase much, even after successive years of no-till. These soils are likely to have a large natural OM content or a high turnover of OM.

No-till improves soil structure. This results when OM combines with soil mineral particles to form aggregates. Conventional tillage destroys soil aggregates. One advantage of the aggregates formed in no-till soils is the large network of pores which form between them. This network serves as a subway system for the movement of nutrients, air and water to crop roots. Like aggregates, these pores are destroyed by tillage.

THE "DUFF LAYER"

The "duff" layer is the thatch of plant material on the surface. No-tilling allows this layer to build up to useful levels. This duff layer suppresses weeds and reduces evaporation. However, the duff can become too thick if the cropping system is not managed properly. This occurs most when high residue and non-legume crops are grown back-to-back.

The duff layer can be managed to optimal levels using diverse crop rotations. Useful crops are those that have low residues, decompose quickly and are rich in nitrogen, like lentils, field peas or beans. Growing these crops will improve subsequent crop emergence. Mechanical shifting of some of the duff layer from the seed row into the inter-row will further suppress weeds and reduce evaporation. It will also create a warmer soil zone, giving quicker crop emergence.

No-till soils evolve over time. Changes in structure and OM affect water and nutrients. Crop management must be adapted to fit these new conditions.

THE ROOT ZONE

Below the duff, in the root zone, important natural cycles of OM formation and decomposition occur. Decomposition occurs through chemical oxidation, when the OM contacts oxygen. Soil aggregates formed with no-till will slow this decomposition by protecting some OM from exposure to oxygen. This soil building process continues until a new equilibrium between OM formation and decay occurs.

SOIL MOISTURE

No-till fields catch more snow and hold more melt water than conventional fields. In addition, the duff layer reduces water losses to evaporation. A properly structured no-till soil should make this extra water available to crops.

Contrary to popular opinion, roots do not seek water. Rather, water moves through soil pores to roots, like electricity through a conductor. Water movement is determined by the distribution of soil pore size and the degree to which soil pores are connected.

No-till soils maintain a network of hair-like pores which are particularly good at supplying water to plant roots. Due to increased continuity among pores, drainage in most no-till soils actually improves over time. Using forages, like alfalfa, in the rotation will further improve deep drainage as after the roots decay they leave channels for the water to move through.

Soil water should be used and not lost, this is possible with good crop management and rotations. An appropriate crop rotation will remove all of the water from the larger pores by harvest time. The cycle of snow catch and infiltration is then able to begin again next spring.

In poorly drained soils, growing high water using crops in the rotation may be required to make no-till viable. This can be achieved by continuous cropping, or by growing crops such as alfalfa, sunflower or corn (see component 4).

SURFACE SOIL TEMPERATURE

The surface 5 cm (2"), or the seed zone, of no-till soils, may warm more slowly in spring and cool more slowly in autumn than in cultivated soils.1 Snow trapped by standing stubble insulates the soil from the cold during winter. This insulation is essential for the survival of winter wheat, but it may delay spring warming if the soil is saturated.

With common hoe-type seeding tools, enough in-row tillage occurs to warm the seed zone, allowing spring sown crops to germinate and emerge in good time. Zone tillage using row cleaners mounted on planters has been used to grow heat responsive row crops like corn, soybeans and dry beans. Low disturbance disc drills may also require row cleaners to encourage soil warming.

The duff layer reduces the frequency of freeze-thaw cycles in the seed zone, maintaining aggregates and preventing crusting.

In clay soils, the seed zone may compact, and the duff layer may prevent the freeze-thaw cycles from effectively loosening it. Ice lenses can form horizontal to the surface and, as they melt, the soil subsides to its original position without being softened.

DEEP SOIL TEMPERATURES

Below 5 cm, in the root zone, no-till soils may be warmer and wetter from fall through to spring. Warmer root zone temperatures in the fall can increase nitrogen and sulphur release from organic matter.

Losses of mineralized and fall-applied nitrogen through denitrification, immobilization and leaching may be somewhat higher in this slightly warmer soil zone. Warmer winter temperatures may also raise the frost line. This can increase water infiltration in no-tilled soils, particularly if a network of large pores has been established.

The cold winters and short summers in the northern plains override tillage effects on root zone soil temperature. In this short growing season, most roots are in the top 60 cm (24") of soil. Temperature differences among tillage systems in the rooting zone are small and probably do not effect the uptake of water or nutrients by plants (Table 1).1

SOIL LIFE IMPROVES

Living things in the soil consist of microorganisms like bacteria, fungi, algae, protozoa, nematodes, and larger organisms like insects and earthworms. These organisms break down OM making nutrients available to plants. Before commercial fertilizer, crop residue breakdown and manure were the main source of nutrients for crops.

In general, soils that grow only grain crops are less biologically diverse than either pasture or undisturbed natural soils. Little is known about the relative importance of soil biodiversity to agriculture.

No-till changes the size and diversity of the community of organisms living in the soil. Tilled soils contain relatively more bacteria than no-till soils. These bacteria rapidly decompose OM, giving a quick release of plant nutrients. In contrast, no-till soils house relatively more fungi, which decompose residue slowly. The result is more gradual nutrient release.

While the moisture in no-till soils favours soil life, saturation reduces it. Early in the season, no-till soils commonly have a majority of their pores filled with water instead of air. If more than 60% of the soil pores contain water, aerobic (oxygen requiring) microorganisms give way to anaerobic (oxygen avoiding) microorganisms. At this stage, some nitrogen will be lost through denitrification.

Research on the heavy clay soils of the Red River Valley shows that, below 10 cm (4"), crops can be starved of oxygen regardless of the tillage system used. In heavy soils, high water using crops like alfalfa and corn are helpful in the rotation.

Standing stubble and the duff layer decompose more slowly than residues in the root zone. This occurs because contact with microorganisms is less and the low N content of crop residues, especially of cereal crops, slows the feeding of microorganisms. The rate of stubble and duff breakdown is also affected by the weather. Dry weather, and therefore soils, slows microbial feeding and activity while moist soil increases these.

The goal of managing soil life is to optimize residue decomposition and nutrient release for crop growth. Although this is easier said than done, crop rotation is critical. Residues from cereal crops contain much more carbon than nitrogen and may stimulate microorganisms to tie up nutrients. In contrast, legume residue has more nitrogen, which favours nutrient release by microorganisms. Legume residue is also a preferred food source for earthworms.

EARTHWORMS INCREASE

The best time to observe earthworms is in the spring or fall. The presence of earthworms is an outward sign of a healthy soil, although their effect on crop production is indirect and long term. Earthworm numbers are highest in pastures and lowest in conventionally farmed annual crop land (Table 2).2 Earthworms help increase pasture production. As a result animals return more manure to the soil which, in turn, also supports earthworms.

There are two types of earthworms; deep burrowing (nightcrawlers) and shallow dwelling. No-till soils have both types, particularly nightcrawlers which use the duff layer for food.

Nightcrawlers form large, permanent burrows into the root zone. As they burrow, the worms perform "biological tillage" by mixing crop residue from the duff layer with the soil below. The burrows improve drainage and increase soil aeration. Plant roots proliferate through the soil using the burrows as easy passage. Earthworm castings cling to the walls of burrows and provide nutrients to plants.

No-till soils support several times more earthworms than conventionally tilled soils. Low soil disturbance keeps the worm burrows intact, and the duff layer provides food and protection from temperature extremes. Any soil disturbance is harmful to earthworms. Even minimum tillage soils have fewer earthworms than no-till soils.

However, no-till does not guarantee more deep burrowing nightcrawlers. For nightcrawlers to become established in a field, they must migrate from a nearby field, ditch or fence-row. Neighbouring conventional tillage fields may not be a good source of nightcrawlers. It may be necessary to collect nightcrawlers from a pasture or roadside and "seed" the target field by placing 4-5 worms under a mulch every 12 m (40') in the field.

Herbicides are considered safe for earthworms. Soil applied insecticides and anhydrous ammonia fertilizer may kill some earthworms in the narrow zone of application. However, the effect on the total soil population is small compared to the killing effect of cultivation.

Earthworm activity is probably a better indicator of soil biological health than soil organic carbon levels. Organic carbon is not necessarily an accurate indicator of a soil's potential to release nutrients or of soil building benefits. Perhaps the following business analogy is useful. A healthy business is one that has a good cash flow regardless of the assets. If soil assets are carbon and cashflow is microbial activity then zero tillage systems are seeing much more healthy soil businesses despite sometimes static organic carbon levels.

NUTRIENTS RECYCLE

Mineralization is a bacteria-driven process which turns the N within OM into ammonium. Nitrification is the conversion of ammonium into nitrate.

In tillage-based systems, mineralization is "boom and bust". Booms occur after tillage with busts following shortly after. In contrast, mineralization in no-till soils is more evenly spread over the season. For this reason, spring sown no-till crops should receive adequate fertilizer early in the season to help establish the crop.

Nitrification in no-till soils may continue even under dry conditions due to more soil moisture. However, in very wet conditions, nitrification may be inhibited sooner in no-till than in a cultivated soil because of a lack of oxygen. Nitrification of OM-derived ammonium, or of ammonium fertilizer, is rapid. If too much nitrate accumulates, then losses to leaching can occur.

Denitrification is the bacteria-driven process which turns N fertilizer into N gas which is then lost to the atmosphere. Compaction in the seed zone can create an oxygen-starved environment which favours denitrification. However, actual losses of N vary a lot within all tillage systems.

Over time, immobile nutrients such as P and K may become layered in no-till soils. For example, banded P may remain in the band for several seasons. This can give false soil test results. In contrast, studies in Manitoba have shown that K becomes concentrated in the upper root zone even when K fertilizer is not added.3

The distribution of the more mobile N and S nutrients may also change in no-till. However, these changes are difficult to predict because they depend on water content, pore size and the rate of OM oxidation.

In general, N mineralized from surface residues accumulates in the top 7.5 cm (3") of the soil. At the same time, some no-till soils show a build up of nitrate below the rooting zone. These N losses may occur because the crop rotation is not intense enough to use either the available N or water. Using deeper rooted or longer season crops may be necessary in some environments to avoid N losses.

What effect does nutrient stratification have on the crop? Under drought stress, nutrients can become stranded near the surface and be unavailable to crops. Conversely, P and K are less likely to be tied-up, making them more available to the crop.

With bands of P remaining intact, a downward shift in pH (toward acidic) may be noticed in the band. This does not mean the soil is being generally acidified. In fact, a reduction in pH of northern plains soils may often increase plant availability of micronutrients like Mn, Cu, and Zn, but not Mo.

Due to a poor environment for mineralization, standing stubble and crop residue in the duff layer are generally not good sources of N in no-till systems. However, surface residue is a better source of K and P since these nutrients are released by leaching from the residue as well as by microbial activity.

"With conventional tillage there is more run off and more puddling in low areas. Cultivated soils with a pulverized surface make it harder for the moisture to penetrate.... If you've got a pretty low spot on a zero till field where a conventional farmer would say 'I better go around that one' - we would lust go through it."

Ron Bell, Birtle, Manitoba

"There is so much that we don't know yet about what is going on under the ground's surface and what's taking place when you start mimicking Mother Nature, utlizing her whole array of plants for cropping...the opportunities ahead of us are amazing."

John Raisler, Beach, North Dakota

Prepared from information provided by:

Cynthia Grant, Agriculture and Agri-Food Canada

Brandon Research Centre

Box 1000A, RR3, Brandon, MB R7A 5X3, Canada

Telephone (204) 726-7650 Fax (204) 728-3858

Email "cgrant@em.agr.ca"

References

1. Gauer LE et al (1982). Can J. Plant Sci. 62:311
2. Kladivko EJ (1995). Purdue University, Indiana

Grant CA and Bailey (1994). Can. J. Soil. Sci. 74:307