Carl Fanning, Extension Soil Specialist, North Dakota State University

The impacts of tillage on land we farm can be seen along any public road. The black prairie soils of yesterday now have exposed subsoil and light colors. Time under cultivation has brought visible change. Technical change has also occurred in the physical and chemical properties of land that’s cropped.

Time under cultivation has brought erosional differences among management systems using different tillage tools. It is natural to scrutinize these systems for strengths and weaknesses. There are two schools of thought on equipment evaluation.

1. It's value as an economical unit for weed control and seedbed preparation.
2. It's value in a management system to control erosion and soil loss.

Erosion in the landscape is conspicuous and in recent years social pressure for resource preservation has grown. It is popular to condemn a number of sound tillage tool used in the past and question new designs that leave the soil surface bare. In this context, two technical points are overlooked.

1. New tillage equipment develops as growers find need to solve a local soil condition management problem.

2. Soil properties have changed with time resulting in tool performance change.

The nature of our farm marketing structure is such that a successful local tool is often produced in quantity and transported to areas where soil conditions and tillage needs are radically different. Examples abound in tools developed for corn production and incorporated in small grain area marketing programs.

Three examples serve to illustrate long term soil property change and provide insight to relative impact of differing tillage systems.

The first is in remarks of Steve Hamburg, Kansas State University as reported in the November 1989 issue of Agrochemical Age.

After planting a field in native grasses and allowing them to grow 30 years, it made no difference whether the site was burned, hayed or simply left alone. All treatments continued to show a dramatic decreased productivity and nutrient level relative to an adjacent unplowed area."

This study underlines the fact that land returned to grass after tillage no longer has potential to produce at its original level. Tillage, in land use, brings about irreversible change. The second example is in the work of Bauer and Black¹, Figure 1. This identifies the rate of change with time of a critical soil component, organic matter. Over about a two-generation period, soils in the northern plains lose about 60% of their organic matter and the loss rate begins to level.

Note: Figure 1 Graft was included in the original proceedings.

Technically, it is important to recognize that equilibrium achieved under grass vegetation was disrupted with cultivation and leveling of the organic matter loss rate is graphic representation of movement toward a new equilibrium under cultivation.

It is also important to recognize the rate of change has been too slow to be a significant factor in year to year crop management planning. Growers using land have been unable to detect annual change.

Technically annual change has also been outside the scope of available analytical technique. Change has not been measurable until a number of years have elapsed. However, over time, significant change has occurred.

As the third example, tillage practices effect rate of change in organic matter degradation. Doran² in Nebraska imposed the treatments shown in Table 1 on sod and measured organic matter content of the 0-3" depth 10 years later.

In this study, no-till production, a minimum soil disturbance production program has precipitated substantial organic matter loss. Although the 10-year loss with moldboard tillage was three fold greater, simply moving from unmanaged native grass to no-till cultivation has shifted equilibrium adequate to initiate organic matter decline.

Technically, soil organic matter levels are a key indicator for a host of subtle problems that develop in production programs over time. Examples include compaction, crusting, cloudiness, erosion susceptibility and excessive seedbed drying. These develop in a gradual progressive manner. Unfortunately our understanding of the mechanisms and relative magnitude of physical problem change is inadequate to index change in discrete steps with relevant management impact. Combined these problems cause more management concern at seedbed preparation and stand establishment than at other times in the production cycle.

Stand establishment failure often reflects the combined impact of soil property change and tillage tool performance. A uniform, rapidly emerging crop is basic to profitable crop production. This factor has done more to prompt examination of tillage and seeding practices than other production needs in the northern plains.

Soil structure deterioration also has a cause and effect relationship with soil erosion. With organic matter loss, soil aggregates lose wet soil strength and degrade easily to erodable sized particles. It follows that topsoil loss with erosion exposes lower organic level subsoil’s even more susceptible to erosion forces.

At the lower end of the physical property degradation cycle specific management problems are easier to identify. This differs among soil textures. In general terms, at about 1% remaining organic matter, structure deterioration of the tilled soil depth is virtually complete; sandy soils reduce to single grain structure, soils crust badly and clay soils clod up. Management for wind erosion control on coarse textured soils becomes paramount for successful cropping. On medium and fine textured soils water intake and erosion control management requires increased attention on all except level and near level soils.

Basically, in terms of tillage needs over time, the soil structural properties that allow preparation of mellow seedbeds through a wide range of soil moisture conditions deteriorates. Technically, crumb structure, common to all organic matter rich prairie topsoil disappears early in the organic matter degradation cycle. As shown by Tabatabai and Hanway⁸, in Figure 2 organic matter loss increases the density of individual soil aggregates. Seedbed drying and increased stand establishment risk accompany aggregate change.

Thus performance demands on tillage equipment segregates to include specific performance for weed control, seedbed preparation and erosion control on combinations of different slopes and textures. Differences in organic matter levels, as shown in Figure 3, have always existed in landscapes. Even modest erosion or organic matter loss turns a sloping soil into a radically different tillage management problem.

The challenge for equipment manufacture as they design larger equipment to match increased horsepower tractors is to design tools that will operate well over abroad range of conditions. Modest success toward this end can be found in field cultivators for small grain production. A similar statement can be made for double disc units for flat land operation in corn-soybean production.

Water dominates as small grain productions yield limiting factor. There is little question tillage can be a valuable management tool for poor physical condition soils. Crusts or shallow layers that restrict water intake can induce runoff and waste usable crop water. These are easily broken with timely tillage. Similarly, roughened surfaces absorb water rapidly and can provide temporary runoff control on modest slopes.

Tillage to destroy crusts and surface roughening for water retention has a major fault in that it is a management intensive practice. Each rain requires field inspection and a decision regarding need for an additional tillage trip to break crusts and reroughen the soil surface. Neglect in this management detail, brought about because of the costs involved, and the fact surface roughening as a water runoff control practice was extended onto strongly sloping land is one of the reasons for extensive erosion damage seen in today's rural landscape. However, to imply tillage is inappropriate management on low erosion risk level and nearly level soils is to ignore both technology and production economics in modern agriculture. Appropriate, timely tillage is the mainstay of profitable production on class 1 and 2 land.

Deep plowing to disrupt natural hard pans compacted layers have potential to increase water intake and yields. However, few data are available showing positive benefit from the practice. Most promising data comes from lower rainfall areas as shown in Table 2 from data by Greb^3.

In this study deep plowing was to 17 inches to break a 4-inch thick natural pan. Average combined yields of winter wheat and barley at this site increased from 23.6 to 28.3 bushel/acre.

Voluminous data are available demonstrating improved water retention when tools that leave small grain residues on the surface are used. Maximum benefit in the northern plains includes tillage operations that leave stubble upright to trap snow. However, across the plains the main thrust of high residue tillage is for evaporation control and water retention on fallow ground. Water retention on fallow ground depends in a large measure on residue levels. Indeed there is a threshold level necessary to affect water retention. Black at Mandan identifies the threshold straw level at about 2000 lbs./a to affect water loss 10 days after a summer rain.

Table 3 as assembled by Greb³ provides insight to water retention expectations with high residue tillage.

Little attention has been given to the effects of residue cover on seedbed moisture retention. Technically it would appear advantages with evaporation control from crop residues could be applied to seedbed drying problems and water loss from coarse aggregate seedbeds. In this respect no-till grain growers have been quick to point out excellent stand establishment in dry years. However, technical attention to crop residues at seeding in no till programs has not progressed past infatuation with a 1^o F seedbed temperature depression in corn production.

The progressive nature of stand establishment problems across eroded ridges and light colored soils and increased management attention these sites require will eventually focus increased research attention to the combined tillage-seeding equipment needs of these soils.

One of the technical features of tillage is the ability to change soil bulk density. Indeed, bulk density manipulation with tillage is so common place it is ignored as a technical phenomenon. Bulk density identifies a volume weight of soil compared with an equal volume of water. Hence a bulk density of 1.4, common to glacial till subsoils, means the soil in a cube foot or any other identified volume weighs 1.4 times an equal volume of water.

The range in soil bulk densities in northern plain prairie soils is from about .9 in undisturbed topsoil of a high organic matter level sod to about 1.5 in dense fine textured subsoils. By harvest, on cultivated land, settling, wetting, drying and traffic normally reduces topsoil bulk density to 1.3 to 1.35. Tillage loosens soil and reduces bulk density. Fresh cultivated land usually has a bulk density of 1.15 -1.25.

The effect bulk density has on water holding capacity is easily illustrated. The arithmetic for converting soil water content to inches of water in a 6-inch plow layer depth is as follows:

1. Inches water = % soil water x bulk density x soil depth inches

2. Untilled - - .15 water X 1.35B^d x 6" depth = 1.21 inches

Tilled - - .15 water X 1.15B^d x 6" depth = 1.03 inches

One of the reasons secondary tillage or packing is beneficial in seedbed preparation is to increase soil bulk density and soil water holding capacity in the seedbed. Similarly, packer wheels on seeding equipment increase bulk density and water holding capacity of soil surrounding the seed. Another, perhaps an equally important soil-conditioning factor with these operations includes reduced pore size to retard water evaporation loss rate.

Organic matter in soil systems also lowers bulk density. Organic matter fills voids that would otherwise allow collapse to higher soil densities. However, organic matter absorbs additional water and the two factors offset such that for practical purposes little water holding capacity change occurs unless extremely heavy organic matter levels are involved.

A great deal of what we know of water holding capacity and bulk density change with soil organic matter levels comes from mulch and manure additions not from soil organic matter loss. Examples are given in Table 4 and Figure 4.

(From: Salter and Haworth, J. Soil Science 12:335,1961)

In this study the site had received 20 tons manure per crop over a 6-year period. The site is extreme in that bulk density levels approach compaction levels found in roadbed construction. However, it demonstrates organic material additions can reduce bulk density and high rates absorb enough additional water to increase crop available water.

Perhaps more important the presence--or absence of organic matter in soil systems affects susceptibility to compaction. This is shown in Figure 4 where manure had been added to a loam soil at the rates shown for 25 years⁷.

As relative data a 1- % loss in organic matter from the plow layer calculates to about a 10-ton/acre-soil organic matter loss. There is substantial difference between1 0 tons of raw manure and 1 0 tons of material accumulated as soil organic matter.

One can speculate on the increased susceptibility to compaction with soil organic matter loss. Although exact effects are not known, there is little question soil organic matter loss over time has predisposed soils to increased compaction susceptibility in today’s production programs.

Cultivation to provide a combination of soil aggregate sizes that retard surface evaporation and pack well for seed-soil contact is desirable in seedbed preparation. In this regard germination is favored by a substantial portion of aggregates in the 1 -2.5 mm range. Cloddiness, or an excess of large aggregates, is a common problem in low organic matter fine textured soils and eroded areas where subsoil is exposed.

In corn production Johnson and Buchele⁴ found seedbed drying increased and emergence decreased as seedbed aggregate size increased from 1.2 to 8.5 mm.

A natural tendency during seedbed preparation on in fields prone to cloddiness is to increase tillage intensity. This has two detrimental effects in the northern plains.

1. Additional tillage dries seedbeds.

Aggregate size following tillage affects susceptibility to both wind and water erosion. In wind erosion aggregates larger than .84 mm are generally non-erodible. Siddowey⁵ reporting tillage study results, Table 5, from a silt loam in southern Idaho, show primary tillage tools leave soils in excellent shape with respect to non-erodible aggregates.

In this winter wheat production area ground worked 5 inches deep in the spring and rod weeded for weed control until fall seeding lost substantial aggregate structure to secondary tillage. These data identify the problem growers’ face between conflicting needs for small aggregates in seedbeds and large stable aggregates for erosion control.

Tillage operations and aggregate size have a measurable affect on soil temperature. This has technical significance in that experience has identified early seeding as a proven money matter in spring grain production. Heat retention when seed is placed in cold soil is vital for rapid uniform emergence.

Thermal properties of soil vary with its density and water content. Heat capacity is expressed by Larson⁶ in the following relationship.

C = 0.46 Vs + Vw

where

C = Heat capacity in calories/cm

Vs = Volume fraction solids

Vw = Volume fraction water

In layman terms this means coarse aggregates that won't pack well don't hold much water and have low heat holding capacity. Conversely, it means that if you want to hold heat next to the seed for rapid germination cover and pack seed with fine aggregates that will hold water. The heat holding capacity of water near the seed exceeds heat-holding capacity of solids in soil by the relationship of 1 to 0.46.

Tillage effects a number of soil parameters that in turn affect crop growth. Technically we have long recognized results on specific soil sites as good or bad but have had limited progress in identifying critical limits. Efforts have proceeded farther in identification of parameter and limits in erosion control than on other problems. However, seeding failures as soil change with time has begun to focus attention to stand establishment plant growth problems.

Larson⁶ identified limits for corn production as shown in Table 6. Compared with small grain production, corn and other row crops, are unique in that inter row spaces do not require the same tillage attention as the relatively narrow seedbed band. In recent years this has lead to ridge till planting. The concept of confined or controlled soil manipulation in a seedbed band extends to small grain production in paired row seeding. Technical opportunities in this area almost assure continued investigation as seeding problems intensify over time.