Fertility Management for Conservation Tillage

Illinois Fertilizer Conference Proceedings
January 27-29, 1992

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Fertility Management for Conservation Tillage

Harold F. Reetz, Jr.¹

Introduction

Illinois leads the nation in conservation tillage. Our farmers are concerned about protecting their soil resources and reducing the potential for erosion contaminating surface water supplies. Improved field equipment, better pest control options, and improved management systems from research and farmer experience have all helped make this possible. Compliance with government program mandates and regulations will force further adoption of conservation practices in the next few years. The shift to reduced tillage, more crop residues on the surface, and changes in crop rotations have raised some new questions and problems relating to fertility management.

Build Fertility First

Farmers should consider taking care of any buildup fertilizer needs before switching to reduced tillage systems. Where serious nutrient deficiencies exist, it is usually important to mix those nutrients within as much of the root zone as possible. Deeper tillage provides best mixing. Moldboard plowing usually mixes deeper and more thoroughly than chisel systems. Chisel plows and field cultivators mix fertilizer materials about half the depth of the tillage. This will vary with different types of tillage implements.

If fertility is high, timing and placement of fertilizer are less critical. More options are available for supplying maintenance fertilizer, providing flexibility. A Minnesota study (Table 1.), evaluating placement systems for ridge planted and chisel plow systems on corn, demonstrates the value of building soil test levels before changing tillage systems. With high fertility, yields were higher in both systems, and with all placement options. There was little difference among the placement options. With low fertility, placement of the fertilizer in a subsurface band was beneficial, although yields were still lower that the high fertility treatments.

The cost of buildup fertilizer should be considered a capital investment to be amortized over a period of 5 years or more. In most cases, the buildup from a medium to a high soil test level will increase yields enough that the cost will be recovered in the first 2 to 3 years. The benefits continue to accrue as long as the soil test is maintained at the higher level. So the cost of NOT building soil tests, as measured in lost opportunity for higher yields and profits, should also be considered in evaluating the economics of building soil fertility.

Stratification of Nutrients

Under reduced tillage, nutrients tend to concentrate in the top 3 to 4 inches of the soil profile. This is particularly true of potassium and phosphorus which are relatively immobile in the soil. Fertilizer applied to the surface moves down very slowly, so over time it accumulates in the top part of the root zone. Nutrients taken up by the plant from deeper in the root zone are also left on the surface in crop residues. As those residues decompose, the nutrients are released at the surface and move only slowly into the soil.

Since there is little downward movement, there is not much opportunity to replenish the nutrients removed in the lower part of the root zone, so the nutrient content of that area decreases, while the accumulations near the surface result in increased nutrient content of the upper part of the root zone. The net effect is a stratification, or layering, of nutrients in the soil. The amount of stratification for each nutrient will depend upon how much each individual nutrient actually moves in the soil. Potassium and phosphorus move very little in most soils, so they will be more likely to remain concentrated near the surface and become depleted lower in the root zone. Nitrogen moves readily in the soil, so it will usually be moved lower in the root zone as water moves downward.

Long-term tillage studies throughout the Midwest show this stratification to be common on many soils, but it is usually not a major deterrent to reduced tillage because the roots are usually more concentrated near the surface in reduce tillage systems, and therefore better able to utilize the shallow supplies of nutrients. Since water is also more likely to be available near the surface in reduced tillage systems, another factor for efficient use of shallow nutrients is taken care of.

As a result, reduced tillage systems do not usually suffer yield loss from the stratification of nutrients. A study at Purdue University that has been in place for nearly 20 years shows that comparable yields can be obtained under a variety of tillage systems---moldboard, chisel, no-till---if plant populations and pest management are adequate.

Starter May Be More Beneficial

In reduced tillage systems, there is usually a greater chance for getting a response to starter, fertilizer. Extra residue left on the surface helps hold soil in place and increase infiltration of water, but it also reflects sunlight and reduces evaporation, so the soil tends to be cool and wet longer into the growing season. Such conditions increase the response to starter fertilizer.

Twenty years ago, researchers and Extension specialists were telling farmers they did not need starter fertilizer , especially on dark-colored soils, if soil tests were in the high range. Under those conditions, that was sound advice. But conditions have changed as farmers have adopted conservation tillage practices. Even the dark-colored soils dry out and warm up more slowly when crop residues are left on the surface. The root system develops more slowly under cool, wet conditions, so nutrients and water must be supplied by a limited number of roots. Starter fertilizer provides a concentrated supply of nutrients that help meet the crops needs until the root system can develop. Starter fertilizer is now more likely to show a yield response than it was 20 years ago, because we have changed the root environment through conservation tillage. Recent research from the University of Wisconsin (Table 2.) shows that the benefits of starter fertilizer (251b/A P₂O₅ + 25 lb/A K₂O) may actually be greater for late May planted corn than for late April planting dates, especially for no-till systems. Starter fertilizer increased plant height and lowered grain moisture at harvest time. Of 24 different treatments in the study, 20 resulted in increased yields, and 19 showed increased profits from starter fertilizer. (University of Wisconsin soil scientists recommend starter fertilizer for corn under most situations. Response is more likely in Wisconsin than in Illinois due to the cooler climate.)

The yield response for later planting dates was greatest under reduced tillage systems. This is likely due to the greater difference in soil temperatures at the later dates between conventional and reduced tillage treatments. Yields were normally depressed for later planting dates, but not as much where starter fertilizer was used.

Teamwork in Conservation Plans

As farmers work toward meeting conservation compliance mandates and regulations, they should develop a team approach. The farmer, any landowners involved in the operation, the local Soil Conservation Service staff, Cooperative Extension staff, and the fertilizer dealer should all be involved in the planning wherever possible. When all of these people work as a team to develop the plan, their expertise and experience can be drawn upon to make sure the plan includes the best management practices for the individual fields involved.

The farmer and landowners ultimately must make the decisions on which practices will be adopted. The fertilizer dealer probably has the most detailed input into the final plan. The role of SCS and CES staff is to provide technical assistance, but they cannot possibly provide detailed site-specific management plans for each farm in their area. Reduction in force in both SCS and CES will make this type of involvement even more difficult in the coming years.

Agronomic consultants will probably play a greater role in farming decisions in the future. These may be consultants working in conjunction with the fertilizer dealer, or they may be independent consultants. In either case, the consultant can provide assistance with soil testing and interpretation, with pest management decisions, and a variety of other technical support. A local consultant can offer more individualized assistance than the SCS or CES staff, because the consultant will have a smaller number of clients. The farmer's own experience and expertise will help determine the role for the consultants he hires.

Summary

Planning fertility management for conservation tillage requires a systematic approach. Changing tillage changes the environment in which roots will grow and in which soil nutrient reactions take place. A team of local support people should be used in working out the details of a conservation/fertility management plan, taking advantage of the expertise and experience available to develop a program that is agronomically sound, economically efficient, and environmentally responsible.