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Soil Fertility Implications When Converting to Direct Seeding | |
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Converting to a direct seeding system can change soil fertility characteristics because of reduced soil disturbance. You will need to adjust your fertilizer management practices to respond to any short- or long-term changes in soil fertility.
Direct seeding systems range from high disturbance, where the entire soil surface is disturbed, to low disturbance, where surface disturbance is minimal. Management practices for high-disturbance systems tend to be more similar to those for conventional systems while low-disturbance practices are less similar.
In general, when fields are converted from conventional tillage to direct seeding systems, several changes affecting soil fertility management tend to occur:
- soil nutrient availability may change;
- crops can have higher yield potentials; and
- nutrients may become concentrated in soil layers (stratified) with time.
The degree of change will be greater for low-disturbance systems than for high disturbance systems.
Soil Nutrient Availability May Change
The changes in soil fertility and nutrient cycling resulting from the move to direct seeding have not been fully explored and are under active research. However, some significant observations can be made.
Direct seeded soils are usually cooler and wetter in spring than tilled fields because the crop residue layer insulates the soil from the sun's heat and from drying winds. Some nutrients, including phosphorus (P) and potassium (K), are less available to crops under cooler temperatures.
However, mycorrhizae (associations between plant roots and fungi that improve the plant's uptake of nutrients, especially P) are more active on roots of direct seeded cereals; this activity improves P nutrition. Mycorrhizae appear to flourish in undisturbed soils.
When tillage is reduced or eliminated, a transition period may occur when nitrogen (N) is less available to plants. With less tillage:
- crop residues remain on the soil surface, which reduces contact with the soil organisms that decompose residues, and
- less oxygen is added to the soil (many decomposers need oxygen).
So with less tillage, crop residues (particularly those low in nitrogen like cereal straw) and soil organic matter are slower to decompose. Crop residue nitrogen stays in the organic form, which is unavailable to plants, for a longer period. However soil organic matter increases. This increase improves the soil's water-holding capacity, nutrient-holding capacity, tilth and ability to resist erosion. It also contributes to better long-term soil fertility.
Crops Tend to Have Higher Yield Potentials
Reducing tillage generally increases yield potential because the seedbed is moister, and insufficient moisture is the major limiting factor for prairie crop production. As well, direct seeding may be done early because of time saved by eliminating spring tillage, and early seeding often has a yield advantage on the prairies.
The key word, however, is potential. Nutrient management must be adjusted to take advantage of this potential or it will be lost. Table 1 gives a few examples of nutrient levels required to produce just 10 bushels per acre of four common crops. Even this low yield requires significant nutrient input. If sufficient nutrients are not available in the soil, then they must be added to reach the desired crop yield. For more information on crop nutrient removal, see Fertilizing Irrigated Grain and Oilseed Crops (Agdex 100/541-1).
Table 1. Nutrients contained in 10 bushels/acre of grain
Crop | Nurtient (pounds per acre) |
Nitrogen | Phosphate | Potash | Sulphur |
| Wheat (hard red spring) | 15.0 | 5.8 | 4.3 | 1.0 |
| Barley | 9.8 | 4.3 | 3.1 | 0.9 |
| Canola | 19.4 | 11.7 | 6.0 | 3.4 |
| Peas | 23.4 | 7.0 | 7.0 | 1.4 |
Nutrients and pH May Become Stratified With Time
Without tillage, some nutrients tend to increase in the near-surface soil and decrease in the lower layers. This stratification is most noticeable for less mobile nutrients such as phosphate, potash and copper. In most parts of Alberta, stratification is a minor concern.
After 15 to 25 years without tillage, the pH of the soil surface may decrease (become more acidic) depending on the specific fertilizer, lime and manure applications.
Nitrogen Fertilizer Management
Nitrogen rates, timing and placement
Nitrogen is the most limiting nutrient for cereal and oilseed production on the prairies, no matter which seeding system is used. Under direct seeding, extra N fertilizer may take advantage of the increased yield potential and offset any reduction in N availability during the transition period. Test your soil each year to determine the actual N fertilizer requirements. Remember that application of unnecessary N can delay crop maturity and increase lodging potential!
Fertilizer efficiency is key. Be aware that the fertilizer placement method affects the crop yield response to N fertilizer far more than the seeding system. If the placement method used under conventional tillage was less efficient than the method being used under direct seeding, then the N requirement may remain the same or even decrease. For efficient N fertilizer use in direct seeding systems, place N with the seed or in a band.
Banding reduces N losses from immobilization (use of N by soil organisms during decomposition), denitrification (breakdown of nitrate and nitrite to gaseous N) and volatilization (breakdown of urea to gaseous N). Spring and fall banding are usually similar in effectiveness. Spring banding can be better than fall banding in wet springs and falls, but a disadvantage in dry springs when the banding operation can dry the seedbed and reduce crop emergence.
Safe rates of seed-placed N fertilizer are critical. Excessive seed-placed N fertilizer damages crop seedlings directly through ammonia toxicity effects and through competition for soil moisture (salt effect). Too much N fertilizer placed too close to the seed can result in reduced crop emergence.
Many factors affect how much seed-placed N is safe. By far the most important factor is soil moisture. The higher the soil moisture, the more flexibility you have in seed-placed N rates. For example, the two cereal plots in Figures 1 and 2 are on similar soil types, and each had 100 lbs/acre of seed-placed urea applied with a narrow opener. Note how the plot in Figure 2 suffered severe crop injury because of a much lower soil moisture content at germination and emergence.
Figure 1. Crop with 100 lbs/acre of seed-placed urea and adequate soil moisture during emergence
Figure 2. Crop with 100 lbs/acre of seed-placed urea and inadequate soil moisture during emergence
Increasing the scatter of the fertilizer and seed (called seedbed utilization) reduces the potential negative effects of seed-placed N. Crop type is also important. Cereal crops are more tolerant of seed-placed N than broad-leaved crops.
Crop residues with higher N contents, such as legume residues, generally decompose quickly whether they are incorporated or not. Adding a legume crop in rotation may compensate for lower soil N during the transition period in N availability. However, most of the benefit of legumes to succeeding crops may not be due to N but to other factors like breaking disease cycles.
Effect of nitrogen placement on weeds
Research with barley has shown the benefits of banding N for the control of some weed species with shallower root systems, such as green foxtail. Crop roots can reach the banded N more easily than the shallow-rooted weeds, giving the crop a competitive advantage. Also, if less soil N is mineralized (changed to a form that plants can use) under direct seeding, then less N may be available for weed growth, giving the crop an additional advantage. In conventional systems when N is broadcast and incorporated, both the crop and shallow-rooted weeds are able to use the fertilizer, reducing or eliminating the crop's competitive edge.
More information
For more information on how N rate, timing, source and placement affect its efficiency, see the Alberta Fertilizer Guide (Agdex 541-1) and Don't Gamble with Fertilizer Rates! (available from Prairie Agricultural Machinery Institute PAMI). Use risk analysis to minimize yield loss with seed-placed N (see PAMI Interim Research Update 718, June 1995).
Phosphorus and Potassium Fertilizer Management
Phosphorus and potassium are less available under cooler soil temperatures. Thus, proper fertilization with these nutrients is important under the cooler soil temperatures inherent in direct seeding. P and K do not move readily in the soil, so placement with the seed or in a band near the seed is the most effective application method. This is especially true in cool, wet springs when root development is slow. In nutrient deficient areas, seed-placed P and K can greatly enhance early spring growth, which reduces weed competition and promotes early, uniform crop maturity.
Seed-placed phosphate is essential for most direct seeded fields. However, the optimal rate of phosphate does not necessarily increase with the move to direct seeding. Mycorrhizae help offset the effects of cooler soil in low-disturbance direct seeded fields. As well, research by the University of Saskatchewan shows that some plant-available P can be leached from undecomposed wheat straw. Thus, leaching of P into the soil may make P available to crops without residue decomposition. So leaving crop residues on the soil surface may have less impact on the cycling of P than on the cycling of N. This is an area that needs further study.
For cereals, safe rates of seed-placed P and K are not usually a concern because the rates normally recommended are not high enough to cause problems. For canola and other small-seeded crops, consult an agronomist for the maximum amount recommended for your particular situation.
You need a good understanding of P and K fertility to maintain optimum yields under direct seeding. For more information on this topic, refer to Phosphorus Fertilizer Application in Crop Production (Agdex 542-3) and Potassium Fertilizer Application in Crop Production (Agdex 542-9).
Sulphur Fertilizer Management
Sulphur (S) is mobile in the soil, so broadcast S is an option under direct seeding. Compared to N, there is not as much advantage in banding S fertilizer because it is not susceptible to volatilization losses (the breakdown to a gaseous state).
Sulphate forms of S fertilizer are immediately plant-available. Elemental S fertilizers must be converted to the sulphate form by soil microbes, so these fertilizers should be broadcast and mixed with soil. For the optimal release of sulphate from elemental S in a direct seeding system, surface broadcasting in the previous year followed by some incorporation during seeding is a good option. More research is needed on the efficiency of surface-applied elemental S in direct seeding systems.
Micronutrient Fertilizer Management
Micronutrient deficiencies, particularly copper, are a concern in many areas of Alberta. Direct seeding may restrict your application options for micronutrients. Along with their low application rates, most micronutrients (except boron) are also very immobile in the soil. Thus, it is very important to place micronutrient fertilizers where the crop can find them. Copper deficiency, for example, can be corrected with either soil or foliar applications of copper. Soil-applied copper fertilizers are more efficient when broadcast and incorporated, so correcting copper deficiency can be more difficult in direct seeding.
If you are concerned about micronutrient deficiencies, have your soil tested for these nutrients. If low levels of micronutrients appear to be causing serious yield reductions, consult an agronomist. For more information, refer to Micronutrient Requirements of Crops (Agdex FS531-1) and Copper Deficiency: Diagnosis and Correction (Agdex 532-3).
Other Fertilizer Management Issues
Soil testing
Nutrient stratification can affect the results from soil tests. Be sure to collect representative samples by:
- taking samples at 0 to 6, 6 to 12, and 12 to 24 inch depths (0 to 15, 15 to 30, and 30 to 60 cm depths), and
- sampling between and within fertilizer bands, and then blend the samples.
Compare this year's soil test results with results from previous years. If the results seem very different, consider retesting.
Soil acidity
After 15 to 25 years without tillage, the surface soil layer may become more acidic due to stratification. Where soils are naturally somewhat acidic, watch for any further increase in acidity. Increased acidity could limit crop selection. For example, wheat yields start to decline when the pH drops to about 5.5. A tillage operation or surface-applied lime will reduce the acidity from stratification.
Summary
Fertility management changes with a move to direct seeding because of changes in crop yield potential and soil nutrient availability.
Yield potential increases mainly because more water is available to the crop. Additional nutrients may be required to take advantage of the higher yield potential.
There may be a transition period in nitrogen availability when changing from a conventional to a direct seeding system. Monitor your soil fertility levels closely to find out if a nitrogen deficit is occurring. If it is, apply extra nitrogen.
Direct seeded fields are cooler in the spring. Nutrients such as phosphorus and potassium are less available under cooler temperatures, so more fertilizer and/or more effective placement and timing are needed.
Without tillage, nutrients may become concentrated in the near-surface soil over the long term. This outcome can affect soil test results. To collect representative samples, blend samples from within and between the fertilizer bands, sample as deeply as possible and average each sample in 6 inch (15 cm) increments. If the soil test results seem unusual, consider retesting.
More Information
For more information on fertility management, contact the Ag-Info Centre at 1-866-882-7677.
Prepared by:
Don Poisson, Elston Solberg and Murray Hartman, Alberta Agriculture, Food and Rural Development.
Source: Agdex 519-10. December 1996.
The contents of this page are no longer available.
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For more information about the content of this document, contact Murray Hartman.
This document is maintained by Ada Serafinchon.
This information published to the web on December 1, 1996.
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