2002 Table of Contents

Nutrient Cycling and Management in Direct Seeding Systems

Jeff Schoenau
Senior Research Scientist and Adjunct Professor
Department of Soil Science, College of Agriculture
University of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N 5A8
schoenau@sask.usask.ca

Background

In a direct seeding system, seeding and fertilizer application are achieved in a single pass across the field without any pre-seeding tillage or soil disturbance. Depending on the type of boot or opener system used on the implement, the direct seeding may be considered as high disturbance or low disturbance. High disturbance direct seeding often involves the use of sweeps or shovels which result in some residue burial during the seeding operation but also achieves partial or complete weed control during the seeding operation. Low disturbance direct seeding, sometimes referred to as no-till or zero-till, utilizes an opener system such as discs or knives which create very little soil disturbance and leaves much of the residue intact on the soil surface, but control of weeds is reliant upon pre-seeding herbicide application.

In the past, fertilizer application in a direct seed system was made either in the seed-row along with the seed or else broadcast for higher rates of nutrient. This posed limitations, especially in low disturbance direct seeding with knives or discs, because of the limited amount of fertilizer that could be safely placed in the seed-row. Broadcasting of fertilizer was not an efficient alternative, due to high gaseous losses of nitrogen, nutrient tie-up during surface residue decomposition, and stranding of nutrient in the dry surface layer of the soil. Advances in product delivery (pneumatic) and opener (double shoot) systems now allow for simultaneous placement of fertilizer in a channel or furrow separate from the seed-row, thus reducing or eliminating risk of fertilizer burn as well as placing the nutrient in the mineral soil for good access by crop roots. As such, it is now possible to seed, seed-place and side or mid-row band fertilizer in a single pass. Various configurations of this type are used successfully in the northern Great Plains for seeding cereals, oil-seeds, grain legumes and forages directly into high residue, standing crop stubble fields.

A successful direct seeding system requires consideration of soil fertility in both the short - term (how to get the most efficient crop use and response from nutrients this year) and longer - term (how to maintain or build soil fertility over several years). To this end, two goals in nutrient management in direct seeding systems may be put forward:

1. Place the fertilizer nutrient for maximum crop response and nutrient recovery.
   
2. Promote nutrient recycling and enhance soil nutrient supply power.
   

Placing the Fertilizer Effectively

One of the main differences between a direct seed system and a conventional tillage system after a few years is the formation of a "thatch" or "duff" layer on the surface of the direct-seeded soil. The lack of incorporation of crop residues, especially evident in low disturbance direct seeding, slows decomposition of the residue and allows their accumulation on the surface of the soil. This is an important factor contributing to the build-up of organic matter in direct seeded soils. The surface thatch layer also reduces evaporation of water, keeping the soil more moist and acts as an insulating layer, moderating temperature fluctuations. In the Canadian prairies, the insulating effect of the thatch layer results in slightly cooler (~0.5 to 1.5 oC) surface soil temperatures in direct-seeded fields in early spring as compared to conventionally tilled fields with low surface residue. The above factors lead to the following recommendations for effective fertilizer placement to maximize crop response and nutrient recovery in a direct - seed system.

1. Put the Fertilizer Nutrient Into the Mineral Soil Beneath the Thatch Layer

Separating the fertilizer from the surface thatch will reduce the potential microbial tie-up (immobilization) of nitrogen, phosphorus and sulfur fertilizer that would occur if the fertilizer was placed directly in the surface thatch. With deeper placement, volatile ammonia losses from urea and anhydrous ammonia will also be reduced as will competition for nutrients from shallow-rooted weeds. Under dry conditions, placement into the moist mineral soil beneath the thatch improves root distribution and access to nutrient, especially for immobile nutrients like phosphorus and potassium.

2. Small Amounts of Starter Nutrient Placed in the Seed-Row Can Be Beneficial

With spring seeded crops, the slower warming up of the soil in a direct seed system can increase the potential response from phosphorus and potassium fertilizers placed close to the seed. This "starter" effect is important to ensure good early P and K nutrition under conditions where cool temperatures can restrict root growth and access to nutrients. However, to avoid crop injury from too much fertilizer placed in the seed-row, one must pay attention to seed-bed utilization (SBU): opener spread / row spacing. SBU affects the amount of fertilizer that can be safely placed in the seed-row. A low disturbance seeder configuration such as using knife openers with a 1 inch spread and 12 inch row spacing (SBU = 1/12 or 8%) means that less fertilizer can be safely placed in the seed row than a high disturbance configuration such as using sweeps (3 inch spread) and 8 inch row spacing (SBU = 3/8 or 38%).

3. Placement of Fertilizer in a Band Increases Availability and Crop Utilization

Having nitrogen fertilizer such as urea placed in a concentrated band in the soil can increase crop use efficiency of the nitrogen by slowing the conversion of ammonium to nitrate, as nitrate is the leachable form of soil N and is also susceptible to gaseous losses (denitrification) under poorly aerated conditions. In prairie soils, substantial amounts of fertilizer N may be lost by denitrification during spring snow melt and run-off as well as saturation following heavy rains. Fixation of phosphorus and potassium fertilizer is also reduced by placement in a concentrated band, as this reduces soil - fertilizer contact. For phosphorus and potassium, the fertilizer band should not be too far away from the seed-row as these nutrients have limited ability to move. Placement of P and K in the seed-row or in a separate band 1" below and 1" side of the seed-row are effective placements to ensure early crop access to these nutrients. Nitrogen, on the other hand, has high mobility in the soil and can move long distances with water drawn towards the roots. Reflecting this, the first year of a three year study underway in Saskatchewan (four sites with three different crops: flax, canola and wheat) has shown no large differences in agronomic performance of fertilizer N (urea and anhydrous) placed 1" below and 1" to the side versus placed mid-way between every second seed-row.

Promoting Nutrient Recycling

A question that is sometimes brought forward by producers regarding direct seeding systems is the effect of tillage elimination on the recycling of nutrients from crop residues and soil organic matter. Research on this issue in Western Canada has indicated that tillage is not a prerequisite for effective short-term recycling of nutrients from residues and soil organic matter back into plant available inorganic forms the crop can use. Over the long-term, increased surface residue cover protects the soil from nutrient losses by erosion. Reduced erosion along with increased soil organic matter content are likely the main agents responsible for observed increases in nutrient supplying power after several years of direct seeding.

Short -Term Effects

A landscape scale study of the short-term effects of eliminating tillage in a wheat-fallow rotation in south-western Saskatchewan revealed no significant differences in soil supplies of available nitrogen between the tillage fallow and no-tillage fallow treatments in years of below - average and average precipitation. However, in very wet years, lower nitrogen availability was observed in the no-till fallow. This likely reflected greater losses of nitrate by denitrification in the no-till fallow in the wet years, as the greater water retention in the no-till fallow resulted in saturated soil conditions for a longer duration. An investigation of the effect of eliminating tillage in a cereal - pea rotation revealed no significant differences in supplies of available nitrogen measured in a wheat crop seeded on no-till versus tilled pea stubble. In the same study, supply rates of available phosphorus measured in - field using PRSTM probes were significantly higher under no-till than tillage treatments, likely due to greater moisture content in the no-till soil. Rapid recycling of phosphorus, potassium and sulfur from surface residues by simple physical leaching of the nutrients from the residues into the mineral soil beneath was also identified as an important recycling mechanism in the direct-seed systems evaluated.

Long - Term Effects

An increase in the ability of soils to release and supply available nitrogen after several years of low disturbance direct seeding has been noted in several studies in Western Canada. This enhancement in soil nutrient supply power was observed to be most pronounced when the implementation of the direct seeding system was also accompanied by reduction of fallow frequency, use of fertilizers at recommended rates and inclusion of legumes in the crop rotation. Reducing erosion and enhancing the mineralizable component of soil organic matter through direct seeding are attributes that work towards enhancing nutrient recycling and soil fertility in direct seed systems.

A Look Ahead

Understanding the processes affecting nutrient use efficiency and cycling is important in development of appropriate nutrient management strategies for a system such as direct seeding. As these processes are often soil, climate and crop specific, these relationships must be also be revealed to make recommendations that are appropriate and accurate for specific regions. The inherent efficiencies in retaining nutrients in a direct seeding system through reducing erosion and building soil organic matter have been complemented in recent years by the development of equipment technology that can effectively place the fertilizer nutrient for improved crop recovery. This makes direct seeding systems attractive not only from a soil and water conservation standpoint, but nutrient conservation and efficiency as well. In Western Canada, technology has recently been developed and successfully used to inject liquid manure into the soil with minimum disturbance using a coulter system. The low disturbance injection of liquid manure is superior to surface application in nutrient retention and crop recovery. Development and implementation of this technology was driven in part by direct seeding Prairie farmers who wished to efficiently and responsibly utilize manure nutrients produced by an expanding swine industry in Western Canada. As such, this represents another example of working towards the goal of placing nutrients effectively and promoting nutrient recycling in a direct seeding system.

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