Agronomy Library > Soil Conservation

Fertility Issues and Long-Term No-till
Author: Adrian M. Johnston, Plant Nutrition Institute
Date Created: March 07, 2005
Last Reviewed: February 01, 2007

Direct-seeding systems are the most rapidly evolving tillage practice in western Canada, with an estimated 63% of crop area in 2001 seeded with minimum or no-till in Alberta (Statistics Canada, 2002). Because direct seeding encompasses a broad range of seeding and fertilization methods, questions have arisen regarding the long-term impact of reduced tillage on soil fertility. The impact of tillage on soil nutrient levels is largely determined by the nature of each specific macronutrient, and the impact that the changes in the soil environment have on transformations of the nutrient. Nutrients like nitrogen (N) and sulphur (S) are mobile in their dominant soil forms, resulting in the lack of soil mixing having little impact on availability to plants. Less mobile nutrients like phosphorus (P) and potassium (K) have the potential to become stratified near the surface of the soil. Fertilizer placement of these less mobile nutrients can become critical to plant uptake and yield optimization. Current soil testing methods are being challenged to consider changes necessary to account for the evolving crop production and soil management practices, in particular direct seeding and increased crop diversity in rotation. The ability to estimate soil N supply with current soil testing methods appears to be limited in some situations and innovative changes to soil testing will be required to improve the ability to predict soil nutrient supply rates. An understand the how nutrients react in the soil, and are impacted by changes in tillage practices, is critical to ensuring that crop production goals can be achieved by balancing soil and fertilizer nutrient management.

Soil Changes In the Absence of Tillage
Likely the most serious consequence of the conventional tillage practices used in western Canada has been the accelerated breakdown of soil organic matter, and the resultant depletion of nutrients available to plant growth. In addition, soil structure and tilth were negatively impacted by excessive tillage, which in some cases changed the soils water holding and infiltration capacity. Moving to no-till has been found to increase the amount of surface soil organic matter and soil N content relative to conventional tillage. However, crop residue placement on the surface of the soil also affects soil temperature, water entry into soil, evaporation loss of water from soil, and as a result the distribution of nutrients in the soil. The retention of surface crop residues also minimizes soil loss by wind and/or water erosion, and the loss of the nutrients in that soil. Where sufficient water exists, soil structure can be improved with the removal of tillage fallow from the rotation and the continuous cropping of lands using no-till. In the absence of fallow to mineralize soil nutrients, fertilizers and manure have been used to meet nutrient deficiencies, promoting both higher yields and crop residues to further protect the soil.In summary, adoption of no-till has had its greatest impact in semi-arid regions. Removing tillage allowed for the retention and conservation of more soil water, allowing the successful establishment of crops each year, which in turn has provided an opportunity for more crop diversity to be introduced into the region, having positive impacts on whole-farm economics. This is clearly demonstrated in a recent summary paper by Dr. Con Campbell, formally of the Swift Current Research Station (Campbell et al. 2002). In a review of agricultural production statistics in western Canada he found that while the area seeded to cereals had not changed, a large reduction in summer fallow area has come about as a result of the expanded area in oilseed and pulse crops in rotation. It is fair to say that this enhanced diversification of the farming system can be largely attributed to the adoption of water conserving, no-till seeding systems.

Understanding Soil Nutrients
The total carbon (C) and N found in soils is critical to the soil tilth, physio-chemical properties, microbial biomass, water holding capacity, and the list goes on. However, when it comes to plant-available N in soil it is the potentially mineralizeable N, or ‘active’ fraction of soil organic matter, that is of interest to us. A reduction in tillage generally has been found to increase this active fraction of soil organic matter and plant-available N (NO3-N and NH4-N) after an initial transition period following adoption of no-till. During this initial transition period plant-available N is often actually lower in no-till as the soil microorganisms adjust (immobilization) to the surface placement of crop residues, rather than soil mixing. It is during this transition period that band placement of nutrients below the residue-covered surface becomes so important.

After a few years the N immobilized in the surface residues begins to be released (mineralization) at a rate faster than the soil microorganisms can immobilize it again.The majority of soil N is in the form of NO3-N, which is soluble in soil water and moves with water in the soil. As a result all of the N that accumulates at the soil surface in the soil organic matter will not become stranded and unavailable to the growing crop. The same applies to the plant-available form of sulphur, SO4-S. Soil P and K are less mobile nutrients due to their reaction with soil minerals (Ca and Mg), and/or soil charge (cation exchange capacity). This may result in an accumulation of these nutrients at the soil surface (0-2”) in the absence of soil mixing by tillage. An understanding of how nutrients move and react in the soil is critical when planning fertilizer additions to correct nutrient deficiencies. While N and S may be applied in random bands in the soil due to the nutrient mobility in soil water, the placement of P and K close to the developing root system is critical for early season uptake by the developing seedling.

Research Results
Soil pHWhen soils increase in organic matter, or carbon, content there is a potential decline in soil pH that can occur. However, the impact is going to be greater on those soils that start out with a low pH reading, and those soils that have the largest increase in soil organic matter due to management. In a review of trial results in western Canada only one reference showed a significant decline in soil pH as a result of long-term no-till management (Arshad et al., 1990). This gray wooded soil in the Peace River region showed a 26% increase in soil carbon (organic matter) after 10 years of no-till management, relative to the conventional till comparison. In this situation the soil pH decline was 0.5 units, a small but significant effect. In general, changes in soil pH with no-till have been minor to nonsignificant.

Soil Phosphorus and Potassium
Some of the original research evaluating nutrient dynamics under no-till was carried out in the corn belt of the USA. It is in this part of the world that most of the fertilizer N was broadcast applied prior to spring tillage. The move to no-till produced some challenges with surface applied N, increasing the potential for loss by both immobilization in the surface residue and volatilization (gaseous loss of ammonia) when urea was the N source. However, the benefits of in-soil banding were quickly documented and adopted to deal with this N management challenge. In this region N is usually added to meet crop demand each year, and they generally fertilize the ‘bulk’ soil with P and K in an attempt to bring soil test levels up to that required for optimum yields. Being that P and K are less mobile in the soil, there was a concern that these nutrients would accumulate in the surface soil horizon. Well, in fact research at Purdue University did find that P and K were considerably higher in the surface 3” of no-till treatments after 7 years, while the plowed treatments had uniform nutrient levels down to 9” depth (Mengel 1984). However, given the density of corn roots in this soil layer, Mengel noted that “The stratification may not be as big a problem as first thought, since by concentrating roots in zones of higher fertility, nutrient availability may increase”. A similar scenario exists for the root development of crops on the Canadian prairies, and the maintenance of surface residues keeps moisture available in these surface soils.

Dr. Fernando Selles from Swift Current conducted some research on acidic Brazilian soils evaluating the effect of tillage on the distribution of soil phosphorus (Selles et al. 1997). While these soils are quite different from our alkaline (higher than pH 7.0) soils in western Canada, the results provide some interesting conclusions to ponder. After a period of 5 years they found that moving to minimum or no-till increased total soil P in the surface 4” by 15%, relative to the conventional tillage, which included plowing to 8”. In the no-till treatment the concentration of plant-available forms of P in the surface 2” was highest, decreasing rapidly with depth. They explained that this could account for the increased plant uptake of P by crops grown in no-till, and the high levels of P being picked up by soil testing. In fact, they suggest that the traditional methods of soil testing, developed under conventional tillage methods, were likely inappropriate for the change to no-till.

Once back from Brazil, Selles evaluated soils for P distribution under tillage studies at Swift Current (Selles et al. 1999). Like Brazil, they found that after 12 years, converting from conventional till wheat-fallow to no-till continuous wheat resulted in an accumulation of plant-available P in the surface 2” layer. However, this was not the case for the no-till fallow-wheat, or the conventional till continuous wheat. They attributed this specific treatment change to the accumulation of surface residue and lack of decomposition in no-till. However, it is important to note that in this study, where 15 lb P2O5/A were seed placed each year, the increased soil P in the surface of no-till continuous wheat fields did not result in increased plant uptake of P. The authors attribute the lack of any difference to the use of starter P at seeding, and the slow release nature of the soil P in the cool spring soils. However, an increased soil supply would be of benefit to mid-season uptake of P by a high yielding crop. Once again they noted that this change in surface soil P would have an impact on soil sampling, and also that it could negatively influence water quality if soil moved from the field into a water body.

In the Black soil zone work has been carried out by Malhi in Alberta (Malhi et al. 1992), and by Dr. Cynthia Grant of Brandon at both Brandon and Indian Head (Grant and Bailey 1994; Grant and Lafond 1994). At Indian Head they found no effect of tillage system after 4 years of a rotation by tillage study in the distribution of P and K in the surface soil profile. With samples collected from the 0-2”, 2-4”, and 4-6” depths, no difference in P and K level in the soil was observed between conventional-, minimum- or no-till. In all cases the N was mid-row banded on 16” centers, while the P, K and S fertilizers were placed in the seed row spaced on 8” centers. In a tillage study near Vegreville, AB, Malhi also found no difference in soil P and K after 8 years. In the Vegreville study soil samples were evaluated for the 0-6” depth, without being divided into 2” increments like at Indian Head.

At Brandon, Grant and Bailey found that after 4 years they could pick up an accumulation of P at the depth of banding under both conventional and no-till, and on sandy loam and silty clay soil types. At this location they used 1” sampling depths down to 6”. The P accumulated at the 4” depth where it had been banded. They attributed this accumulation of P to the repeated application of the N+P bands in this study. In the sandy loam soil the concentration of P at the banding depth was higher with no-till than conventional-till, something they attributed to mixing on this soil type. Soil K levels were found to be higher under no-till in the 0-6” depth on the sandy loam soil and 0-1” depth in the silty clay soil. The increased movement of K relative to P in the sandy soil is illustrated with the increased depth that K was increased. They attributed the retention of K near the soil surface to lack of mixing of the crop residue in no-till.

Finally, an Alberta project that evaluated the response of forage grass to long-term fertilizer management found that while surface soil pH decreased with increasing N rate (> 100 lb N/A/yr), and the effect of N rate on the soil pH declined with soil depth (no change below 2 inches) (Malhi et al., 2001). Only at the highest N rate of 300 lb N/A did soil pH show any change below the 4-inch depth. Soil test P concentration in the surface soil were unaffected by annual application of fertilizer P at rates equivalent to crop removal (34 lb P2O5/A/yr). When fertilizer P rates were applied at twice crop removal, soil test P more than doubled (15 to 34 ppm) in the surface 2 inches of soil. No impact was measured below that depth.

In summary, it would appear that the P and K do tend to accumulate near the soil surface of no-till treatments, relative to conventional tillage. Could this pose a problem to future production on these soils remains uncertain. The accumulation of surface crop residues does an excellent job of maintaining higher soil moisture levels at the soil surface. This will keep roots active and in a position to access these accumulated nutrients. However, under drying conditions, a deficiency of P or K may mean that plants cannot access these surface nutrients. This may place an increased importance on in-soil band placement of these nutrients.

Nitrogen and Sulphur
At Brandon, Grant and Bailey (1994) found that the concentration of NO3-N was higher in the 0-3” depth for no-till than conventional-till on the fine sandy loam soil. They speculated that this increase might be due to the release of N from organic matter residues retained near the soil surface in the absence of tillage, and/or fertilizer N under the dry conditions of the project. A similar pattern was observed on the silty clay soil, however, only in the surface 1” was the NO3-N level actually higher for no-till relative to conventional-till.

At Indian Head, Grant and Lafond (1994) reported that total N was higher in the 0-2” depth with no-till and minimum-till on this heavy clay soil. A similar trend was observed at the 2-4” and 4-6” depths, however they were not significantly different relative to conventional-till. They speculated that the reduced soil mixing under minimum- and no-till, compared to conventional tillage, would lead to an accumulation of organic N materials near the soil surface. There was a numerically higher accumulation of NO3-N under conventional-till than minimum- or no-till in the surface soil, however, this was not significantly different. The tillage associated with conventional tillage would be expected to mineralize more of the soil N relative to minimum- and no-till.

In the Alberta study, Malhi et al. (1992) reported that there was no impact of tillage system on soil NO3-N levels down to a depth of 24”. However, fertilizer N rate was used in this study, and increasing N rate did result in increased soil NO3-N content. An evaluation of SO4-S level in these soils also found no effect of tillage on S concentration or distribution in the soil. At the Indian Head site tillage was again not found to have any effect on SO4-S concentration in the soil. However, there was a minor increase in SO4-S in the 6-12” depth with conventional-till relative to minimum- or no-till (Grant and Lafond 1994).

An interesting project at Swift Current evaluating crop response to tillage on three different soil types found significant effects of tillage on soil N supply (Campbell et al. 1995; McConkey et al. 1996). On a low fertility fine sandy loam soil they found higher soil-test N with no-till in both the fallow-wheat and continuous wheat rotations in the 0-3” soil layer. While significantly different only in the fallow-wheat rotation, this increased soil N supply would lead one to believe that there was potential for improved yield and quality benefits from adoption of no-till. However, this was not the case, and rarely did the no-till system provide for increased yield or grain N (protein) content. In fact, on the silt loam and heavy clay soil sites the grain N content was frequently lower with no-till relative to conventional tillage. While this difference could be explained in some instances with higher yields with no-till (grain N dilution in more yield) or lower spring soil-test N, frequently it could not be related to these factors. Similar results have been observed on a Dark Brown soil at Scott Experimental Farm (Brandt, personal communication). The authors concluded that this loss of N could not have been due to immobilization in the additional surface crop residues with no-till. Given that this impact on grain N was observed in years of above-average growing season precipitation, one could speculate that the increased soil N supply in no-till systems was lost by denitrification in the spring of the year.

Implications for Fertilizer Placement
Fertilizer management with reduced and no-till seeding requires careful attention to placement to optimize efficiency of fertilizer-use by the crop. There is little doubt that broadcast application of nutrients onto the residue covered soil surface is not a realistic management option in most instances. Early season broadcast application of N onto winter cereals when air temperatures are low is an exception. However, as temperatures rise the potential for loss of urea N applied in this manner increases. In-soil band placement of N is likely the most effective means of minimizing any immobilization of N in no-till fields. Similarly, the application of P and K in bands either with, or close to the seed minimizes tie-up by the soil and increases early season uptake by the crop.

One of the challenges facing no-till farmers is fertilizer placement. When side banding is not the choice of seeding opener, the placement of starter P, K and S fertilizer with the seed can often be limited by the width (spread) of the opener selected. Limiting the amount of these nutrients can often reduce the response to applied N, the result of an imbalance between these macronutrients. In the absence of building soil P and K levels prior to adopting a no-till program, deficiencies in these nutrients should be addressed with low disturbance banding operations carried out independent of seeding. It is important to remember that implementing the best conservation tillage practices on your farm will not make up for deficiencies, or imbalances, in soil nutrients required to optimize crop yield and quality.

Soil Testing Issues
Soil testing is our best available tool to establish some estimate of soil nutrient levels. However, the results of soil testing should not be used alone, but rather along with details on the fields history (last years yield, previous fertilizer management, manure application, forage crops grown, legume crops grown, etc.) and the next year’s crop production plan (crop type, yield goal, etc). Much of the information currently used to estimate crop requirements based on soil test level was established 20-40 years ago. While these databases are updated when information is available by the labs, the impact of changing management practices may also be influencing the results we obtain from soil testing. Recent research results comparing soil testing with crop nutrient uptake is challenging the nitrate-N test as a good indicator of soil N supply on higher organic matter soils. So do not be surprised if you begin to read and hear about research trial results that are addressing this issue in the near future. While the soil testing methods we are now using are the best tools we currently have, new and innovative means of evaluating soil nutrient levels will be available commercially once supported by research results. An example of this is the Plant-Root Simulator probes, marketed by Western Ag Innovations. This is the first commercial system of estimating soil nutrient supply rate available to farmers.

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Campbell, C.A., Zentner, R.P., Gameda, S., Blomert, B, and Wall, D.D. 2002. Production of annual crops on the Canadian Prairies: Trends during the last two decades. Can. J. Soil Sci. 82: 45-57.
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Mengel, D. 1984. Quote in article “Tillage for the times”, In IMC and Agrichemical age explore series. Agrichemical Age, California Farmer Publishing Co.
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Statistics Canada, 2002. Census of agriculture. On line data on October 25, 2002 found at: