Soil properties currently limiting crop yields in Swedish agriculture – An analysis of 90 yield survey districts and 10 long-term field experiments

https://doi.org/10.1016/j.eja.2020.126132Get rights and content

Highlights

  • Among manageable soil properties, soil pH and extractable soil P had the largest impact on Swedish crop yields.

  • Current recommendations on soil pH and soil P require upward adjustment.

  • Surprisingly, increasing soil organic matter (SOM) reduced yields, most probably due to lower pH in soils richer in SOM.

Abstract

This study evaluated yield statistics and data from a comprehensive national soil inventory representing 90 yield survey districts and from the Swedish long-term soil fertility experiments. The aim was to identify the most yield-limiting soil properties and best agronomic practices in order to further improve yield. Principal Component Analysis, multi-variate analysis, step-wise regression, and tree partitioning analysis identified the same variables affecting yields in the national and experimental datasets. Crop yields were significantly affected by soil pH, soil organic matter (SOM), plant-available soil phosphorus (P-AL) and mean annual temperature. Contents of plant-available potassium and magnesium in soil had no significant impact on yield, except in potatoes. Soil pH was found to have the greatest potential to affect crop yields, even at values >6.5 (pH(H2O)). Soil organic matter ranging from 3 to 6% in Swedish arable soils had an indirect negative effect on crop yields by lowering soil pH values with higher SOM content. To fully exploit the known benefits of SOM, liming requires more attention. One important finding was that current Swedish agricultural recommendations require updating. High-yielding crops demand more plant-available soil P and a range of 60−100 mg P-AL kg−1 soil is needed for sufficient P supply. A new target value of pH 7 for all crops except potatoes is recommended.

Introduction

Crop yield is of fundamental importance for food security. Yield increases must keep pace with population growth in order to provide sufficient food and avert famine (Malthus, 1798). The expansion in the global population from 1.6 billion people in 1900 to 7.8 billion today was possible through industrial synthesis of ammonia and use of inorganic fertilizers and lime leading to higher crop yields (Smil, 2000). Plant breeding has also improved the yield potential of crops and new varieties are the driving force behind current yield increases (Foulkes et al., 2007). Continuing yield increases are vital in many parts of the world due to population growth (Lobell et al., 2009; Godfray et al., 2010).

It has been observed in many European countries that yield increases are stagnating, as indicated by FAO statistics (http://faostat3.fao.org/home/E). For example, Wiesmeier et al. (2015) analyzed FAO data for three of the most important crops world-wide (wheat, barley, and maize) and found that production in both Central and Northern Europe levelled out after a breakpoint in the early 1990s. Several reasons for this have been proposed in different countries: Denmark: Regulations limiting nitrogen (N) application to crops and growing more wheat after wheat in rotations (Petersen et al., 2010). Finland: Environmental programs to reduce fertilizer inputs and nutrient emissions, compared with the modest economic incentive to increase production due to low prices for wheat (Peltonen-Sainio et al., 2009). France: Reducing N fertilizer rates, growing less legumes in rotations, experiencing high temperatures during grain filling, and drought during stem elongation (Brisson et al., 2010). Germany: Agricultural policies and economic reasons to reduce nutrient supply to crop (Laidig et al., 2014). Switzerland: Agricultural policies for adoption of extensive farming, lowering the input of mineral fertilizer (Finger, 2010). In summary, agricultural policies aiming at lowering environmental impacts by limiting nutrient input are a main reason for the stagnating yield increase in European countries.

Unlike in several other European countries, application rates of mineral fertilizer are not legally limited in Sweden, although animal manure application is limited to a maximum of 170 kg total N ha−1 in vulnerable areas (Jordbruksverket, 2015). Inputs of mineral N fertilizer have steadily increased in Sweden (SCB, 2016a), accompanied by steady yield improvements (SCB, 1982-2010). However, total crop production has been reduced due to large-scale transformation to organic farming reducing yields significantly (SCB, 2016b). Instead of N fertilizer regulations, measures to reduce discharge of nutrients from arable land to the surrounding environment have been prioritized in Sweden. These include farming more autumn-sown crops, subsidies for catch crops, creation of wetlands and buffer strips, use of N sensors and/or crop satellite images for split N application, etc. (Bergström et al., 2015). These measures have succeeded in reducing N leaching losses, as shown by long-term environmental monitoring (Fölster et al., 2012).

Short-term field trials are useful when investigating the effects of soil management, fertilizer treatments, or crop varieties on crop yields (Bootsma et al., 2005; Speirs et al., 2013). Long-term field experiments are required when analyzing soil properties requiring decades to change (Powlson et al., 2014; Ridley and Hedlin, 1980), such as SOM, pH, and plant-available phosphorus, before affecting soil functioning (Bai et al., 2018; Bünemann et al., 2018). National soil-monitoring programs and databases created from routine soil analyses by farmers and agricultural statistics are useful for assessing spatio-temporal changes in critical soil properties (Baxter et al., 2006; Lemercier et al., 2008).

In this study, data from long-term field trials and an intensive Swedish soil monitoring program, similar to the soil inventory of German arable soils (Jacobs et al., 2018), and yearly agricultural statistics were used for identification of soil properties currently limiting crop production. The objective was to identify which, and to what extent, soil properties (other than N input) limit yields of cereals, oilseeds, and root crops under various pedo-climatic conditions in Sweden. The data included physical and chemical soil properties from a detailed national soil inventory representing 90 yield survey districts, climate data and crop yield statistics, and results from long-term field experiments across the country.

To identify the most important soil properties limiting productivity and to estimate optimum soil conditions required for highest achievable yields, data on soil properties and crop yields were used to address the following research questions:

i) What general information about Swedish arable soils can be retrieved from the national soil inventory?

(ii) What type of yield response pattern in relation to soil variables (linear or breaking points) can be distinguished?

(iii) Which soil properties limit crop yields most significantly in current agriculture?

(iv) What are the critical levels of the main soil properties reducing yields?

(v) Do the data obtained in the soil inventory and yield survey districts match the outcome from long-term field experiments?

(vi) Are current agronomic recommendations appropriate, given the results of the above analysis?

An overall objective was to identify best agronomic practices in order to maintain yield increases, make best use of nutrients, and reduce negative environmental impacts so as to meet national environmental quality goals (Naturvårdsverket, 2017).

Section snippets

Soil inventory and crop yield data

Soil data were derived from the most comprehensive inventory to date of topsoil characteristics of Swedish arable land (Paulsson et al., 2015; Djodjic, 2015). This soil-monitoring program was performed in 2013 by the Swedish Board of Agriculture, in order to create a better map of Swedish arable soils. It included 12 554 topsoil samples distributed all over southern Sweden (Fig. 1), enabling site identification for specific agricultural measures. In this study, we focused on mineral soils only,

Overview of the soil inventory

Texture: The survey revealed that the dominant soil texture class was sandy loam (30 %), followed by silt loam (17 %) and loam (14 %). Fine-textured soils such silt (1%), silty clay (8%), and clay (5%) comprised of around 15 % of arable soils, while sand (2%) and loamy sand (9%) comprised around 11 % (Djodjic, 2015). The geographical distribution map revealed that clay soils were most prevalent in east-central parts of the survey area (Fig. 2), which is in good agreement with a previous texture

Why did yields respond negatively to higher soil organic matter content?

It is widely accepted that SOM is a very important indicator of soil fertility and crop productivity (Körschens et al., 2013; Lal, 2013). Meta-analyses using long-term field experiments and annual N-fertilizer trials have confirmed the positive impact of SOM on yields (Liu et al., 2014; Wang et al., 2015; Lu, 2015; Zhang et al., 2016). Positive yield responses to gradual increases in SOM have also been observed in a Swedish long-term field experiment (Henryson et al., 2018). However, negative

Conclusions

Availability of two comprehensive data sets on Swedish arable soils and regional crop yield statistics enabled us to evaluate soil variables currently limiting crop yields. Multi-variate analysis, PCA, step-wise regression, and tree partitioning analysis identified the same variables affecting yields. One main finding was that current agricultural recommendations require updating. Considering that no legislative restrictions were put on N fertilizer use in Swedish agriculture, the following

CRediT authorship contribution statement

Holger Kirchmann: Conceptualization, Writing - review & editing, Validation. Gunnar Börjesson: Data curation, Validation. Martin A. Bolinder: Data curation, Writing - review & editing. Thomas Kätterer: Methodology, Writing - review & editing. Faruk Djodjic: Data curation, Formal analysis, Writing - original draft.

Declaration of Competing Interest

This article has not be inflicted by affiliations, memberships, funding, financial holdings, interests of industries, individual companies or retailers affecting the objectivity of the data analysis.

Acknowledgements

The national data were collected by the Swedish Board of Agriculture, which is greatly acknowledged. The evaluation of data sets was sponsored by the Royal Swedish Academy of Agriculture and Forestry (KSLA), (Grant no. VX2016-0004) and the Swedish Farmers' Foundation for Agricultural Research (Grant no. O-18-23-141), which we are thankful for.

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