Potassium assessment of grain producing soils in North China

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Abstract

Agricultural land around the world is often potassium (K) deficient, and decreasing soil K availability is a global challenge for grain producers. In China, negative K budgets are ubiquitous and K depletion is accelerating. Previous studies have mainly focused on the effect of insufficient K fertilizer on single soil types and cropping systems but have not evaluated the effects of balanced fertilizer application on K budgets. In this study, soil K budgets of grain producing soils and the effects of K fertilizer application on crops under different cropping systems and soil types were evaluated from 1992 to 2006. Two treatments each at nine locations in North China were replicated four times, four plots received nitrogen (N) and phosphorus (P) fertilizer application exclusively, and the other four plots received N, P, and K fertilizer application. Basic soil properties, available K, slowly available K, crop rotations, crop K absorption, and yield were monitored and evaluated and K budgets and K use efficiencies were calculated. Average annual K absorption by crops with K fertilizer application was 230.2 kg ha−1 K2O, and the average annual deficit was 46.8 kg ha−1 K2O. Long-term application of K fertilizer increased available soil K on all nine sites by an average of 35.7 mg kg−1. Annual K absorption by crops averaged 99.0 kg ha−1 in the northeast region, 181.2 kg ha−1 in the north-central region, and 220.9 kg ha−1 in the northwest region. Annual K fertilizer use efficiency in the northeast (39.9%) and north-central (38.8%) regions were higher than in the northwest (17.8%). Correlations between annual K budgets and K use efficiency with fertilizer application, crop rotation, and soil type were weak. High spatial variability and low correlation between variables is a result of the complex mechanisms of K cycling. Research directions are proposed which would contribute to a better understanding of these mechanisms and processes and how they affect agronomic and economic objectives.

Highlights

► Soil K budgets of grain producing soils were monitored. ► K fertilizer interaction with cropping systems and soil types were evaluated. ► Soil K budgets decreased in every trial. ► Available soil K and K use efficiency had high spatial variability.

Introduction

Potassium (K) deficiency is a world-wide problem (Dobermann et al., 1998). In Europe, 25% of soils routinely test low in available K (Johnston, 2003) and the majority of soils in sub-Saharan Africa test low in available K (Smaling, 1998). At the same time, the K status of agricultural soils is also decreasing across the globe, from Europe (Fagerberg et al., 1996, Bengtsson et al., 2003), to Africa (Harris, 1998, Wortmann and Kaizzi, 1998), to Asia (Hedlund et al., 2003, Wijnhoud et al., 2003), and North America (Malo et al., 2005).

In China, due to limited arable land and the necessity to increase production to ensure national food security, nitrogen (N) and phosphorus (P) fertilizer recommendations and subsequent application have increased annually (Gao et al., 2006). At the same time, K fertilizer application is increasingly inadequate. Zhao et al. (2008) found that in a county-wide survey in the Yangtze Delta Region, 38% of samples had K values below recommended levels according to Chinese testing standards. A study by the National Soil Survey Office (1998) found that 40% of agricultural land in South China was deficient in K.

Potassium deficiency in China has been well correlated with excess N and P application (Wang et al., 2008, Liu et al., 2009). This is partially because the majority of K fertilizer in China is imported and therefore cost prohibitive (Wang, 1996, Sheldrick et al., 2003) and because farmers are not aware of the economic benefit of K fertilizer application. Some farmers suppose that N and P application increases yield infinitely (Hu et al., 2007); and application of K fertilizer does not produce results as quickly as N and P fertilizer application, causing farmers to question the value of K fertilizer application (Jin, 1997).

Soil K is not only low in China, it is also decreasing (Li et al., 2002, Suo et al., 2002, Zhang et al., 2010). Soil fertility tests have shown that available soil K is declining, nation-wide, especially in already infertile soils (Xie and Zhou, 1999). Wang et al. (2008) found that on intensively cultivated vegetable plots in the Yangtze Delta Region, K balances in some areas were as low as −500 kg ha−1. Available K decreased by 21% on a rice–wheat cropping system in Zhangjiagang County (Darilek et al., 2009), where decreasing soil K is, reportedly, one of the top fertility concerns (Sheldrick et al., 2003, Gao et al., 2006) and TK decreased by 9% on agricultural land in Suzhou over an 18 year period (Li et al., 2007). Zhang et al. (2010) reported negative annual K balances on rice/wheat systems, even when K fertilizer application was applied at rates which resulted in a 20% increase of yield.

Recent studies have also shown that K fertilizer application is greatly improving production in China (Jiang et al., 2006), and particularly in the major grain producing areas of North China (Liu et al., 2000, Lin et al., 2006). Other studies have highlighted the importance of balanced K application for sustainable agricultural development (Singh et al., 2002, Liu et al., 2009). In Northern Shandong Province, K balance in one wheat–maize rotation study was −163 kg ha−1 year−1 (Ju et al., 2007) and in Heilongjiang, K balance was −20.1 kg ha−1 year−1 (Zhou et al., 2000). Another county-wide study in Hebei found that available K decreased by 64.9 mg kg−1 from 1980 to 1999 (Kong et al., 2006). However, these studies have mainly focused on single soil types and cropping systems. There is still a lack of understanding of K balance in grain producing soils of different soil types and under various popular fertilizer and crop rotation programs in North China. Additionally, although these studies called for balanced fertilizer application ratios and have evaluated the results of improper nutrient input, few of these studies have evaluated the results of fertilizer application which follows these recommendations. Without this knowledge, the economic and agronomic benefits of K fertilizer application for different regions and under different management strategies cannot be fully evaluated.

In this study, soil K budgets of grain producing soils and the effects of K fertilizer application on crop yield and soil K availability under different cropping systems and soil types were evaluated from 1992 to 2006. This investigation contributes to a more comprehensive understanding of the soil K status under current and recommended agricultural production practices, and provides a new, robust foundation for further research to answer critical soil K issues in North China and similar intensive grain producing regions around the world.

Section snippets

Experimental sites

Nine sites, representative of different areas of North China, were selected for research (Fig. 1). Shuangcheng, Gongzhuling, and Shenyang represented the northeastern provinces of Heilongjiang, Jilin, and Liaoning, respectively; Xinji, Luoyang, and Linfen represented the north-central provinces of Hebei, Henan, and Shanxi, respectively; and Yinchuan, Xining, and Changji represented the northwestern provinces of Ningxia, Qinghai, and Xinjiang, respectively (Table 1). Each crop was sampled

Potassium absorption by crops

Regardless of fertilizer application, crop rotation, and soil type, K balances on all sites were negative. Average annual K absorption by crops was 230.2 kg ha−1 K2O and the average annual deficit was 46.8 kg ha−1 K2O. There was no discernable trend to explain annual K budgets and K use efficiency on the nine sites. The K deficit was higher in the north-central region than the northeast region. Annual K absorption by crops was higher in treatments with K fertilizer application than those without K

Soil K balances

Negative K budgets on every site in the study, regardless of K fertilizer application, underscores the necessity of adequate and appropriate K supply to the soil. Lin et al. (2006) also found negative K balances for wheat and maize on the North China Plain despite K fertilizer application, however, application rates in that study were 105 kg ha−1 and 45 kg ha−1 for wheat and maize, respectively. Similarly, Ju et al. (2007) found a negative K balance of 163 kg ha−1 year−1 on a wheat/maize rotation

Conclusions

This research confirms that soil K budgets continue to decrease, that soil K availability and K use efficiency are highly variable, and that the mechanisms of K cycling are highly complex. Soil K in North China becomes seriously depleted without application of K fertilizer and soil K balance continues to decrease with conventional NPK fertilizer application. The highest removal of soil K was found in the field trial of winter wheat–summer maize double cropping system. Maize yield was more

Acknowledgments

We thank the National Natural Science Foundation of China (31101601), the National Basic Research Program of China (973 Program: 2007CB109306), the China Agriculture Research System (CARS-03), and the IPNI-Shandong Project for their financial support.

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