Soil Fertility in Soybean Production

Linda Benedict, Wang, Jim Jian, Tubana, Brenda S.  |  7/31/2013 1:08:23 AM

Brenda S. Tubaña, Donald Boquet and Jim Wang

Soybeans emerged as a major Louisiana crop in the 1970s and now occupy the largest proportion of cultivated land in the state. Poor soil fertility was one of the yield-limiting problems encountered during the rapid expansion in the 1970s and 1980s. The major soybean-producing parishes are in northeast and central Louisiana. Soils from these areas were formed from loess (wind-blown) and alluvium (flowing water) deposits, giving rise to a wide range of soil types and growing conditions.

Soybeans grow best on soils with a pH of 6.0-6.5 and a sufficient level of essential nutrients. Correcting soil pH by liming is a fundamental requirement to maintain balanced proportions and necessary amounts of essential nutrients in the soil. Solubility of several nutrients increases as the soil becomes acidic, which could lead to nutrient toxicity problems in plants if not treated. This group of nutrients is called micronutrients, which plants take up in small quantities. On the other hand, the level of most macronutrients – the group of nutrients which are taken up in large amounts – becomes deficient as soil pH decreases.

Phosphorus and potassium are among the essential plant macronutrients. Their unique roles in energy storage and transfer, enzyme activation, protein synthesis and water use efficiency in plants underscore the need for their sufficient supply to ensure vigorous soybean growth and development.

Fertility information related to soil pH and guidelines for phosphorus and potassium fertilization tailored for areas where soybeans are commonly grown in Louisiana are valuable to producers. This led to a long-term field study initiated in 2004 near Winnsboro on a Gigger silt loam, an acidic Mississippi upland loess soil. The findings from this study have been used to document the benefits of correcting soil pH through liming in combination with a sufficient supply of phosphorus and potassium to soybean production under an irrigated, three-year soybean-corn rotation system with minimum tillage.

The soil types in this area are commonly known for their low organic matter content, permanent hard pan layer and moisture-limited conditions. The treated plots were structured by combining different phosphorus and potassium fertilizer rates – 0, 30, 60, 90 and 120 pounds per acre – with or without lime. Every year, yield data and soil samples are collected from each plot. Soil samples are analyzed for pH and nutrient content.

Plots that received lime produced better soybean stands (Figures 1 and 2). This can be attributed to a combination of benefits from liming, which improves nutrient availability as well as microbial activity, soil structure and nitrogen fixation. All of these contribute significantly to the productivity potential of a soil. Liming also is a low-cost source of magne sium and calcium. Except in 2004, soybean plots that had been receiving lime consistently yielded higher than those without lime (Figure 3).

When averaged over the years, lime application resulted in a three-bushel-per-acre yield increase. The highest yield increase of nine bushels per acre came in 2011 (Figure 3). In 2004 when this study began, there was no notable difference in soybean yield. The absence of yield benefit from liming during the first year is perhaps expected because some soil types, like Gigger silt loam, may require higher amount of lime and more time for reaction to raise the soil pH to a desired level. In subsequent years, limed plots have shown statistically significant and greater increases both in yield and soil pH when compared with plots that have not received lime (Figure 3).

The soil from this long-term study initially tested low for phosphorus and potassium, which explains why in most cropping years, soybeans performed better when phosphorus and potassium fertilizers were applied. However, this study highlighted the value of including lime in a nutrient-management program.

In 2011 and 2012, the positive impact of phosphorus and potassium fertilization on soybean yield was enhanced with liming (Figure 4). This is encouraging. With a small investment in lime, the net return to both phosphorus and potassium fertilizers could be substantially higher. For example, an application of 60 pounds of phosphate fertilizer per acre without lime improved yield by six bushels per acre in 2011, but the same phosphorus fertilizer rate with lime produced an additional 12 bushels per acre. With a soybean price of $13 per bushel and a lime price of $19 per ton, net return to lime is roughly equivalent to $137 per acre.

Apart from increasing soil pH, noticeable changes in soil chemical properties due to the cumulative effect of liming included higher calcium and magnesium content – both essential plant macronutrients. On the other hand, there were indications of enhanced solubility of manganese and copper in plots that have not received lime. Although the levels in the soil are not toxic, with a few more years of cropping, this could be yield-limiting to soybeans.

The findings of this study have demonstrated the benefits of maintaining correct soil pH in combination with effective nutrient management to achieve competitive soybean yields. Soil fertility-related problems that may arise if an acidic soil is left untreated were also highlighted. In the future, this study should produce more invaluable insights and pertinent information regarding the long-term effects of phosphorus, potassium and lime applications. Thus, continuing this long-term soil fertility study is of unquestionable importance to the Louisiana soybean industry.

Brenda S. Tubaña is an associate professor in the School of Plant, Environmental & Soil Sciences, Donald Boquet is a retired AgCenter research agronomist, and Jim Wang is a professor in the School of Plant, Environmental & Soil Sciences.

(This article was published in the spring 2013 issue of Louisiana Agriculture magazine.)

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