Influence Of Gypsum on Crop Yields

R. Gelderman, A. Bly, J. Gerwing, H. Woodard, and R. Berg

Introduction

Gypsum, calcium sulfate (CaSO₄ . 2H₂0), is naturally occurring mineral that is mined for many purposes. Gypsum has calcium content of 23% and sulfur content of 19%. In agriculture it is used for treating sodium affected soils. The calcium in the applied gypsum will displace sodium on the soil cation exchanged capacity. This is a mass action process; therefore large amounts of calcium are required. Drainage within the soil profile must also occur for the displaced sodium to be leached out of the soil profile. Sodium is part of soil salt compounds (NaCl, NASO₄ and Na₂CO₃). Many other forms of soil salts also exists (KCl, MgCl₃, CaCl₂, MgSO₄,CaSO₄). Gypsum can also be used to supply sulfur. Questions about the effectiveness of gypsum in alleviating salt effects plants are common. Therefore this study was conducted to determine if gypsum could significantly increase crop yields in soils that are not affected by sodium adsorption ration (SAR < 9) but have somewhat higher than average salt levels.

Materials and Methods

Corn:

A research site was selected on the northeast quarter of the Southeast Research Farm, located near Beresford, SD. This site has been managed in a corn-soybean rotation. Conventional tillage practices have been used on this site that consists of chisel plowing in the fall and field cultivation in the spring. There is some white salt formation and at the soil surface. Composite soil samples from the 0-6, 6-12, and 12-24 inch soil depths were obtained prior to gypsum application and analyzed for selected soil parameters (TABLE 1)

Gypsum rates including a control were randomized in three replications. The gypsum rates were 0, 300, and 1500 lbs/ac/ and applied in a pelletized form with a Gandy Orbit Air applicator and incorporated with a field cultivator. Corn (Pioneer 34M95) was planted in 30-inch rows on May 14, 2002 at 27,900 seeds/ac. Plot size is 15 x 40 feet. No phosphorous or potassium was applied because the soil tests were not limiting these nutrients. Recommended nitrogen for the 150 bu/ac yield goal was knife applied as UAN (28-0-0) at the five-leaf stage of growth. The center two rows of each plot were harvested with a small plot combine on November 7, 2002. Grain yield was adjusted to 15% moisture.

Soybeans:

A research site was selected on the northwest quarter of the Southeast Research Farm located near Beresford, SD. The previous crop was spring wheat. Conventional tillage practices have been used on this site whenever possible. Areas of this site have had wetness problems in some years and there is significant white salt formation on the soil surface of the lower areas when soil dries. During some years, crop emergence is affected by the salty conditions. Composite soil samples representing the whole site were taken from 0-6, 6-12, and 12-24 soil depths prior to gypsum application. Detailed soil samples of the 0-6 inch depth were taken from each replication at 50, 150, and 250 feet from the south border of the research area.

Gypsum rates including a control were randomized in four replications. The gypsum rates were 0, 300, and 1500 lbs/ac and applied in a pelletized form with a Gandy Orbit Air applicator and incorporated with a field cultivator. Soybean (Prairie Brand 2141RR) was planted in 30-inch rows at 148, 800 seeds/ac on May 21, 2002. Plots measured 15 x 300 feet. No phosphorous or potassium was applied, because soil tests indicated these nutrients were not limiting. Soybean from all fifteen feet of plot width (6 rows) were harvested with a field scale combine and measured with a yield monitor on October 18, 2002. Seed weight from each plot was also measured with a weigh wagon to cross check the grain yield values given by the grain yield monitor.

The salt effect on plant growth was extremely variable and resulted in many areas where no soybean grain was produced especially in reps of 3 and 4 (east side). It was, therefore, decided that total plot weight could not be used to estimate grain yield and that grain yield values from the combine monitor would be most representative. Very good correlation of estimated grain yield between the weigh wagon and combine as observed (FIGURE 1).

Wheat:

Two spring wheat sites were chosen on farmer cooperator fields in Brown County. One site near Houghton was on sandy soil and one near Warner on heavy soil typical of the James River valley. The Houghton site was no tilled while the Warner site was clean tilled. Soil tests for the two sites are listed (TABLE 5). Neither site had elevated salt levels in the surface. Gypsum and fertilizer treatments were broadcast on the surface shortly after planting. All treatments received 50 lbs/ac of nitrogen and either 45 lbs/ac chloride as potassium chloride, 50 lbs/ac of sulfur as ammonium sulfate, or 300 lbs/ac of gypsum (TABLE 6). The cooperators uniformly applied all other fertilizer. Plot size was 20 feet x 20 feet at both sites and treatments were replicated four times. Plots were hand harvested by cutting heads from a 2.5 x 10 feet area in each plot and thrashed in a small plot combine.

RESULTS

The corn site had slightly higher than normal salt levels with very little calcium carbonate, but high sulfate-sulfur levels (TABLE 1). Gypsum did not significantly influence corn yield although there is a trend for increased yield with higher gypsum levels (TABLE 2). A sulfur response would not be expected here because of very high soil test levels. The dealer cost of this product (coated) is $120 per ton. Therefore the economics are not favorable even if this is a real effect and it lasts for several years. The 300 lb application rate will be repeated next year and no gypsum will be applied on the 1500 lb rate for the coming year.

The soybean site had high salt levels. Although sodium levels should not be negatively affecting the soil, levels are higher than normal. Moderate calcium carbonate levels are present in the 6-12 inch depth. Sulfate-S is very high at this site. (TABLE 3). Sulfate salts are very common salty areas. Gypsum did not statistically influence soybean yields at this site (TABLE 4). Variability within the soil and plant growth is common in salty areas. This can noted by the CV, % (coefficient of variability) which is over 9 and 18 percent for the west and east areas of the site, respectively. There is no consistent influence on yields with the addition of gypsum. This site will be planted to corn next year. The 300 lbs gypsum rate will be repeated next year and no gypsum will be applied on the 1500 lb treatment.

The detailed soil samples for the soybean site were analyzed for salts and correlated to combine yield monitor data (FIGURE 2). A fairly good relationship exists as would be expected. High salts limit soybean yields.

Wheat yields at the Brown County sites were not influenced by either gypsum or an equal rate of sulfur applied as ammonium sulfur (TABLE 6). The sulfate sulfur soil test was high at both sites and a response to sulfur was not expected. The chloride treatment at these sites did not influence yield. The chloride soil test was medium at Houghton and very high at Warner.

Overall the addition of gypsum statistically significant in 2002.

While not statistically significant, the addition of gypsum at the 300 lbs/a added 4 bushels of corn The addition of of gypsum at the 1500 lbs/a increased yields by 11 bushels of corn