Sulfur (S) is taken up by plants in the sulfate (SO₄⁼) form. This ion with the negative electrical charge is an anion – just like nitrate (NO₃^-). The movement of NO₃^- through soils has been discussed by many and has been the focus of numerous research projects.

It's common knowledge that the NO₃^- ion moves most rapidly in sandy soils and more slowly in the fine textured soils. Does SO₄⁼ move as rapidly as NO₃^-? Will SO₄⁼ in fertilizer bands move downward and away from young corn plants? If SO₄⁼ moves below the root zone, will split applications of fertilizer S be beneficial. These are some of the more common questions that revolve around the use of fertilizer S for corn production.

Movement of SO₄⁼ in a corn production system had not been studied in the past. Therefore, downward movement of SO₄⁼ was measured in trials conducted in 1999 and 2001. The objective of the trials was to measure the effect of banded ammonium sulfate (21-0-0-24) on growth and yield of corn.

A detailed soil sampling pattern was used to measure the movement of SO₄⁼_-S applied in a band. Soil cores were collected in the row then at distances of 3, 6, and 9 inches from the row. In 2001, soil was collected from depths of 0 to 3, 3 to 6, and 6 to 12 inches. In 1999 the depths were 0 to 3, 3 to 6, 6 to 12, and 12 to 18 inches. The samples were taken from treatments where the 21-0-0-24 had been applied in a band to supply 18 lb. S per acre. The bands were either with the seed or the more traditional starter placement of 2 inches beside and 2 inches below the seed.

The samples were analyzed for SO₄⁼ and the results for 1999 and 2001 are summarized in Tables 1 and 2 respectively. The soil at the 1999 test site had a loamy fine sand texture. The soil at the 2001 site had a silt loam texture. The samples were collected at approximately four weeks after corn emergence.

Rainfall was not measured at the test sites. However, rainfall measured at nearby locations was near normal in both years.

The concentrations measured in the control treatments can be used as a basis for comparison. When the 21-0-0-24 was applied, high concentrations of SO₄⁼_-S were measured both under the row and at a distance of three inches from the row. This indicates downward as well as some lateral movement of SO₄⁼_-S. This movement was measured at both sites when the 21-0-0-24 was placed either with the seed or in the traditional starter band.

It's important to note that there was no movement below 12 inches in 1999 when the soil had a loamy fine sand texture. This observation indicates that early in the growing season SO₄⁼_-S does not move below the corn root system if applied in a band.

Soil below 12 inches was not sampled in 2001. However, if there was no deep movement in a loamy fine sand, deep movement should not be expected in a soil with a silt loam texture.

Table 1. Concentration of SO₄⁼_-S in a loamy fine sand approximately four weeks after corn emergence.

Table 2. Concentration of SO₄⁼_-S in a silt loam soil approximately four weeks after corn emergence.

The results from these two test sites also indicate that downward movement of SO₄⁼_-S is not as rapid as NO₃^-N. Although NO₃^-N was not measured in this study, results from other trials indicate that NO₃^-N can move below 12 inches in a period of four weeks after emergence.

Measurement of SO₄⁼ movement in two different soils provides no indication that fertilizer S should not be applied in a band at planting. When soil texture indicates a need for fertilizer S, measurements of corn yields show that there is no substitute for a banded application of this essential nutrient.

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