NC State Extension Publications

Introduction

Sulfur (S) is found in certain amino acids and is a necessary component of certain proteins, vitamins, and oils. It is considered a secondary nutrient and is taken up in the sulfate form. Plants do not require as much S as they do nitrogen, phosphorus, and potassium (N, P, and K); but S is still needed in greater quantities than are the micronutrients boron, copper, iron, manganese, zinc, and chloride. Plant tissue should contain 1 part sulfur for every 15 to 20 parts nitrogen for optimum growth and quality. Deficiencies typically occur in deep, coarse-textured soils where sulfate leaches easily. Today, S deficiency may be more of a concern due to several factors that farmers may not have considered: (1) tighter air quality standards for atmospheric emissions mean less sulfur falls onto the landscape, (2) past use of single superphosphate fertilizers (12% S) led to S accumulation, but the fertilizer industry switched decades ago to triple superphosphates (1% S), and (3) heavy rainfall leaches more S downward in the soil profile. Sulfur deficiencies are being correctly diagnosed more often due to plant tissue analysis, while in years past they were often mistaken for nitrogen deficiency.

Sulfur is an immobile element, and as a result S-deficient plants have green lower leaves and yellow upper leaves (Figure 1).

This is in contrast with N, a mobile element. N-deficient plants have yellow lower leaves and green upper leaves. S deficiency is most likely to occur during early growth, before extensive root development. Crops deficient in S show little or no response to N fertilizer until the S deficiency is corrected.

Photo of different indicators of sulfur deficiency in plants.

Figure 1. Sulfur deficiency can be seen on the right in stunted growth and pale green/yellow upper leaves in a) corn, b) tobacco, c) cotton, and d) wheat.

Factors Influencing Crop Responses to S Fertilizer

Cropping system

Corn, cotton, and wheat, when planted on S-deficient soils, have shown yield increases in response to the addition of S fertilizer (Figure 2). Crop species are not all equally sensitive to soil S levels. Sulfur deficiency is rare in soybean, even though it is grown in rotation with corn, which often responds to S fertilizer. Soybean is a tap-rooted crop that may acquire S from deeper depths than more shallow-rooted grass crops such as corn. Peanuts are typically fertilized with gypsum, which also supplies S (1,000 lb CaSO4 contains 190 lb S), so additional S should not be necessary for other crops in this rotation.

Since not all crops respond to S fertilizer, awareness of crop S uptake and removal will help decide if the additional expense of S fertilizer is necessary. Table 1 lists nutrient removal rates for different crops at typical yield levels. Crops vary widely in S removal rates. Silage corn (24 lb S/ac/yr in grain + stover) and burley tobacco (18 lb S/ac/yr) remove more S than other agronomic crops; while wheat (3 lb S/ac/yr) and cotton (5 lb S/ac/yr) remove relatively little S. Orchardgrass removes the most S among forages, even with biomass yields lower than those of coastal bermuda. Among vegetables, cabbage (and probably other cole crops such as collard greens and broccoli) removes more S (44 lb S/ac/yr) at typical yield levels than any other North Carolina crop. Producers may need to supply more S fertilizer when growing high S-demanding crops, especially in high-yielding environments.


Table 1. Sulfur removal rates by North Carolina crops.
Crop Component Yield Level S Removal (lb S/ac)
Field crops
Wheat grain 40 bu 3
straw 1.5 tons 5
Cotton seed + lint 2,600 lb 5
residue 3,000 lb 15
Corn grain 150 bu 10
stover 4.5 tons 14
Tobacco, flue-cured leaves 3,000 lb 12
stalk 3,600 lb 7
Tobacco, burley leaves 3,000 lb 18
Soybean grain 50 bu 11
residue 6,100 lb 12
Forages
Red clover 2.5 tons 7
Fescue 3.5 tons 20
Coastal bermuda 8 tons 32
Orchardgrass 6 tons 35
Vegetables
Sweet potato root 300 bu 6
Sweet corn ears 9,000 lb 11
Tomato fruit 20 tons 14
Onion bulbs 7.5 tons 18
Cabbage heads 20 tons 44
Source: SoilFacts: Nutrient Removal by Crops in North Carolina, Zublena, 1991, AG 439-16.

Soil

Responses occur most often when soil test S levels are low in both the topsoil and subsoil, typically in deep sandy soils. Sulfate, the plant-available form of S, can leach through sandy surface soils, but usually accumulates in clay subsoil. The negatively charged sulfate ion is attracted to soil clay particle surfaces. Even subtle changes in clay content, such as a transition from a loamy sand topsoil to a sandy loam subsoil can result in S accumulation. Unfortunately, most producers submit only topsoil samples for soil test analyses, when sufficient S may be present in the subsoil. Typical topsoil samples for fertilizer recommendations are collected from a 6- to 8-inch deep layer (4-inch if continuous no-till). It is possible for a soil to have very little S in this upper surface layer, but sufficient S present at a lower depth to support plant growth. Response to S fertilizer is more likely if the depth to increased clay content is greater than 18 inches below the soil surface. Natural Resource Conservation Service (NRCS) county soil surveys and field observations should be used to identify problematic fields. When a loamy sand topsoil with low S content overlays a sandy loam subsoil, significant S accumulations may occur (Figure 3). Sulfur previously applied as gypsum or other materials leaches downward in the profile, but may still be within reach of roots.

Soil variability complicates fertilizer management decisions. Note that for each region of the state, certain soils have deeper and/or sandier surfaces, and are thus more likely candidates for S leaching and fertilizer S response (Table 2). In the mountain floodplains, leaching and S depletion are more likely in coarser soils such as Biltmore and Dellwood than in finer soils such as Rosman. Many upland piedmont soils such as Cecil and Vance are eroded and have significant clay near the surface, while floodplain soils, such as Chewacla and Congaree, have more sandy surface layers due to deposition and therefore are more prone to deep leaching of S through surface layers.


Table 2. General soil profile characteristics of selected North Carolina soils. Consult your Natural Resources Conservation Service county soil survey for soil properties in your area.
Region Soil Series Topsoil Texture Subsoil Texture Typical Depth in Inches to Clay
Mountain floodplains Biltmore Sand Sand 40+
Dellwood Cobbly sandy loam Loamy sand/sand 50+
Rosman Loam Sandy loam/sand 50+
Piedmont Cecil Sandy loam Sandy clay loam 7 (less if eroded)
Chewacla Silt loam Loamy fine sand/silt loam 36+
Congaree Sandy loam / loam Silty clay loam 38
Vance Sandy loam Clay 5
Upper/Middle Coastal Plain Candor Sand Loamy sand 25
Norfolk Loamy sand Sandy loam 15
Rains Sandy loam Silty clay loam 25
Lower Coastal Plain Conetoe Loamy sand Sandy loam 25
Craven Silt loam Silty clay loam 10
Goldsboro Loamy sand Sandy loam 13
Roanoke Silt loam Silty clay loam 25
Tidewater Belhaven Muck Sandy loam 26
Bojac Loamy fine sand Fine sandy loam 8
Newholland Mucky loamy sand Sandy loam 27
Portsmouth Fine sandy loam Sandy clay loam 24
Tomotley Fine sandy loam Sandy clay loam 15

Of the coastal plain soils listed, the Candor and Conetoe are most prone to deep S leaching. Most crops grown on tidewater region soils are less likely to suffer S deficiency. Some soils have mucky surfaces that can supply S as organic matter decomposes (Belhaven, Newholland). Sulfur deficiencies on these organic soils are more likely in the winter months due to slower decomposition rates with cooler temperatures. Other tidewater soils have either finer-textured surfaces (Portsmouth, Tomotley) or a coarse-textured surface layer that is relatively thin (Bojac), so less leaching is expected.

Weather

Heavy rains can leach S to lower profile depths, while cold weather and/or high water tables can reduce crop rooting depth and residue decomposition rates, thus reducing sulfur supply.

Tillage management

Long-term no-till cultivation is increasingly popular in North Carolina. Without cultivation, crop residue remains on the soil surface; a subsurface network of dead roots and animal burrows persists; and surface-applied lime and fertilizer may concentrate near the surface. While the surface cover and subsurface channels can reduce overland water flow and soil erosion, more water may percolate and leach soluble nutrients deeper in the profile. Producers should consider the possibility that soil stratification and slower residue decomposition may reduce S availability, especially when low subsoil pH or compaction limits rooting. Occasionally, the producer should sample the subsoil to detect chemical limitation, and probe soils and observe roots to detect physical limitation.

Figure 2.

Figure 2. Corn S response in Bladen County, NC, 2006, on a Norfolk loamy fine sand. Note yield increase with S fertilizer, while leaf N:S ratio decreased. Initial soil S index (S-I) was 21, below the Mehlich-3 soil test critical level of 25 (12 ppm).

Photo showing different soil layers.

Figure 3. At a Bertie County, NC, site with a Conetoe soil similar to the profile in the photo, soil Mehlich-3 S-index level increased dramatically with depth: from 45 in the loamy sand topsoil, to 194 in the sandy loam layer at a depth of 24 inches.

Diagnostic Laboratory Data

Although visual nutrient deficiency symptoms can be very distinct, there are cases where S deficiency looks similar to N, manganese (Mn), or other nutrient deficiencies. Waiting until you see visual symptoms is often too late to recover from crop economic loss. Laboratory analysis of soil and plant tissue samples will confirm the diagnosis.

Soil testing

Sulfur is reported in the routine North Carolina Department of Agriculture & Consumer Services soil testing procedure, and a sulfur index of 25 or less is indicative of a need for fertilizer. Actually, the need for fertilizer S depends not only on laboratory results from a topsoil sample, but also an awareness of the nature of the soil profile as mentioned previously in the soil discussion. Periodically take a deep sample to measure the amount of subsoil S. When S is needed, 15 to 25 lb S per acre should be adequate; the lower rate is recommended on soils with higher levels of organic matter.

Plant Tissue Analysis

Both the percent S and the ratio of N to S (N:S ratio) are used to evaluate crop S status. Find more information at NCDA&CS's Plant Tissue Analysis Section website.

S Fertilizer Materials, Costs, and Typical Application Scenarios

Several fertilizers are available to North Carolina farmers that can be applied in preplant granular blends or included in N solution applications (Table 3).


Table 3. Commonly used S fertilizers in North Carolina.
Source Formula % S
Ammonium sulfate 21-0-0-24 (also ammonium sulfate liquid, 8-0-0-9S; or industrial by-product liquor, 7-0-0-9S) (NH4)2SO4 24 (or diluted)
Gypsum (calcium sulfate) CaSO4·2H2O) 19
Potassium sulfate K2SO4 18
Potassium magnesium sulfate (Sul-Po-Mag or K-Mag) K2SO4-MgSO4 18-23
Normal superphosphate Ca(H2PO4)2·H2O + CaSO4 12
Ammonium thiosulfate (NH4)2S2O3 26
Sulfur-coated urea CO(NH2)2-S 10
N-S solutions (24-0-0-S) UAN mixed with ammonium+ sulfate, polysulfide, bisulfite, or thiosulfate 3-5
Elemental Sulfur (powder or 50% suspension in water) S 90-100

The most economical applications of S are in combination with other fertilizer materials. Some convenient options for North Carolina crops are:

  1. 24-0-0-S to supply 12.5 - 21 lb S with each 100 lb N. This could be at planting, or as a sidedress/topdress. Based on a 2006 survey, this should cost approximately $0.15/lb of S, as compared with applying N alone. The S adds about 10% to the cost of N fertilizer alone, so it is important to document cases when the extra expense is justified.
  2. Ammonium sulfate (21-0-0-24) to supply 20 lb S with each 17.5 lb N. This could be at planting, or as an early sidedress/topdress. Based on a 2006 survey, this should cost approximately $0.20/lb of S, as compared with applying N alone.
  3. Sul-Po-Mag (0-0-22-23% S) broadcast to supply 1 lb S with each 1 lb K2O. This would usually be at planting, unless also correcting a mid-season K or Mg deficiency. Based on a 2006 survey, this should cost approximately $0.33/lb of S, as compared with applying K alone as muriate of potash (0-0-60).
  4. Sulfate of potash (0-0-50-17% S) broadcast to supply 1 lb S with each 3 lb K2O. This would usually be at planting, unless also correcting a mid-season K deficiency. Based on a 2007 survey, this should cost approximately $0.56/lb of S, as compared with applying K alone as muriate of potash (0-0-60).
  5. Manure broadcast preplant. See approximate composition values in Table 4.

Incidental S inputs from rainfall, dry atmospheric S deposition, P fertilizer, and especially animal wastes (Table 4) can represent substantial amounts of S. The balance between these inputs and crop S removal rates indicates likely fertilizer needs.


Table 4. Incidental S inputs potentially available to North Carolina farms.
Source Material application rate (lb/ac/yr) Rate (lb S/ac/yr)
Rainfall 6
Dry atmospheric deposition 6
Triple superphosphate fertilizer 75 lb P2O5 0.75
Swine lagoon liquid 100 lb plant-available N 8
Dairy lagoon liquid 100 lb plant-available N 26
Stockpiled broiler litter 100 lb plant-available N 42

Summary

Adequate S is necessary for high crop yields and profitable farming. However, one size does not fit all, and not all crops benefit from S fertilizers. The best management plan considers S removal and incidental S inputs for the entire crop rotation, soil type and profile depth layers, and soil and plant analysis results. Consult the Natural Resources Conservation Service county soil survey and look at your own soils to determine the likelihood of S leaching or accumulation in your fields. Send soil and plant tissue samples to Agronomic Services Division, North Carolina Department of Agriculture & Consumer Services to confirm S fertilizer needs.

Authors

Cooperative Extension Soil Science Specialist
Crop and Soil Sciences
Professor and Extension Specialist (Soil Management/Env. Quality)
Crop and Soil Sciences
Agronomic Division
North Carolina Department of Agriculture & Consumer Services

Publication date: May 7, 2014
AG-439-63

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