William Hallmark, Greg Williams, Lester Brown and Gert Hawkins

Soil fertility and plant nutrition research are important components of the LSU AgCenter’s sugarcane research efforts. With tight economic conditions and increasing concern for the environment, it is important that the nutritional needs of sugarcane be met without applying excess nutrients. To meet this challenge, the LSU AgCenter maintains a rigorous program for examining the nutritional needs of the recommended sugarcane varieties on the major soil groups where sugarcane is grown.

Nitrogen Fertilizer

The biggest fertilizer expense for sugarcane is nitrogen (N). Because sugarcane is a member of the grass family, it requires large amounts of nitrogen to produce optimal yields. Nitrogen is provided by atmospheric nitrogen, soil nitrogen and the decomposition of soil organic matter. The largest source of plant nitrogen, however, is that of commercial inorganic nitrogen, which is usually applied to the sides of sugarcane rows each spring that a crop is grown. Nitrogen can be supplied in either dry or liquid form and is usually covered with soil after application.

The high yields obtained with sugarcane variety LCP 85-384 (20 percent to 25 percent more than the next best variety) have raised questions about whether this variety should receive the same amount of nitrogen as other recommended sugarcane varieties. Some producers apply more than the recommended N rate to LCP 85-384 because it produces higher yields; others apply less in an effort to reduce lodging, which can cause yield losses at harvest.

Yield Response to Nitrogen

To answer the question of whether variety LCP 85-384 should be fertilized with the same nitrogen rates as other sugarcane varieties, 11 years of research (five years for plant cane, four for first-stubble and one each for second- and third-stubble cane) were conducted with LCP 85-384 on heavy-textured soil in the Teche region of Louisiana. Economic analyses were based on a sugar price of $0.19 per pound, nitrogen fertilizer cost of $0.30 per pound of N, fertilizer application cost of $4 per acre, and the producer’s giving half of the crop to the landlord and sugar mill.

Our research shows it is important for sugarcane producers not to apply more than the recommended N rate to LCP 85-384 because doing so can reduce sugar yields and net profits.

Results with plant cane show that applying 50 to 60 pounds of nitrogen per acre at four test sites resulted in as good a sugar yield and producer profits as where the recommended rate (100 to 120 pounds of nitrogen per acre) was used. Applying more than the recommended rate (150 to 180 pounds of nitrogen per acre) did not result in higher sugar yields or producer profits at four test sites, and actually reduced sugar yields by 630 pounds per acre ($83 per acre) at one site. Consequently, our results indicate that sugarcane producers should avoid overfertilizing with nitrogen and that they could reduce nitrogen rates with plantcane.

Results with first-stubble cane showed that applying 100 to 120 pounds of nitrogen per acre resulted in as high a sugar yield and producer profits at two test sites as where the recommended rate (140 to 160 pounds per acre) was used. At one site, overfertilizing with nitrogen (180 pounds per acre) resulted in lower sugar yields (420 pounds per acre) and reduced producer profits ($51 per acre). Our research indicates that LCP 85-384 first-stubble cane should not be over-fertilized with nitrogen, and it may require less nitrogen than is currently recommended.

Results with second- and-third-stubble cane indicate that the recommended rate of 140 to 160 pounds per acre was consistent with optimal sugar yields and producer profits.

Nitrogen Placement and Timing

Sugarcane in Louisiana is usually fertilized in the sides of the row in April or May each year that a crop is grown. Some producers, however, have expressed interest in applying nitrogen fertilizer on row tops. Also, little research has been done to determine the effect of using urease (slows the rate that urea is converted to ammonium) and nitrification (reduces the rate that ammonium is converted to nitrate) inhibitors with urea nitrogen fertilizer on sugarcane yields. It is not known whether using urease and nitrification inhibitors with urea would allow it to be put out earlier, since there would be less potential for nitrogen loss caused by urea volatilization, nitrate leaching and denitrification. To provide information on the above, research was conducted on heavy-textured soil in the Teche region of Louisiana.

Our work showed that placing dry urea nitrogen (120 pounds per acre each year) on row tops (of sugarcane rows where cane trash was burned the previous fall) in early May resulted in equivalent sugar yields across four years (plantcane through third-stubble) compared to where dry urea was applied to the sides of sugarcane rows (in early May) and covered with soil.

Results further showed that applying liquid nitrogen stabilized urea (containing calcium chloride and urease and nitrification inhibitors) in a 1-inch band in the row furrows between sugarcane rows in late December to early January resulted in as good a sugar yield across two years (plant and first-stubble cane) as where the same liquid urea N rates (60,100,140 and 180 pounds per acre) were applied to the sides of the rows in early May and covered with soil.

Organic Nitrogen Fertilizer

While inorganic nitrogen is the overwhelming choice of nitrogen fertilizer applied by sugarcane producers, organic fertilizer can be used on a limited basis to help meet the nutritional requirements of sugarcane. Sources of organic nitrogen fertilizer include composted municipal and agricultural waste and municipal sewage sludge. Research at the Iberia Research Station shows that these materials are safe and effective sources of nutrients for sugarcane production. They are also good sources of organic matter, which helps improve soil structure and water infiltration and storage in soils. Since root diseases of other crops have been reduced with organic matter application, root disease in sugarcane (which depresses yield and can reduce the number of stubble crops) has the potential to be reduced by organic fertilizer as well.

At present, organic fertilizer is available only on a limited basis for commercial use, but as landfills close because of environmental concerns and high costs, municipalities will need an alternative means of disposing their organic waste in a socially acceptable manner. Making these wastes into organic fertilizer for use in agricultural production would be a responsible means of dealing with the solid waste problem.

Managing Combine Residue

About 85 percent of the sugarcane acreage in Louisiana is harvested with combine harvesters. Much of this cane

is harvested green chopped, which results in a trash (residue) blanket on the soil surface that can reduce sugar yields from 500 to 1,000 pounds per acre for the following crop if it is not removed or burned. However, removing the residue blanket from the row tops and placing it in the furrow can cause cultivation problems the following spring.

Many producers burn the trash blanket after harvest, resulting in air quality problems. Burning the sugarcane residue also results in loss of nitrogen and organic matter that could improve soil fertility and soil manageability if the trash blanket were not destroyed.

The sugarcane combine residue blanket is at present more of a liability than an asset. Research in progress at the Iberia Research Station seeks to determine if spraying the combine residue with nitrogen-stabilized urea (containing a urease and nitrification inhibitor) can convert the trash blanket into organic fertilizer, which could increase soil fertility and manageability. At this time, research results are too premature to make a recommendation.

Fall Fertilizer

Fertilizer is usually applied to sugarcane in the spring for each year that the crop is grown. Research at the Iberia Research Station, however, showed that applying a limited amount of inorganic fertilizer under cane at planting (fall fertilizer) resulted in profitable economic returns for heavy-textured soil. Our work showed that the fall fertilizer rate of 15 pounds of nitrogen per acre, 60 pounds of phosphate per acre and 60 pounds of potassium per acre resulted in increased sugar yield and profits. Applying more than 15 pounds of nitrogen per acre as fall fertilizer resulted in less sugar and a net loss in profits.

Potassium Fertilizer Research

Sugarcane requires large quantities of potassium. The recommended rate of potassium application to sugarcane in Louisiana is based on the crop year (plant or stubble cane), soil type and the level of exchangeable soil potassium. Potassium soil fertility research at the Iberia Research Station is used to test the validity of the potassium fertilizer recommendations.

Potassium is usually applied as potassium chloride, though some sugar-cane producers prefer to use potassium sulfate. Our research is comparing the efficacy of the two potassium sources on sugarcane yields. To date, our results indicate that potassium sulfate is not superior to potassium chloride in growing sugarcane.

Industrial Byproducts

Sources of byproducts for possible use in growing sugarcane are byproduct lime from sugar refineries, byproduct gypsum from fertilizer producers, calcium silicate slag produced by steel processors and waste from aquaculture processors.

Our research shows that byproduct lime and gypsum are very fine in texture and react quickly with soil to obtain their desired effect. Often these materials can be obtained for little more than the cost of shipping and are wisely used by some sugarcane producers.

Calcium silicate slag is an effective liming material for reducing soil acidity. It provides silicon, an essential element for sugarcane growth and reproduction. Research at the Iberia Research Station is evaluating the effects of calcium silicate slag and fish emulsion in sugarcane production.

Fertility Research Summary

LCP 85-384 plantcane and first-stubble cane should not be overfertilized with nitrogen.

Applying dry urea nitrogen fertilizer in a band on row tops can be an effective way to fertilize sugarcane (where cane trash had previously been burned).

Liquid urea nitrogen (stabilized to prevent nitrogen loss) applied to row furrows in a narrow 1-inch band in the winter produced as good a sugar yield as where N was applied to the sides of rows in the spring.

Applying a limited amount of fall fertilizer under cane at planting can increase sugar yields and producer profits.

Municipal and agricultural waste can be safe and effective sources of organic matter and nutrients for producing sugarcane.

Industrial byproducts offer a cheap source of nutrients and liming materials.