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Virginia Cooperative Extension - Knowledge for the CommonWealth

Crop and Soil Environmental News, February 2002

Fertilizer Strategies for Dry Weather

Greg Mullins and Steve Donohue
Extension Nutrient Management Specialist, Virginia Tech;
Extension Soil Testing and Plant Analysis Specialist; respectively

The recent extensive drought across Virginia has raised many questions about managing fertilizers for forage production. Frequently, fertilizer application cannot be adjusted during a drought year, because fertilizers are often applied before the true extent of a drought is known. However, fertilizer use can become a significant issue the year after a drought, due to low forage yields during the drought year. Low yields during the recent drought mean that significant amounts of unused nutrients could remain in the soil, potentially affecting fertilizer needs for the upcoming growing season.

A basic premise is that the nutrient-holding capacity of soils makes it possible for those who have had a good fertilization program over the years to utilize some of this "stored" fertilizer when the situation dictates, and just such a situation may be here for many forage producers. Several methods are available to help growers determine nitrogen, phosphorus and potassium carryover and current needs.

Phosphorus and Potassium Needs.

Regardless of the fertilizer form, phosphorus and potassium will remain as residual forms in the soil unless plants take it up or if potassium is leached by rainfall in sandy soils. If phosphorus or potassium was applied but not used because of lower than expected yields, it usually remains in the top few inches of soil and the unused portion will be carried over as residual soil forms that are potentially available for future crops.

A routine soil test is the best tool for determining the current levels of available phosphorus and potassium and to obtain fertilizer recommendations. In Virginia we believe good pasture and a good crop of hay can be grown without broadcast applications of phosphorus and potassium and with little or no reduction in yield if the soil tests High to Very High in these two nutrients. Thus, during the year after a drought (i.e., 2002), the cost of producing forage crops can be reduced by eliminating applications of phosphorus and/or potassium on soils testing Very High in phosphorus and/or potassium. For soils testing High in phosphorus and/or potassium, application of these nutrients can be eliminated for pastures and hay crops, especially if they were applied during the drought year. Thus soil testing is the most accurate method of determining the phosphorus and potassium needs of forages in Virginia following a drought year. If phosphorus and potassium fertilizer applications are eliminated on high testing soils the year following a drought, a new soil sample should be collected and tested the next year.

In productive forage systems nutrient utilization during the drought year can be estimated based on the actual drought-year yield and nutrient removal by the harvested forage.

Approximate nutrient content of selected forages.

Crop (Unit of Yield) Pounds Per unit of Yield
  N P2O5 K2O
Alfalfa (ton) 60 15 60
Cool Season Grass (ton) 45 12 50
Warm Season Grass (ton) 35 10 35
Pasture (ton) 60 5 17
Source: 1997 Missouri Grazing Manual


This example shows that the actual yield during the drought year was 1/3 of the yield goal based on VALUES and the expected nutrient removal would be 12 lbs P2O5/acre & 50 lbs K2O/acre. When applying potassium and phosphate fertilizers an important point to remember is that plants do not have a high efficiency for taking up recently applied fertilizer phosphorus and potassium even under optimum growing conditions. A low uptake efficiency is due to the retention of phosphorus and potassium in soils. Using the estimated phosphorus and potassium removals in the above example, one would expect to see little to no decline in soil test levels for either phosphorus or potassium on high testing soils after the hay is harvested. Little change would occur since fertilizer input and crop removal of phosphorus and potassium versus changes in soil test levels is not a 1:1 relationship. This is especially true for phosphorus. For example, in a field trial conducted in Orange, Virginia on a clay loam soil, an average of 10.4 lbs P2O5/acre were required to increase the soil test P level by 1 lb/acre in treatments receiving annual applications of 100 lbs P2O5/acre from 1953-1967. In a long-term residual phosphorus study conducted on a fine sandy loam soil in North Carolina, crop phosphorus removal of approximately 3.5 lbs P2O5 was required to reduce the Mehlich 1 soil test phosphorous level by 1 lb/acre over a 15 year period without any additional phosphorus fertilizer. Thus, soil testing is the best tool available for determining the carryover of residual fertilizer phosphorus and potassium in Virginia soils.

Nitrogen Needs

Nitrogen is generally the most expensive nutrient we apply and its chemical form, soil conditions and method of application affects its fate when applied to soils. Following a drought year, most nitrogen carryover exists as nitrate in the plant root zone. However, the possibility of overwinter loss of residual nitrate makes estimation of carryover of nitrogen more difficult than for phosphorus or potassium. Except for the pre-sidedress nitrate test on corn, there is no good way to accurately estimate the amount of nitrogen available to the crop by a soil test. Therefore, accurate determination of the amount of nitrogen carryover into the 2002 crop season is not possible.

In estimating the amount of nitrogen carryover one needs to estimate the amount of residual nitrogen in the soil at the end of the growing season as well as take into account factors affecting overwinter nitrogen losses. Nitrogen carryover from the previous drought year will depend on several factors. Nitrogen carryover is likely where:

Nitrogen rates following a heavily fertilized but poor yielding forage crop can be reduced by 20-30% for the spring application if rainfall during the fall and winter is below normal or if the soil stays frozen during the winter. If rainfall and temperatures are normal or above, probably the best strategy will be to apply normal nitrogen rates in the spring.

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