published in


April 2012

Getting Ready for Spring Fertilization

by Maria Silveira

Ona Report - Dr. Maria Silveira

For questions or comments regarding this publication contact: Dr. Maria Silveria, University of Florida, IFAS

It is that time of the year again. Nitrogen fertilizer is often applied to pastures in Central and South Florida in late February to early March to increase forage production and nutritive value. Despite the relatively low forage production during the months of March to May, fertilizer can boost forage production to meet the animal requirements during this critical period of the year. However, due to the historically high prices of fertilizer, early planning is a key to successful spring fertilization. 

Soil and Tissue Testing
Soil and tissue testing are critical components of a successful soil fertility program. Soil should be tested before spring fertilizer application to ensure that soil pH is within the optimum range for each specific forage crop. The majority of the forage species cultivated in Florida requires soil pH around 5.5. Excessively high pH can be as detrimental as low pH.  Bahiagrass, for instance, is particularly sensitive to high pH and will not perform well when soil pH exceeds 6.5. Most soils in Florida are acidic and will likely require lime application to increase the pH to the desirable levels. 

Soil test results are only as good as the sample taken. It is very important to submit a soil sample to the laboratory that truly represents the area of interest so that a reliable test and recommendations can be made for the entire area. A minimum of 15 to 20 subsamples (0 to 6 inches in depth) should be collected from each field and composite into a single sample. Samples should be taken at random in a zigzag pattern over the entire area. Areas that are managed or cropped differently should be sampled separately. Similarly, areas that show clear signs of a problem (i.e., poor forage production, disease) should also be sampled and analyzed separately. Collecting a good, representative soil sample is well worth the time and effort it requires. Soil samples can be taken using a soil probe or a shovel. The most important point is to be consistent and collect every sample as close as possible to the same depth. Place all the subsamples (15-20) for each area in a clean plastic bucket and mix thoroughly. A hand full (approximately 1 pint) of soil should be sent to a reputable laboratory for analysis. If multiple samples are sent to the lab, pack them in sturdy containers to avoid cross-contamination among the samples.

Plant tissue analysis has been recently incorporated into the revised UF/IFAS fertilizer recommendations as a management tool to guide proper phosphorus fertilization in established bahiagrass pastures. Unlike soil sampling that can be done at any time of the year, tissue sampling should be done when the forage is actively growing. This can be challenging, especially during late fall and winter months when the majority of the forages cultivated in Florida are dormant. The best approach is to collect both plant tissue and soil samples at the same time of the year (preferably before the first frost in the winter). Similar to soil testing, tissue samples must be representative of the field. The number of plants to sample in a specific area will depend on the general conditions of plant vigor, soil heterogeneity, and forage management. A truly representative sample can be obtained by sampling a large number of plants so that the sample represents the entire field. Collect at least 1 ounce (30 g) of fresh material. Sampling is not recommended when plants are injured by insects and diseases. To avoid contamination, plants should not be sampled soon after spraying pesticides or herbicides. Care should be taken to minimize soil contamination on the sampled plant material. In addition, plants should not be sampled under temperature or moisture stress. Ideally, samples should be collected during a time of the day when climatic conditions are mild, generally early to mid-morning or early evening. The plant part, maturity stage and time of sampling are also important factors that can affect plant nutrient composition. Forage grasses and hay fields should be sampled prior to seed head emergence. Care should be taken to select the plant part that accurately reflects the nutrient status of the plant. The top portion of the plant (similar as the cattle would graze) should be sampled. Do not sample seeds as they are not useful for assessing nutrient status of forage crops and may introduce large errors in the report interpretation. If deficiency symptoms are suspected, plants showing these symptoms should be sampled and analyzed separately from “normal” or healthy appearing plants. After sampling, tissue should be placed in properly labeled paper bags and sent immediately to a reputable laboratory for analysis. Avoid plastic bags because they can hold heat and moisture. The same precautions taken for collecting the plant material should be followed for handling the samples. Because fresh plant material may start decomposing shortly after collection, it is important that plant material be sent to the laboratory as quickly as possible. Prior to transporting the samples to the laboratory, plant material should be stored in a refrigerator at 41oF (or 5oC).

Lime Application 
When planning lime application, one important aspect to consider is that time that the liming material will require to react with the soil. In general, liming materials such as dolomite or calcitic limestone react relatively slowly in the soil. Depending on the moisture conditions and the characteristics of the liming material such as purity and particle size, it may take from 3 to 6 months for the liming material to react with the soil and increase the pH. The finer the particle, the faster the material will react. There are a number of by-products such as lime-stabilized biosolids that contains alkaline compounds (i.e. calcium carbonate, calcium hydroxide) and may also be used as a soil amendment to increase soil pH. The decision to whether use a slow- or quick-acting liming material can be challenging. If lime can be applied at least 3 months prior to the spring nitrogen fertilization, then more coarse liming materials can be used. On the other hand, pulverized lime, slaked lime, and burnt or hydrated liming materials can react quickly in the soil, so the soil acidity is reduced faster. The down side is that the effects of these “fast-reacting” materials typically last less than one year. Lime application is relatively easy unless the liming material is exposed to rainfall or excessive moisture. When liming material contains excessive moisture, it may not flow easily through the spreading equipment, which may result in a non-uniform distribution. 

Soil type, fertilizer management and cropping intensity are factors that determine the liming frequency. In general, hayfields require more frequent lime applications as compared to grazing pastures because of the greater crop removal and fertilizer application rates. Despite the fact that sandy soils typically require lower lime application rate, more frequent lime application may be needed to maintain the soil pH at the desirable levels. 

The amount of lime required to raise the pH will depend on the soil acidity and the solubility and purity of the material. The lower the soil pH (more acidic conditions), the more lime will be required. In addition, soil organic matter levels and soil texture also affect the amount of lime needed. Lime recommendations based on soil testing generally assumes that a pure liming material (calcium carbonate equivalent or CCE of 95 to 100%). Application rates should be adjusted according to the CCE of the liming material that will be applied. For instance, if soil test report recommends 2 ton per acre and quick lime with a CCE of 160 is used, then application rate should be reduced to 1.2 ton per acre.

Nitrogen Fertilization
Both commercial fertilizer and organic sources such as biosolids and animal manure can be used as nitrogen sources for pastures and hayfields. Several fertilizer sources are commercially available to supply nitrogen, phosphorus, potassium and micronutrients to forage crops. In this section, we will focus on nitrogen fertilization, but the same considerations should be applied to other essential nutrients.

Ammonium nitrate, ammonium sulfate, and urea are the major nitrogen sources used on pastures in Florida. Organic sources such as biosolids and animal manure also represent important sources of nitrogen that can be used in pastures. When choosing the right fertilizer source, it is important to consider important factors, such as price, fertilizer effectiveness, method and rate of application.

Cost of fertilizer should be calculated in terms of dollars per pound of nutrient. So if ammonium, nitrate contains 34% of nitrogen and ammonium sulfate has 21% nitrogen, the cost of each source should be based on how much you are paying per pound of nitrogen. Below is an example how this can be easily calculated. Please note the fertilizer prices used here are just an example, so please check with your local fertilizer dealer the current fertilizer cost.

  • Ammonium nitrate (34% N) costs $350/Ton.
    • 2000 lb ammonium nitrate contains 680 lb N (2000 X 0.34 = 680). Thus, the price per lb of N is $0.51 (350/680= 0.51)
  • Ammonium sulfate (21% N) costs $300/Ton
    • 2000 lb ammonium sulfate contains 420 lb N (2000 X 0.21 = 420). Thus, the price per lb of N is $0.71 (300/420= 0.71)

In addition to fertilizer costs, it is also important to consider the acidity potential of each nitrogen fertilizer source. Regardless of the source, nitrogen fertilization typically reduces soil pH. However, some nitrogen sources can cause a reduction in soil pH more rapidly than others. Thus, when choosing a nitrogen source, it is also important to account for additional costs associated with lime application. For instance, ammonium nitrate requires 0.61 lb of lime per lb of fertilizer, while ammonium sulfate requires 1.10 lb of lime per lb of fertilizer to maintain soil pH. Although this may be an important issue in hayfields where more nitrogen fertilizer is applied on an annual basis, for pastures that normally receive less than 60 pounds of nitrogen per acre, the source of commercial nitrogen fertilizer has minimum effect on soil pH or forage production.Commercial fertilizer mix often provides multiple nutrients, which can be most economical in some situations. However, the N:P:K ratio of the fertilizer formula should coincide with the soil test and tissue recommendations to avoid unnecessary nutrient application. 

Organic fertilizer sources such as animal manure and biosolids can satisfactorily provide nitrogen and other nutrients to forage grasses. When properly applied, these organic sources can be beneficial to agriculture with no negative impacts on the environment. Another advantage of organic sources is that, because of the alkaline nature of some of these materials, they can increase soil pH and reduce costs associated with liming.

One important aspect to consider when using organic amendments is that the nitrogen present in these sources is not readily available to plants and total nitrogen is often a poor indicator of nitrogen availability. While only 40% of the total nitrogen in some biosolids materials may become available in the first year, up to 80 to 90% of the total nitrogen present in chicken manure may be available during the same period. As the organic compounds mineralize, nitrogen and other essential nutrients become available to the plants. Factors such as source, time and rate of application and environmental conditions can impact the effectiveness of organic materials in providing nitrogen to pastures. From both agronomic and environmental prospective, it is important to monitor soil fertility after organic amendment application to avoid nutrient buildup in the soil or soil pH above the desirable range.

Timing and rate of fertilizer application
Fertilizer should be applied when the forage is actively growing. For most warm-season grasses commonly used in Florida such as bahiagrass, growing season does not start until night temperatures reach 50 to 60oF, which typically occurs in early spring. For establishment of new plantings, fertilizer should not be applied until plants have emerged. Nitrogen and potassium should be split-applied into two applications: after emergence and 30 to 50 days later. For hayfields, nitrogen and potassium should be applied after each cutting. 

Unlike phosphorus and potassium recommendations, nitrogen application rates are not based on soil test results, but rather they are calculated based on expected yields.  Grasses such as bermudagrass, limpograss, and stargrass usually require higher fertilizer application rates than bahiagrass pastures. Beside the forage species, another important aspect that should be considered is how much grass is needed. Do not fertilize pastures if forage production will not be consumed by grazing animals and/or harvested for hay. Nitrogen fertilization will likely increase forage production and nutritive value but these benefits may not be economical if not converted into animal product. Thus, adequate stocking rate is another important variable that should be considered when choosing nitrogen rates.