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Virginia Cooperative Extension - Knowledge for the CommonWealth

Crop and Soil Environmental News, March 2001

Fertilizers for Forages

Mark Alley
W. G. Wysor Professor of Agriculture
Dept. of Crop & Soil Environmental Sciences
Virginia Tech Blacksburg, VA 24061

Fertilizer use is an important factor for producing profitable forages and for maintaining soil productivity over years. Nitrogen (N), phosphorus (P), and potassium (K) are needed in most fields to support forage production at economic levels, produce quality forage, and to replace nutrients removed in forage harvests. The objectives of this paper are to: (1) discuss the concept of soil productivity; (2) illustrate nutrient removals with various levels of forage production; (3) establish the importance of soil pH and liming; and (4) list the properties of the major commercial N, P, and K fertilizer sources.

Soil Productivity and Nutrient Removals

All soils are not created equal. The yields that can be obtained on a specific soil will vary with the soil's capacity to supply plant-available moisture and nutrients. For example, soils that have the highest productivity (Group I) have the potential to produce 6 tons/acre of alfalfa or alfalfa-grass, greater than 4 tons/acre of tall grass clover hay, and carry 1.0 animal unit per acre with grazing (Table 1). Characteristics of Group I productivity soils are the following: (1) deep soil profiles with little restriction on rooting depth; (2) loam, silt loam, and/or clay loam textures that provide high plant-available water-holding capacity; (3) slope such that erosion and rainfall runoff are minimal; and (4) the soils are well-drained. In contrast, productivity Group IV soils have much lower yield potentials (Table 1). Soil characteristics associated with productivity Group IV soils are: (1) shallow soil profiles with restricted rooting depth; (2) high stone content or sandy texture which results in low plant-available water-holding capacity (droughty soil); (3) slope may be steep corresponding to high erosion and runoff potentials; and (4) level fields that may be poorly drained. The physical properties that lead to varying yield potentials must be recognized and management adapted to optimize the productivity of each soil.

Table 1. Forage yield potentials and grazing capacities associated with soil productivity groups.

Productivity Group Alfalfa or alfalfa-grass (t/a) Tall grass-clover hay (t/a) Acres per animal unit
I >6 >4 1.0
II 4-6 3.5-4.0 1.1-1.5
III <4 3.0-3.5 1.6-3.0
IV Not suited <3.0 3.1-6.5

Nutrients are required to grow forage plants, and different forage plants contain different levels of nutrients. Nutrient contents of various forages have been measured and some example values are shown in Table 2. Legumes generally contain higher amounts of N and K than non-legumes while P content does not vary greatly. Nutrient removals by forages can be estimated from the average nutrient content of the forage and the yield per acre. Nutrient removals per acre by forages will increase as yield levels increase (Table 3). Thus, over time, nutrient applications will need to be the greatest on soils that produce the highest yields. The greatest return on money spent for fertilizer can be expected on soils with the greatest yield potentials. These soils are capable of the highest yields, and yields will also be more consistent over years due to Group I productivity soils having high water-holding capacities.

Table 2. Estimated nutrient removals by various forages.

Crop N P2O5 K2O Mg
Alfalfa 60 15 60 5
Red clover 56 13 45 6
CS grass* 45 12 50 5
WS grass* 35 10 35 5
Grass pasture 60 5 17 2
Source: 1997 Missouri Grazing Manual
*CS: Cool season; WS: Warm season

Table 3. Estimated nutrient removals by alfalfa and cool season grass at yield levels associated with various soil productivity groups.

Productivity Group Yield(ton/ac) N(lbs/ac) P2O5(lbs/ac) K2O(lbs/ac)
--------------------------------------------------- Alfalfa ---------------------------------------------------
I 6.0 360 90 360
II 5.0 300 75 225
III 3.0 180 45 180
-------------------------------------------- Cool-Season Grass --------------------------------------------
I 4.0 180 48 200
II 3.5 158 42 175
III 3.0 135 36 150

Soil Acidity and Liming

Soil acidity is a natural situation in high rainfall (humid) climates, i.e Virginia and the entire Eastern United States. "Bad things happen to good forage plants" when soil acidity is not corrected with liming. In acid soils, N fixation by legumes is reduced, P availability is reduced, K leaching losses increase, and weed competition increases. The results of these factors are low yields, low fertilizer use efficiency, and low-quality forage. Figure 1 shows the first cutting yield response of alfalfa to lime applied at the time of alfalfa seeding. At a pH of 5.1, yields increased from approximately 400 lbs/acre to over 1600 lbs/acre with 2 tons/acre of limestone. Legumes require high available calcium (Ca) and magnesium (Mg), high available molybdenum (availability increases with increasing pH), high available P and high available K. Liming acid soils supplies Ca and Mg, increases the availability of molybdenum, and increases the efficiency of applied P and K fertilizers.

Legumes are not only higher quality forages than most grasses, they also provide N to associated grass species. Thus, the value of legumes for replacing N fertilizer can be estimated, especially in regard to higher soil pH requirements for legumes versus grass species, and the relatively high price of N fertilizers. Table 4 compares the cost of N fertilizer at $0.30 per pound of N (for three different N rates that might be applied to grasses) to the potential extra cost of 2 tons of lime per acre in order to adjust the soil pH so that legumes will be productive. Over the three-year period, dollars available for legume seed and establishment range from $68/acre to $122 per acre. Moreover, these figures do not consider the increased quality of forage associated with legume-grass mixtures compared to pure grass stands fertilized with N.

Table 4. Estimated value of legumes for replacing N fertilizer at three N rates.

N Rate(lbs/ac) N Cost @ .30/lb 3 Year Total N Cost ($/ac) Potential Extra Lime Costs - 2 t/a @$20/ton Money for legume $/acre
180 $54/ac $162 $40 $122
150 $45/ac $135 $40 $95
120 $36/ac $108 $40 $68

Fertility for Forages Basics

Soil test all fields on a regular basis, i.e. every 2 to 3 years, and apply lime according to soil test recommendations. Soil samples should be collected at the same time during the year in order to avoid seasonal variation in soil pH levels. Apply lime according to soil test recommendations. Soil pH will not increase without lime applications and thus there is no reason to wait to apply lime once a need is determined. The sooner the lime is applied, the sooner plants will respond. Also, do not over apply lime. Excessive lime applications are expensive and can result in reduced micronutrient availability. Finally, apply lime uniformly in order to obtain the greatest benefits. Uneven application will result in areas with low pH and yields will be reduced in these areas. Areas receiving the higher than needed applications will not have increased yields.

Phosphorus and potassium fertilizers should be applied according to soil test recommendations. These recommendations provide for building soil fertility levels when soil samples show that the levels are low and maintaining high levels when the high fertility has been achieved. Forages remove much more K than P, and thus when high levels of P and K are achieved the maintenance fertilizer applications will generally require much more K than P.

Nitrogen fertilizer applications to grass for hay or pasture should be split to increase efficiency. Fall applications to grass can promote tiller development and root growth. However, these applications must be carefully timed to avoid causing excessive fall growth, but be applied early enough so that temperatures will be warm enough for root growth. Late November is a general time that works for these applications, but I recommend that growers consult with their Extension Agent and initially try only a few acres to determine the best time for their specific location. The standard N fertilizer application schedule would be to apply the first N application in March, and then follow each cutting with an additional application if soil moisture conditions are good for regrowth and forage is needed. Nitrogen application rates should vary between soils, with Group I productivity soils receiving the highest applications. Application rates will also vary between years with the highest applications being made during years with high rainfall. Moisture is required to utilize nutrients. We can not overcome a lack of rainfall with increased fertilization.

Nitrogen fertilizers must be applied uniformly. Overlapped areas will lodge and not have enough extra yield to overcome those areas that are skipped or under-applied. Spreaders must be well calibrated and operators trained and required to do a good job of spreading.

Commercial Fertilizers and Manures

Commercial N-P-K fertilizers supply plant-available nutrients according to their nutrient content. Manures and other organic nutrient sources contain readily available N and slowly available N as well as P, K, and sulfur (S). Manures are excellent sources of nutrients and should be analyzed and applied according to their nutrient contents. For example, poultry litter generally contains almost as much P as N, and thus is an excellent material for building P levels in low P soils. Extension agents and Nutrient Management Specialists can arrange for manure analysis and provide recommendations on the proper use of these materials.

Commercial N Fertilizer Sources. Several commercial N fertilizer sources are available for use on forages and other crops. These materials have various characteristics that are described in the following paragraphs.

Ammonium Nitrate:

Ammonium Sulfate:
Urea-Ammonium Nitrate Solution (UAN): The selection of a N fertilizer is based on the cost per pound of N and the potential for volatilization losses. For example, Table 5 shows a cost per lb of N for various materials at a constant cost/ton for the fertilizer materials. The cost per acre for applying 80 lbs N/acre could vary from $17.60/acre to $27.20/acre, even though all fertilizers cost the same per ton. In many situations, the more expensive cost per ton material may be the best buy if it contains more N per ton than a lower cost/ton fertilizer. Finally, if the potential for volatilization losses is high, i.e. warm temperatures and no rainfall soon after application, urea should usually be avoided as losses can reach as high as 20 to 25% of the applied N. However, if the cost of urea is 25% less per pound of N than other N fertilizers, then the urea application rate can be increased to compensate for loss and the cost per acre may still be less.

Table 5. Sample values for determining the cost per pound of N and cost per acre for different N fertilizers at a constant fertilizer cost per ton.
Source lb N/ton Constant Cost/ton Cost per lb of N Cost/acre for 80 lbs N
Ammonium Nitrate 680 $204 0.30 $24.00
Urea 920 $204 0.22 $17.60
UAN - 30% 600 $204 0.34 $27.20

Commercial phosphorus fertilizer sources: Several commercial P fertilizer sources are available for use on forages and other crops. These materials have various characteristics that are described in the following paragraphs.

Diammonium phosphate:

Monoammonium phosphate: Triple Super Phosphate: Ammonium polyphosphate: Ammoniated Superphosphates: Rock Phosphate: Manures: Commercial potassium fertilizer sources: Several commercial K fertilizer sources are available for use on forages and other crops. These materials have various characteristics that are described in the following paragraphs.

Potassium Chloride or Muriate of Potash:

Potassium Sulfate: Potassium Magnesium Sulfate:


An efficient forage fertilization program adds to the profitability of producing forages while maintaining the productivity of the soil for future generations. The basic aspects of an efficient forage fertilization program are outlined as follows:

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