The phosphorus cycle in soil is a dynamic system involving soils, plants, and microorganisms. Major processes include uptake of soil phosphorus by plants, recycling through return of plant and animal residues, biological turnover through mineralization-immobilization, fixation reactions at clay and oxide surfaces, and solubilization of mineral phosphates through the activities of microorganisms (Stevenson 1986). Looking at the plant-soil-microbiological relationships in which the phosphorus (P) is partitioned into "pools" is based on availability of various organic and inorganic forms to plants (Figure 1)(Stevenson 1986). In the figure, soil solution phosphorus is shown to be in equilibrium with a given quantity of labile (unstable) inorganic phosphorus such that as phosphorus is taken up by plants, or immobilized by microorganisms, additional inorganic phosphorus is solubilized (Stevenson 1986). Microbial activity has been depicted as a "wheel" that rotates in the soil, simultaneously consuming and releasing phosphorus to the soil solution (Stevenson 1986).
Response of mixed prairie vegetation to annual applications of nitrogen (N) and P fertilizer was studied during an 8-year period at the Northern Great Plains Research Center near Mandan, North Dakota (Lorenz 1970). Fertilizer rates of 0, 40, 80, and 160 pounds of elemental N per acre and 0, 18, and 35 pounds of elemental P per acre were used. An increase in dry matter production in response to N was found to be highly significant and nearly linear in nature for the 40-N and 80-N levels, but 160-N often produced no more forage than did 80-N. Response to P was often not significant during the first three years of the study, however, over the eight-year period, each increment of P produced highly significant yield increases. Without N, response to P was small, but as N level increased, response to P increased.
In research at Dickinson, North Dakota, Goetz (1976) used a single application of 200, 300 and 400 lb of N per acre. Seven years after application, yields from the 400 N plots were 18% higher than yields from unfertilized native mixed-grass prairie, the 200 lb rate showed increases of 50-55% the first two years, but by the fourth year yields were lower, and the 300 N treatment showed an increase of 14% over five years after application.
Rogler and Lorenz (1974) found that 30 lb of N per acre gave more production per pound of N than did 90 lb of N per acre. Other studies also indicate that 30 to 40 lb per acre of N are more efficient than 80 to 90 lb of N (Nyren 1979). In yet another example, Smika et al. (1965) found that 80 lb per acre produced more pounds of forage per pound of N than did 40 or 160 lb of N. Nyren (1979) states that variability shown in these results is partially explained by the fact that these studies were conducted on different range sites, or on similar range sites which had been subjected to different grazing pressure.
In addition to improving forage quality and quantity, fertilization can also improve the palatability of range plants to livestock (Nyren 1979). Fertilization can also increase utilization of some low palatability plants. Ryerson et al. (1974) found that 200 lb N per acre increased utilization of a porcupine grass community from approximately 50 to 90% or greater.
Lorenz and Rogler (1974) studied root production under moderate and heavy grazing treatments and showed that the total root weights were nearly equal; however, the heavily grazed pasture had a greater percentage of the roots within the top 6 inches than did the moderately grazed one. It was found that the addition of 30 and 90 lb of N per acre significantly increased root volume over that of an unfertilized area within the same pasture. Added root production has the effect of increasing the amount of soil water available to the plant. Since water moves slowly through most grassland soils, the more root volume a plant has the more water that is readily available at any given time (Nyren 1979).
Botanical composition is also a factor when applying fertilizers. When N fertilizer is placed within the range ecosystem, those plants which begin their growth early are able to make use of these added nutrients (Nyren 1979). The increased growth uses up a large portion of the water stored within the soil and the roots are more vigorous, thus occupying a large portion of the soil. While there are distinct advantages to having more forage production earlier, there are reasons for not wanting to convert a native range completely to cool-season species (Nyren 1979).
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