Discussion and Summary
A few cases have been cited to illustrate the relationship of soils and their fertilization on the chemical composition of hay and pasture. Because of the mixed vegetation usually occurring in meadows and pastures, including the experimental plots, it was necessary to omit references to classic examples of the remarkable variations in the botanical and chemical composition of forage of different areas and of the well-being of the animals kept on them. However, enough comparable data have been tabulated or quoted to show that of the so-called major elements—N, P, K, and Ca—P is the one which is most frequently present in very small amounts in hays and pasturage. In different regions, varying circumstances make the P situation acute. In some the soil is too acid, in others too alkaline and in still others, the total supply of P in the soil is so low that only small amounts of P are available for plants and therefore only a sparse vegetation can exist. The liming of acid soils has in many instances, but not all, increased the availability of P to plants. A practical remedy for the unavailability of phosphates in alkaline soils does not seem to have been found, although some benefit has been obtained from liberal applications of S. Therefore, in some regions, means of overcoming deficiencies of available P constitutes a serious problem.
On the other hand, this paper contains many examples of the remarkable effects of phosphatic fertilization on grassland. The lasting qualities of such treatments should be emphasized. Thus, it has been demonstrated that an application of 16% superphosphate at 500 pounds on the runout pastures of northeastern United States will increase the quantity and quality of the herbage for at least 14 years (5). Moreover, when superphosphate is the source of P, there is usually an increase in the Ca content of the grasses or legumes.
In most of the humid areas, Ca is the next most important limiting element. The prevalence of legumes, particularly, is closely related to the level of exchangeable Ca in the soil. It has also been shown in this and other papers that indirectly the N content of grasses in mixed vegetation is dependent on a lime supply sufficient to permit a thrifty growth of legumes. Otherwise, recourse must be had to frequent applications of nitrogenous fertilizers.
Although the pH of soils is not an accurate criterion of whether a species will thrive on them, it may be the most easily determined guide. The opinion is held here that pH should not be much below 5.5 for red clover and white clover and Lespedeza or below 6.0 for alfalfa and sweet clover. The value of Ca as a nutritive element has probably been under-emphasized. Better results may be expected with less acid conditions, not only directly, but indirectly through making it less likely that added phosphates will be fixed as more insoluble iron and aluminum compounds.
Evidence of the adequacy of the available K supply in soils for forage plants is much more conflicting than it is for P and Ca. The tables contain many data showing wide differences in the K content of plants because of variations in soils or fertilization. Of the three major minerals, plants appear to absorb more K, both absolutely and relatively, than P or Ca. This is especially true with more mature plants such as are harvested for hay. In other words, “luxury” consumption is accentuated in the case of K.
Another important consideration is that animals void a large part of the K consumed in their feeds. In this connection, there is some evidence that land runout by grazing is much less in need of K than adjacent areas depleted by removing the hay. However, on light soils or on soils under tillage for many generations, the successful growing of forage plants, particularly legumes, depends upon conserving and returning manure to the land or on applying potassic fertilizers. For example, alfalfa is very intolerant of low levels of available K in the soil and in some of the eastern states will not persist long unless K is supplied frequently by manure or fertilizers. In the middle west and especially the far west, the need for much fertilization with K has not yet become evident.
The growth of grasses is likely to be increased appreciably by adding nitrogenous fertilizers. Usually the protein content of grasses, grazed or mowed in the vegetative stage (before heading) will be increased by such treatments. The chief disadvantages of this practice are that the effects only last a few weeks, the materials are relatively more expensive and the greatest returns per unit of N are obtained from spring applications or in a period when pastures are at their best. A nitrogen carrier may appreciably affect the concentrations of other elements in plants.
Among forage crops, there are very few reported instances of plant deficiencies of other certainly necessary plant nutrient elements—Mg, Fe, Mn, S, B, Cu, and Zn. (Animals need still other elements such as Na, Co, Cl, and I, but usually it is necessary or expedient to supply them directly, not via soils and plants.) Probably this is due to the very small quantities needed and also to the fact that they are frequently present in fertilizers as impurities or parts of compounds containing major elements. However, soil deficiencies of these elements for pasture and hay crops have been reported. The availability to plants of many of these elements depends on the natural reaction of soils or amounts of lime added to them. This is especially true of Mn and B and only recently the so-called “over-liming” injury has been shown to be due, at least in some circumstances, to the unavailability of B on heavily limed soils (12).
Minor element deficiencies appear to have been more common among legumes than grasses. From limited analyses one would conclude that normal legumes are richer in some of these elements. For example, McHargue (11) reports three or four times as much B in alfalfa as in bluegrass.