The preceding discussion of the factors affecting the dynamics and loads of P in runoff and streams reveals various gaps in our knowledge and illus- trates the problems in interpreting the data thus far obtained. The first and major difficulty in data interpretation and comparison is the lack of uniformity in the forms of P measured. In many cases this makes compari- son between different studies virtually impossible, thereby prohibiting esti- mations of the relative importance of any particular source. In many studies, particularly those relating to surface runoff from agricultural land, the measurement of total P has been favored. This has led to the concept of nutrient budgets for P, whereby nutrient input and output for an eco- system are used to determine whether P is lost. This approach is favored by Frink (1967, 1971 ). If the estimates of P input and output are based on measurements of total P, little information is gained because such deter-
minations override any knowledge of the distribution of P between various forms in runoff and streams, some of which will have a greater or lesser effect on the biological productivity of surface waters.
Although relatively few studies have been conducted on the P loads of streams and surface runoff from forest and urban watersheds respectively, there is considerable agreement in the results so far obtained. The situation is quite different for P loads in runoff and streams from agricultural water- sheds. Frink (1971) stated that an “average” agricultural watershed with respect to P loss is a “useless fabrication.” It would appear, however, that the major problem arises from the lack of relevant information upon which reliable estimates can be made, a situation which has arisen largely because of an apparent lack of definition of the system being investigated.
The use of surface runoff plots to determine losses of P from agricultural watersheds presents several problems. Surface runoff is a spasmodic rather that a continuous phenomenon, its composition at any location being highly heterogeneous and likely to change over short distances because the energy of the aqueous component, and therefore its ability to carry particulate material, varies with slope. The studies cited previously (Tim- mons et al., 1968; Nelson and Romkens, 1969), in which attempts were made to measure the distribution of the P load between the solid and aque- ous phases of surface runoff appear to have limited value. When surface runoff enters streams, a much greater degree of homogeneity will be as- sumed, resulting in a new and probably more stable distribution of P be- tween the aqueous and sediment phases, as discussed previously. Measure- ment of dissolved P fractions in surface runoff itself may lead to erroneous conclusions regarding its impact on the dissolved P status of streams due to the transitory nature of surface runoff.
In order to obtain more meaningful estimates of P loss from agricultural watersheds, detailed studies of the P load of streams draining the water- sheds are required. Some such studies have been conducted (Minshall et al., 1969; Witzel et al., 1969; Campbell and Webber, 1969; Taylor et al., 1971); these will be referred to as watershed analyses herein. None of the watershed analyses cited, however, covered more than a 2-year period of monitoring; the duration of the study could lead to considerable varia- tion in P loss estimates, due to yearly differences in weather patterns as noted by Timmons et al. (1968) for surface runoff studies.
Future studies must be based on the watershed analysis approach in order to avoid bias in estimates of the P loss obtained in plot studies due to differences in the energy of surface runoff imparted by slope variations within the watershed. Furthermore, it is essential that studies be long-term to minimize yearly variation in weather patterns and that the forms of P measured be standardized. Although watershed analyses combine the P
40 J . C. RYDEN, J . K. SYERS, A N D R. F. HARRIS
loads of surface, subsurface, and groundwater runoff, these may be sepa- rated by determining P loads under various flow conditions in a way similar to that used by Minshall et al. (1969) and to some extent Taylor et al.
( 1971 ) .
With careful selection of small watersheds in the same geographic and climatic area, accurate records of fertilizer practice, and cognizance of less diffuse or even point sources of P (e.g., effluent from animal-rearing or industrial operations) within the watershed, it should be possible to obtain meaningful estimates of the effects of various land use and fertilizer prac- tices as well as physical variables on the loss of P from agricultural water- sheds. This approach is similar to that which has been used to evaluate P loads in streams draining forest watersheds. It is also important that this be coupled with investigation to define diffuse sources of P more ade- quately in terms of the components which constitute such sources. At- tempts have been made in this direction, as illustrated in the studies con- ducted by Taylor and Kunishi (1971), Cowen and Lee (1972), and Ryden et al. (1972a,b). Studies similar to these are necessary if any remedial steps are to be taken to reduce the magnitude of man-induced diffuse P sources and will be particularly valuable if carried out in conjunc- tion with watershed analyses. Only by adopting such an approach will it be possible to provide adequate estimates of the potential of soil and fer- tilizer P for the P enrichment of streams; a topic which is currently sur- rounded by considerable controversy.
Comparative tables of the relative magnitude of various P sources have been drawn up for individual watersheds (Miller and Tash, 1967; Lee et al., 1969; Jaworski and Hetting, 1970). Although such tables are useful for identification of problems within a specific watershed, extrapolation of this concept to a national basis is dangerous. Local and regional varia- tions in land use can seriously distort the relative impact of any source of P on water quality. The way in which P source data are presented can also lead to different conclusions as to the impact of one source as opposed to another. This is particularly true for comparative tables of P sources compiled on a nationwide basis. McCarty (1967) estimates that in the United States, 4.9 X loG to 77.2 X loG kg of P per year is lost to surface waters through urban surface runoff, whereas 54.5 x lo6 to 544.8 x loG kg of P per year originates from agricultural runoff. If losses are expressed on a per area basis, relative contribution estimates are very similar if not reversed, losses being 0.23 to 3.59 and 0.12 to 1.23 kg/ha per year, re- spectively. These figures show the need for careful evaluation of problems within any given watershed or group of watersheds. Watershed analyses will provide more useful data than estimations of the magnitude of various P sources from a national standpoint.
ACKNOWLEDGMENTS
Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, by the Office of Water Resources Research Project No.
WRC 71-10 (OWRR A- 038- WIS), and by the Eastern Deciduous Forest Biome Project, International Biological Program, National Science Foundation subcontract 3351, under Interagency Agreement AG-199, 40-193-69, with the Atomic Energy Commission, Oak Ridge National Laboratory.
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W . E . Knight and E . A . Hollowell
U S . Department of Agriculture. Mississippi State. Mississippi.
and U.S. Department of Agriculture. Beltsville. Maryland
I . Introduction . . .
A . Origin . . .
B . Distribution . . .
C . Economic Importance . . .
I1 . Morphology . . .
A . Root. Stem. and Leaf ...
B . Flower . . .
C . Pollination and Seed Development ...
I11 . Physiology ... ...
A . Growth and Development . . . B . Flowering . . .
C . Seed . . . . . . A . Adaptation . . .
B . Soils and Soil Fertility . . .
C . Inoculation ...
D . Establishment . . . . . .
E . Companion Grasses and Cro uences . . . F . Weed Control ... . . . G . Diseases . . . H . Insects . . .
I . Seed Production . . .
IV . Culture ...
V . Utilization . . . . . . A . Pasture . . . . . . B . Hay and Silage . . . C . Green Manure . . .
D . Seed ...
VI . Genetics and Cytology . . . . ...
A . Cytology . . .
B . Inheritance of Characters . . . A . Objectives . . .
B . Variability . . . C . Seed Shattering . . .
D . Seedling Vigor . . . E . Inbreeding and Hybridization . . . F . Cultivars . . .
References . . . . . . VII . Breeding . . .
VIII . Conclusions . . .
47
48 48 48 49 50 50 50 51 52 52 54 55 57 57 57 59 59 61 62 63 63 64 65 65 66 66 67 68 68 68 69 69 70 70 70 70 71 72 73
48 W. E. KNIGHT AND E. A. HOLLOWELL
I . Introduction
A. ORIGIN
Crimson clover, Trifolium incarnatum L., of the section Trifolium, be- longs to the Leguminosae (Ascherson and Graebner, 1906-1910; Coombe, 1968; Zohary, 1970). Numerous botanists have recognized many varieties, based on wild populations. The authors believe, however, that these are nothing more than variations of morphological characteristics found in large populations of plants.
Crimson clover is a winter-annual clover. It is native to Europe, where it was cultivated as a forage and green-manuring crop in Italy, France, Spain, Germany, Austria, and Great Britain during the eighteenth century.
In 1818, this clover was introduced into the United States. By 1855, seed was widely distributed by the United States Patent Office (Kephart, 1920).
This clover has been called “scarlet clover” because of the rich scarlet flowers. It is also known as “French clover,” “Italian clover,” “German clover,” “incarnate clover,” and “annual clover” (Westgate, 19 1 3, 19 1 4 ) . Foury (1950) lists more than twenty common names by which crimson clover is known throughout the world.
B. DISTRIBUTION
The genus Trifolium consists of some 250 described species of annual, and perennial forms that are widely distributed. Pieters and Hollowell (1937) listed crimson clover, Trifolium incarnatum L.; with red, T . pratense L.; alsike, T . hybridum L.; and white, T , repens L.; as one of the four Trifolium species of primary importance in the United States.
Crimson clover is grown widely as a winter annual from the Gulf Coast region, except peninsular Florida, and as far northward as Maryland, southern Ohio, and Illinois. It spread rapidly throughout the southeastern states after 1880. By 1900, it was considered a good crop as far north as Kentucky. It also is grown in the Pacific Coast states and is an impor- tant seed crop in Western Oregon (Rampton, 1969; Williams et af., 1957;
Williams and Elliott, 1960). If planted late in May or early in June, it can be grown as a summer annual in northern Maine (Westgate, 1924;
Kephart, 1920) and is a promising crop for high altitudes. Initially, crim- son clover was used as a winter cover and green manure crop (Duggar, 1897; von Horn, 1936; Westgate, 1914; Kephart, 1920). Since it grew during the off-season of the year, it was considered to be one of the most economical legumes for green-manuring (Duggar, 1897; Kephart, 1920).
Before 1942, the largest acreage of crimson clover was located in Tennes- see, Georgia, Alabama, Kentucky, and Oregon (Hollowell, 1943-1 947, 1947, 1950). After 1942, a rapid increase in use of crimson clover oc- curred. Contributing to this increase are: ( a ) the development of reseeding or volunteering varieties, ( b ) recognition of the requirements of crimson clover for substantial amounts of mineral fertilizers for rapid stand estab- lishment and vigorous growth, (c) an appreciation of its value for winter grazing, and ( d ) an understanding of its need for thorough inoculation
(Hollowell, 195 1 ; Hollowell and Knight, 1962).
C. ECONOMIC IMPORTANCE
Crimson clover is probably the most important annual legume in the rapidly expanding winter grazing program of the South (Stewart and Boseck, 1947; Hollowell and Knight, 1962). One of the most important characteristics of crimson clover is its ability to grow rapidly during the fall and early spring when the land is not occupied by the ordinary sum- mer-grown crops. It, therefore, fits well into cropping systems and se- quences. Other characteristics that make crimson clover the most impor- tant winter-annual legume in the South are: ( a ) it will grow under a wide range of climatic and soil conditions; ( b ) it has many uses; ( c ) it produces large yields of easily harvested seed; and ( d ) it thrives in association with other crops (Hollowell, 1951 ; Hollowell and Knight, 1962).
The total acreage of crimson clover is not known. The domestic disap- pearance of seed reached a peak in 1951 with 37,812,000 pounds of seed used in the United States. Since 1960, domestic use of seed has declined from an annual disappearance of 16,724,000 pounds to 10,116,000 pounds in 1970. Several factors contribute to this decline: ( a ) a sudden increase in seed losses in the mid 1950’s from clover seed weevils, ( b ) more than 6 0 % of the crimson clover acreage was in reseeding cultivars that did not require annual reseeding, thus reducing demand for seed, (c) a decline in price of seed as seed production moved to the West and per- acre yields of seed increased, and ( d ) an emphasis during the 1960’s on high per-acre yields of grass forage produced with mineral nitrogen.
Since 1965, considerable emphasis has been placed on arrowleaf clover.
This has resulted in a shift in acreage formerly in crimson clover to arrow- leaf clover. Unless some of the hazards involved in the production of ar- rowleaf clover are overcome, crimson clover will continue to be the reliable standby in the winter-grazing program in the South (Kight and Well- hausen, 1968).
Crimson clover has several advantages over arrowleaf, Trifolium vesicu- losum Savi. (Beaty and Powell, 1969; Hoveland et al., 1569; Knight