Many grasslands show phosphorus deficiencies. Clover plants growing without sufficient phosphorus are small, with small, dark-green leaves. Soil phosphorus levels are rated as very low (5 ppm); low (5 to 10 ppm); intermediate (10 to 20 ppm); and high (over 20 ppm). Based on those ratings, fertilizer should be applied as follows:
Available Soil P Amt. Single Superphosphate (ppm) or Equivalent (lb/A) Less than 5 500 every third year 5-10 250 every third year 10 or over 100 every second yearAfter two or three heavy applications on low-phosphorus soils, only maintenance amounts should be required. If soil phosphorus is high, light applications every second year increase clover production by replacing phosphorus and sulfur removed by grazing (Murphy et al., 1976).
Single superphosphate also supplies sufficient sulfur (about 12%) except where rainfall is greater than 30 inches. In wet areas, apply about 50 lbs/acre of sulfur in alternate years. If a high percentage of the sulfur will not pass through a 50-mesh screen, use from 100 to 200 lbs/acre. The larger particles will last longer. After the first year, the clover can be more heavily grazed if the soil phosphorus and sulfur supplies are adequate (Murphy et. al, 1976).
Kendall and Stringer (1985) reported that competition for sunlight is a major aspect in both intraspecific and interspecific dynamics, leading to domination of stands by clovers that express rapid regrowth or tall stature.
White and Scott (1991) in New York found that yield of cereal rye was less affected by living mulch of white clover, 'Ladino' clover, or red clover than of crown vetch, birdsfoot trefoil or alfalfa.
Taylor (1985) wrote that forage crops like clovers protect soil from rain damage and reduce runoff, erosion, and pollution of surface waters.
Insects can cause periodic local damage but do not usually eliminate the legume attacked; chemical control is usually impractical. In spring, various phytophagous mites can cause severe local damage. Along the Sierra foothills, a small leaftier caterpillar ties the folded clover leaf together with a web and lives inside, feeding on the leaf. Larvae of alfalfa weevil and clover leaf weevil (green, legless, and about 1/2 inch long) damage growing tips and young leaves in late spring (Murphy et al., 1976).
Murphy et al. (1976) detailed several important diseases of clovers. The causal agents of damping-off disease are the common soil-borne fungi Pythium spp. These usually attack young plants below the soil line, invade plant cells, and kill seedlings rapidly. Seed treatment does not protect small-seeded legumes; varietial and specific susceptibility varies.
Sclerotinia, a soil-borne fungal pathogen commonly termed water mold, leads to the appearance of dead, watery vegetation in scattered spots about 6 inches in diameter. This mainly occurs from late February through early April and is most severe in rank, ungrazed fields. The spots rapidly increase in size to several feet in diameter. A dense white mycelial web covers the dying clover. Within 10 days, small hard, black sclerotia (lumps of mycelium) appear on the soil surface. These look like large, irregularly shaped subclover seeds but are soft enough to cut open with a thumbnail and are white inside. With drying weather the disease disappears. A wide range of legumes is affected; treatment is impractical (Murphy et al., 1976).
Pepper spot (causal agent the fungus Pseudoplea trifolii) is common on clovers during February and March. Symptoms include black spots on leaflets and petioles. Severe infection causes the leaves to turn yellow, then brown, and the dead tissue is dotted with tiny black fruiting bodies of the fungus (Murphy et al., 1976).
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