Tropical Legumes for Green Manure. I. Nitrogen Production and the Effects on Succeeding Crop Yields

The energy shortage of the 1970s and knowledge that N prices may continue to escalate renewed interest in growing legume green manure crops to furnish N. The objective of this study was to evaluate the ability of selected green manure crops to furnish N to succeeding crops. This study evaluated seven tropical legumes and a marigold (Tagetes erecta L.) cv ‘Cracker Jack’ grown for different lengths of the summer under fumigation (F) and nonfumigation (NF) on Arredondo fine sand (loamy, silicious, hyperthermic Grossarenic Paleudults) during 1979, 1980, and 1981. Split-plot and randomized complete block designs with five replicates were used. In the basic experiment, fumigation and nonfumigation were main plots and tropical legumes and fallow were the subplots. The green manure crops were grown for three different periods, early summer, full summer, and late summer. Early and late summer plantings usually included only the legume crops and fallow in a randomized complete block design. Average top growth N yields in kg ha⁻¹ were 40 for ‘PI 305070’ mungbean [Vigna radiata (L.) Wilcz.], 250 for ‘Norman’ pigeonpea [Cajanus cajan (L.) Millsp.], 190 for ‘FL 81d’ pigeonpea, 170 for showy crotalaria (Crotalaria spectabilis Roth.), 220 for hairy indigo (Indigofera hirsuta L.), 170 for jointvetch (Aeschynomene americana L.), 190 for velvetbean [Mucuna deeringiana (Bort.) Merr.1, and 60 for marigold. The dry matter yield of legumes (harvested before seeding) with higher N yields usually exceeded 10 000 kg ha−1 for full-season production in all 3 p. Fumigation increased dry matter yield of legumes and marigold, but not their N yield. The grass crops, rye (Secale cereale L.), ryegrass (Lolium multiflorum L.), maize (Zea mays L.), and wheat (Triticum aestivum L.) produced significantly higher dry matter (up to 100%) when planted after green-manure summer legumes than when planted after summer fallow. Calculated uptake of N from green manure by these grass crops was relatively low, averaging from −2 to 23 kg ha−1. Applying 50 and 100 kg N ha−1 as ammonium nitrate to the green-manured plots did not increase recovery of N from green manure by wheat.