Green Manuring for Soil Productivity Improvement [antikvár]

Preface Green manuring — a practice of ancient origin — is defined as the use of green plant material, grown in situ or cut and brought in for incorporation to improve soil productivity. The concept is distinct from, although having elements in common, with short- or long-duration cover-cropping and rotational ploughing-in of green crop residues such as mungbean. The species commonly used are of the Leguminosae with Crotalaria juncea (sunn hemp) and Sesbania aculeata (sesbania) being the most popular in India, and Vicia spp., Trifolium spp. and Astragalus sinicus in cooler climates. Chapter 2 outlines the standard cultivations involved. A small starter-application of N may be beneficial on some soils, as also of P instead of applying it to the following rice crop. There is some flexibility in age at which green manure should be incorporated; 7 to 8 weeks, i.e. just before the onset of woodiness, is a guideline. The importance of thorough incorporation is stressed and various mechanical devices are mentioned; the most efficient, a tractor/disc-harrow system, however, is beyond the reach of most small farmers. An efficient but labour-intensive method involves traditional ploughing-in of sesbania in standing water and trampling the herbage into the paddy field. Time for decomposition before establishing the main crop is also an important consideration; evidence is variable but seems to suggest up to 8 weeks for rainfed conditions ?nd up to 2 weeks for irrigated rice areas. In addition to the root-nodulating, Nj-fixing system of leguminous green manures', mention is made of S. rostrata that bears nodules on the stem. Another source of atmospheric N2 is the floating fern Azolla pinnata which, in symbiosis with a N2-fixing blue-green alga, is an efficient carrier of N to rice. Techniques for its multiplication and use in wetland rice are given. Perennial legumes such as Gliricidia sepium and Leucaena leucocephala grown separately in rows or on bunds provide another source of green manure from prunings taken five or six times a year. ¦jj^'i. Chapter 3 and Annex 2 report biomass and N content for various species from worldwide locations. Values for the former vary greatiy within species and location, ranging from 2.9 to 9.9 t/ha dry-matter basis for annual species, and up to 80 t/ha/year fresh material for Leucaena. Yields of over 100 kg N/ha in the herbage are common in 60-day-old sesbania, but values exceeding 200 kg N/ha are reported from the Philippines, China and the USA. The proportion of plant N derived from the atmosphere can amount to 90 percent in S. rostrata and S. cannabina. Azolla pinnata contains 4-5 percent N on a dry-weight basis and can fix an average of 2 kg N/ha/day. Differences between the six Azolla spp. in ability to fix N2 vary up to 8-fold according to season. The fates of individual nutrients released from incorporated green manure are discussed in Chapter 4. Factors affecting the mineralization kinetics of N include N content of the herbage, C/N ratio and lignin content; release is retarded by high values of the last two, and accelerated by high temperatures and anaerobic conditions. Under waterlogged conditions the time of peak release is at least two weeks after incorporation. Mineralization of P re-cycled from the subsoil follows a similar process to that for N, except that no losses by volatilization