Growth rates and nutrient accumulation in aerial biomass of sunn hemp juncea and spectabilis sown in spring

Abstract

Crotalaria juncea and spectabilis have been widely used in green manuring, mainly due to the fact that they incorporate large amounts of N into the soil, through biological fixation of atmospheric N2, and also because they have a vigorous and branched root system, characteristics that help to recycle nutrients from the deeper layers of the soil. The present work was carried out in the municipality of Mercês, in the Mata Mineira area. Growth rates and nutrient accumulation were evaluated in the aerial part of sunn hemp and sunn hemp, grown in soil with good nutrient availability. The soil in the chosen area had good fertility, with no exchangeable aluminum in the 0 to 20 cm layer. Soon after the first spring rains, the soil was subsoiled, plowed, and harrowed, and the study was then installed, conducted in an experimental design in randomized blocks with five replications. The plots consisted of six furrows of five meters in length with 0.50 meters of spacing. Sowing was carried out in the first week of October. Evaluations of dry matter accumulation and plant height were carried out in the four central furrows of the plots, at 30; 45; 60; 75; 90 and 105 days after emergence (D.A.E.), with a sample area of 1.0 m of furrow (0.5 m2). In the biomass collected at 105 D.A.E, phenological stage of grain filling, the contents of nitrogen, phosphorus, potassium, calcium, magnesium and sulfur were determined, with the objective of quantifying the accumulation of these elements in the aerial part of sunn hemp. The results obtained in this edaphoclimatic environment led to the conclusion that sunn hemp juncea had faster vegetative development than sunn hemp spectabilis, therefore, a plant with greater soil protection potential against the impact of raindrops than sunn hemp spectabilis. Although they have very different growth rates, both for sunn hemp and sunn hemp, the highest rates of growth and dry matter accumulation in the shoots occurred in the periods from 45 to 60 D.A.E., with mean values, respectively, of 406, 7 and 140.0 kg per hectare per day. The accumulation of dry matter in the aerial part, as a function of the age of the plants, can be described by the quadratic equation, y = 0.0024 x2 + 0.5267x -11.8086 (R2= 0.9888), for sunn hemp, and y = 0.0008 x2 + 0.2003x -4.4657 (R2= 0.9928), for sunn hemp spectabilis. For the adopted production system, and in this climatic environment, sunn hemp was a better recycled/fixing plant than sunn hemp spectabilis, thus, green manuring with sunn hemp was more advantageous than with sunn hemp spectabilis.