S. Wiethölter

Assessing Indices for Predicting Potential Nitrogen Mineralization in Soils under Different Management Systems

A reliable laboratory index ofN availability would be useful for making N recommendations, but no single approach has received broad acceptance across a wide range of soils. We compared several indices over a range of soil conditions to test the possibility of combining indices for predicting potentially mineralizable N (N 0 ). Soils (0-5 and 5-15 cm) from nine tillage studies across the southern USA were used in the evaluations. Long-term incubation data were fit to a first-order exponential equation to determine N 0 , k (mineralization rate), and N 0 * (N 0 estimated with a fixed k equal to 0.054 wk -1 ). Out of 13 indices, five [total C (TC), total N (TN), N mineralized by hot KCI (Hot_N), anaerobic N (Ana_N), and N mineralized in 24 d (Nmin_24)] were strongly correlated to N 0 (r > 0.85) and had linear regressions with r 2 > 0.60. None of the indices were good predictors ofk. Correlations between indices and N 0 * improved compared with N 0 , ranging from r = 0.90 to 0.95. Total N and Hush of CO 2 determined after 3 d (Fl_CO2) produced the best multiple regression for predicting N 0 (R 2 = 0.85) while the best combination for predicting N 0 * (R 2 = 0.94) included TN, Fl_CO2 Cold_N, and NaOH_N. Combining indices appears promising for predicting potentially mineralizable N, and because TN and Fl_CO2 are rapid and simple, this approach could be easily adopted by soil testing laboratories.

Épocas e métodos de aplicação de nitrogênio em milho cultivado no sistema plantio direto

Corn (Zea mays L.) is usually cultivated after winter cereals under no-tillage in southern Brazil. During the decomposition of plant residues, nitrogen (N) can be imobilized and limit plant growth. In order to evaluate the effect of timings and methods of N application on corn yields, five experiments were carried out during 1997 to 2002, under no-tillage. Black oat (Avena strigosa Schrieb) was used as a preceding cover crop, beeing desiccated at anthesis. Randomized blocks were used as experimental design, with four replications, and 8 to 12 treatments. N was applied to the soil by different methods (on the surface, broadcasted and incorporated) and timing (before seeding, at seeding, and at seeding + topdressing), at the rate of 100kg ha-1, as urea. At the first year (1997/98), a very high amount of rain fell during the months of September and October, before and shortly after corn emergence. In this season, higher grain yield was obtained by the treatments with topdressed N than by all other treatments, which were lower and similar among themselves. In the subsequent years, with less rainfall, no significant differences were observed among treatments, except in the 2000/2001 season, for the treatments in which N was broadcasted before seeding. Considering the average yield, no differences were observed among the timings of N application. On the average, incorporation of N, at any of the three timings of N application, indicated a tendency to generate about 5 % higher grain yields than soil surface application.

Bases teóricas e experimentais de fatores relacionados com a disponibilidade de potássio do solo às plantas usando trigo como referência

The degree of soil nutrient availability to plants is one of the major factors determining crop production. Mathematical models have been proposed to simulate nutrient flux toward roots, taking into account several physical and chemical soil parameters as well as anatomical and physiological root traits. The objective of this paper was to establish a relationship between Baldwins theoretical diffusion model and experimental field data involving the response of wheat to K fertilization. Based on simulations with the theoretical model, it was observed that soils with a 3.6 times higher K buffer power would result in 3 times lower K root absorption. An empirical model derived from three wheat field experiments indicated significant interaction among the following factors: grain yield, soil clay content, applied K rate, and soil K content. The higher the clay content, the greater the K rate necessary for a given wheat yield. The model indicated that for every 10 % more clay the K2O demand would increase by 8 to 9 kg ha-1. When the soil K content was above 40 mg dm-3 there was a tendency to gradually decrease the amount of K to be applied for any soil clay content, indicating that the minimum K level for wheat development for the studied soils was about this concentration. These observations may imply that soil clay content, or any other soil factor related to it, such as the soil K buffer power or cation exchange capacity, could be incorporated into K fertilizer recommendation, once these factors present a relationship with Baldwins diffusion model variables.

Dose de esterco de ave poedeira e suprimento de nitrogênio à cultura do trigo

Poultry manure has been used as fertilizer in grain crops, however, its efficiency to supply nutrients to the plants is not well known. The objective of this study was to evaluate the effect of poultry manure doses on nitrogen (N) supply and on grain yield (GY) of wheat. The doses tested were (wet basis): 2.8, 4.2, 5.6, and 11.2 t ha-1, which were compared to treatments with no manure (control), but with or without urea application. These last treatments were complemented with mineral application of P, K, Ca, and S in amount equivalent applied by 5.6 t ha-1 of manure. The experiment was carried out in Passo Fundo, in the State of Rio Grande do Sul, on a Red Latosol (Typic Haplorthox). A randomized block design was used with four replicates. Poultry manure supplied the amount of N required by the plants, mainly when the application rate was 2.8 t ha-1. Plants fertilized with this rate and with urea show the highest N accumulation in anthesis. The application increased N mobilization to the grains, increased number of spikes per area and grains per spike, and, consequently, GY. Poultry manure applied at 2.8 t ha-1 was enough to supply N and to increase GY similar to observed by applying N as urea.

Resposta da aveia-preta à adubação nitrogenada em semeadura direta sobre pastagens nativas

The effect of topdressed N on the dry matter production of no-till oat (Avena strigosa Schreb) planted onto native pasture, without desiccation with herbicides, in soil classified as Dark-Red Latosol (Haplorthox), was studied in three experiments. Experiment 1 was carried out in 1994 to evaluate the effect of rates of nitrogen (0, 20, 40, 80, 160, and 320 kg ha-1 ), split into three applications (1/3, 30 days after emergence of plants-DAE, 1/3 after the first cutting, and 1/3 after the second cutting), on the dry matter production of oat. Experiment 2 was carried out in 1996 with the objective of determining the effect of time of N application on the production of dry matter of oat. Experiment 3 was carried out in 1996 to evaluate the effect of N applied 27 DAE, and after the first and second cuttings, with rates of 0, 40, 80, and 160 kg ha-1. The treatments applied after the first cutting received 40 kg ha-1 of N at 27 DAE and those applied after the second cutting received 40 kg ha-1 of N at 27 DAE + 40 kg ha-1 of N after the first cutting. In all cases the experimental design was completely randomized blocks, with four replications. The results showed considerable N limitation for the introduction of oat under no-till into native pasture, economically supporting its application at levels exceeding the recommended amounts when this forage is grown under conventional tillage system. Splitting the rate of N before the first cutting did not increase the efficiency of fertilization for the production of oat forage, under no-till onto native pasture. Residual effect of topdressed N was observed, especially on dry matter production of the second cutting after N application.