Organic fertilization ofpotato with manure and, or, Crotalaria juncea. II - Soil N, P, and K availability throughout the growing season

Soil fertility levels in the Agreste region, state of Paraiba, Brazil are generally low. Usually, animal manure is applied to soils to supply agricultural crops with nutrients. However, depending on the quality of the manure applied, incorporation can lead to soil nitrogen immobilization in the beginning of the growing season. It was hypothesized that planting and incorporating crotalaria (Crotalaria juncea) together with animal manure could synchronize the nutrient mineralization pattern more specifically with crop demand. In 2003, field and green-house experiments were carried out to test this hypothesis. Prior to the current study, the field plots were cultivated with potato from 1996 to 2002 and subjected annually to the following organic fertilization treatments: planting and incorporation of crotalaria during flowering (C), manure incorporation, 15 t ha(-1) (E), planting and incorporation of crotalaria + 7.5 t ha(-1) manure (CE); and control plots (T). During 2003, these treatments were repeated and the decomposition dynamics evaluated in samples contained in 0.5 mm-mesh nylon decomposition bags with organic materials buried at 20 cm soil depth. Soil nutrient availability was also evaluated in soil samples (0-20 cm) during the growing season. In the green-house, buffel grass (Cenchrus ciliaris L.) was grown for 300 days in pots containing soil from the field plots and submitted to successive cuts of the shoots. Dry matter and nutrient losses from the incorporated material in the decomposition bags peaked in the first 30 days in all treatments. At the end of the growing season, dry matter and nutrient losses were lower for the treatments with manure, compared to treatments C and T. Manure incorporation increased soil P and K levels throughout the study period, but led to net N immobilization during the first few weeks after incorporation. Treatment C increased soil mineral N concentration shortly after incorporation, but decreased later in the growing season. Soil P and K levels remained unchanged. In the green-house study, 35 days after planting the biomass accumulation and nutrient uptake by buffel grass was the highest in treatment CE. In the following harvests, treatment E led to a greater biomass and nutrient uptake by buffel grass, indicating that N immobilization after manure incorporation limited plant growth in the first weeks. These results show that planting and incorporation of crotalaria coupled with the application of only half the usual manure rate was enough to avoid soil net N immobilization and increased soil P and K levels. The combined application of animal and green manure leads to availability of soil nutrients more synchronized with the plant demand.