Beatrice van Saan-Klein1 und Richard F. Fisher2
1Abt. Spezielle Botanik, Universität Ulm, Albert-Einstein-Allee
11, D-89069 Ulm
2Department of Forest Science, Texas
A&M University, College Station, Texas 77843, USA
Loblolly pine (Pinus taeda) plantations exist in the eastern part of Texas on a large variety of soil types, where the nutrient supply often is the limiting factor for the pine growth. The presence of and depth to an argillic horizon generally determines the ability of a soil to retain and release fertilizer nutrients.
This investigation focuses on the nitrogen, phosphorus, calcium, magnesium and potassium availability on soils after N-, P-, and N+P-fertilization compared to control sites. During a one year period, the in situ soil coring method was used to determine NH4 and NO3 availability in the top 10 centimeters as well as net mineralization. Exchangeable cations and phosphorus contents were analysed on the bulked soil samples.
The investigated pine stands are limited by phosphorus and addition of N simply leads to immobilization of nitrogen in the soil. Addition of P leads to accumulation of phosphorus in the soil, because of a limited N supply. Combined fertilization with N and P results in increasing nitrogen and phosphorus availability, but levels are lower than with single element fertilization.
P and N seem to be so limiting as to reduce cation uptake. Exchangeable calcium, magnesium and potassium contents in soil decrease after fertilization. On some sites, needle contents indicate Ca deficiency after addition of phosphorus.