Keywords: Fertiliser placement, no-tillage, nitrogen, phosphorus, potassium, nutrient stratification.
Nutrient uptake efficiency of applied fertilisers is of economic and environmental concern. Reducing tillage in agronomic crop production is a way to decrease production costs, reduce erosion and improve soil quality. Fertilisers cannot be mixed with the surface soil horizon in reduced and no-tillage systems. However, fundamental mechanisms of nutrient movement to plant roots are the same in reduced tillage as in extensively tilled systems. Mass flow is the most important mechanism for nitrogen (N) movement to plant roots, while diffusion is responsible for most phosphorus (P) and potassium (K) movement. In addition, plant roots tend to proliferate and increase nutrient uptake in zones of nutrient enrichment. No-tillage systems increase soil organic matter and N contents in the surface layer, and generally have stratified P and K zones. Large volatilisation losses of ammonia can result from surface broadcast applications of urea and urea-ammonium nitrate solutions, especially in no-tillage systems. Subsurface band-placement of N fertilisers essentially eliminates volatilisation losses and reduces immobilisation by reducing the interaction of fertiliser and soil. Surface broadcast applications of P and K fertilisers appear generally to maintain adequate nutrient uptake levels in reduced tillage systems. Apparently increased moisture levels in the soil surface layers and proliferation of roots in these zones creates an environment that supports adequate diffusion rates and nutrient uptake in humid regions. Reduced nutrient uptake may occur during cool periods early in the growing season with no-tillage. Band-placed application of P and K near the seed overcomes low temperature effects on nutrient uptake. For semi-arid climates, surface broadcast applications may result in nutrients in surface layers that can not be taken-up due to lack of moisture, but subsurface band applications results in adequate uptake in these environments. Reduced and no-tillage crop production systems do not change the basic principles of fertiliser application and nutrient movement to plant roots. These systems only require consideration of the principles and how they should be applied in specific soil and crop situations.
Mark M Alley, W G Wysor Professor of Agriculture, Dept. of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA.
31 pages, 3 figures, 4 tables, 79 references.