Fertiliser Nitrogen and Factors Affecting Pasture Responses

Abstract

Nitrogen (N) is an essential plant element. However, its supply from soil compared to its demand by crops as well as pasture plants is the most limiting amongst soil nutrients. Pastures respond well to N application. N utilization efficiency can be 9-28 kg dry matter per 1 kg N applied. As a result, N fertilisers are increasingly applied for high pasture production. How pastures respond to N and the factors affecting responses are crucial to the efficient use of N fertilisers. After fertiliser N is applied, N is rapidly absorbed into plants and growth stimulated via improvement of root systems and photosynthetic activity. Pasture production increases depend on botanical composition, cultivars and physiological state. Pasture growth is improved immediately after N application. This effect can last into the next growing period following initial defoliation. The carryover N effect may increase plant growth, but can be negative in some cases. New Zealand studies showed positive N carryover effects present for first two cuts, inconsistent at third, and negative for fourth and fifth cuts. Pasture composition, N fixation by legumes and herbage nutrient concentration all respond to N application. Pastures response to N flux varies with various factors, including N form, rate applied, and frequency and timing of application. Dry matter yields in pure grass pastures increases linearly with N application rate up to 200-400 kg N ha-1 per year. Split N applications improve annual yield and seasonal yield distribution. Spring applications gave greater N responses than autumn applications. N responses are also affected by climate, geographical factors, and soil factors, such as type, texture, drainage, pH, fertility, moisture and temperature.