Within the manual Protocols for experimental plot sampling, handling and processing of cereals in field experiments by G.J. Rebetzke (Greg.Rebetzke@csiro.au), A. van Herwaarden, B. Biddulph, C. Moeller, R. Richards, A. Rattey and K. Chenu.
Knowledge of starting soil nutrients is critical to ensure application of appropriate levels of fertiliser (especially nitrogen – N) to attain desired yield levels, or where nutrients (eg N, P) are to be applied as an experimental treatment. Nutrient measurements can be performed on soil cores performed for initial soil water (see Appendix 3.1), using sub-sections 0-10cm, 10-20cm, 20-40, 40-60 and 60+ cm. The cores of one trial can then be pooled for further processing and analysis. Depending on the size and nature of the trial, these cores could be pooled could be a single pool (eg for multiple genotypes at a single N rate), or into multiple pools (eg 3-5 for a N-by-genotype interaction trial). For trials where P is a factor, the top 0-10cm of soil is the critical sample. Pooled samples should be stored, dried and then sent to an accredited laboratory for chemical analysis.
Nutrient budget: Many experiments are nutrient-limited, confounding inference to treatment effects to reduce confidence in the research. Perhaps the most commonly limiting nutrient in research experiments is nitrogen. Inadequate nitrogen slows growth to reduce biomass. Simple budgeting will provide meaningful baseline data necessary for predicting nitrogen requirements for individual experiments. Below is a worksheet demonstrating information required and calculations for predicting N-requirements for a wheat experiment with a predicted, or target yield of 4 t/ha at 13% grain protein. The crop N requirement is 121 kg/ha with the soil N analysis indicating net N supply of 90 kg/ha. We can then estimate (by difference) a need of 31 kg/ha N which equates to 62 kg/ha of urea (assuming urea fertiliser is 50%N). The framework can be modified simply to account for different yield, or target grain protein; different starting soil N or mineralisation (previous crop, soil type and soil moisture); and different fertiliser used.