Now that spring is here, many growers will be heading to the field, probe and bucket in hand, to sample their soils. Soil testing is a vital first step toward successful nutrient management. Analysis results indicate the concentration of nutrients present in the soil, as well as any deficiencies that could potentially limit plant growth. Using this information to inform fertilizer application most often results in healthier, higher yielding crops. Soil testing also contributes to efficient fertilizer use that maximizes return on investment and protects the environment. That said, soil analysis and fertilizer recommendations are only as good as the samples submitted to the lab. The way that samples are collected can impact how accurately they represent the soil in question.
Sample uniform areas
Soil samples are small examples intended to reflect the nutrient status of a much larger management zone. In general, one composite sample of 20 cores mixed together can be used to represent 10 to 15 acres. However, differences in soil type, texture, drainage, topography and management occurring across a field should all be considered when delineating sampling areas. Building each composite sample with subsamples from a relatively uniform area increases the likelihood that the soil in your bucket accurately represents the rest of the management zone.
Soil survey maps are an excellent reference to the basic variability in a field. Field history records can provide additional information regarding crop rotation, fertilizer or manure application, and yield patterns. It is also important to note any atypical areas such as field edges near roadways or spots where lime or manure were once piled. Incorporating all of this information in a simple field map will make it much easier to identify uniform soil areas for sampling.
Soil samples can be taken using a probe, auger or spade. Probes and augers are handy because they extract a standard volume of soil, but a spade can do the same with a little extra effort. Subsamples should be collected and mixed in a clean, plastic pail. It is best to avoid metal containers due to their potential to contaminate samples.
Research has indicated that moving through each uniform sampling area in a zigzag pattern will generate the most representative sample. Select 20 evenly distributed sampling sites along the sampling pattern to visit (Figure 1). At each site, brush aside any crop residue, remove a core or slice of soil approximately 0.5 to 0.75 inches think and add it to your bucket. Take each sample at a consistent depth to accurately reflect the vertical distribution of nutrients. In tilled fields, sample to the average depth of tillage. No-till or reduced tillage fields should be sampled to 8 inches.