Establish a Sampling Schedule
Most soils should be sampled every 2 – 3 years; more often for sandy soils, high-value crops, or problem areas.
To avoid seasonal variation, try to sample at the same time every year for a given field or garden. Fall is generally considered to be the most reliable time to pull samples, especially when it comes to pH. Soil pH fluctuates and tends to be lower in the summer when temperatures are higher and soils are dryer. When soils dry out, salt concentrations increase allowing Ca++, Mg++, K+ to replace H+ and Al+++ on the soil surface. The extra H+ and Al+++ in the soil solution will temporarily decrease soil pH hence pH determination is more reliable in the Fall when soil moisture is a bit higher.
Use the Right Tool
Use tools that are clean and free of rust. Avoid brass or galvanized tools or containers that can contaminate samples with zinc or copper. Stainless steel probes or augers are best because they collect a continuous core through the entire sampling depth with a minimum disturbance of the soil. Avoid shovels or trowels.
Collect samples in a clean plastic bucket or plastic bag. Avoid collecting or shipping wet samples in plain commercial paper bags or boxes that are often treated with a product containing boron. Wet samples can leach boron out of the paper and contaminate the sample. If possible, send air-dried samples in a sample container.
Please see Taking a Soil Sample for instructions on taking commercial or homeowner soil samples.