At ALGL we are seeing an increase in spring soil sampling. Spring soil samples have increased from 15% to 28% of annual soil samples in the past 10 years. Perceived concerns of seasonality impacting soil test data are being overshadowed by increased management flexibility.
Tradition has held soil samples to the fall sampling season. This works very well if the plan is to apply fertilizer in the spring, thus allowing time to make management plans over the winter months. More commonly the plan has been to collect the soil samples and turn the resulting data into fertilizer recommendations as fast as possible. This plan leaves no time to make key management decisions. The growing trend in our market is to separate soil sample collection, and fertilizer application, into separate seasons.
Soil test results will vary through the year. Often, nutrient levels are highest early in the crop growing season and decline through the growing season, with a recovery as nutrients begin leaving crop residues at harvest time. It is often argued that fall soil sampling shows the seasonally lowest soil test levels to ensure adequate crop fertility. A similar argument can be made for spring soil sampling in that it directly reflects what the starting point for crop fertility at the beginning of the growing season.
The greatest concern is with potassium soil test levels as it has the greatest chance of variability through the growing season. Demonstration samples collected over the last few years by ALGL shows that potassium levels vary less than ± 6-7% between spring and fall soil samples. This is often less than the cumulative sampling error between sampling events. The difference will have little to no impact in the resulting fertilizer application rates in most cases. It is assumed that soil test potassium will be higher in the spring, this same demonstration data set shows that is often not correct.
Figure 1 - Management Cycle
True management of anything is a cyclic pattern. We often start with a soil test in the analysis phase, then make decisions on how and what to correct/improve. Followed by planning on how to implement those decisions. Then implementing the plan takes place. The real value in this model is on the next sampling cycle, taking the time to review the new data and determining if your goals of the previous plan were met, and if not, determining why. When soil samples are collected repeatedly at the same time of year, location, after the same crop, etc. to reduce the sampling variation, the changes in soil test values based on management become clearer.
When soil samples are collected in the same season as fertilizer is applied, all the management steps need to take place in 7 to 10 days during one of two busiest and stressful seasons, with no time to critically evaluate the data. The risk for mistakes, or less than ideal management decisions, increases using this short time frame. Sampling a season ahead of fertilizer application provides more time to evaluate the overall fertility management plan. For example, soil sampling in the spring provides the entire growing season to evaluate/refine the plan, make fertilizer purchases, prepare prescriptions ahead of time (yield data crop removal can still be added at harvest time), evaluate the crop through the growing season, and adjust based on crop prices, yields, and fertilizer prices. This leaves the only management step taking place during fall harvest is implementation of the plan.
When we let go of the notion that we need to collect soil samples in the fall and start looking to collect soil samples a season in advance of fertilizer application, the opportunities for increased and advanced management of soil fertility becomes possible.