When Should We Take Soil Sample After Rice And Corn Harvest?
RASEL PARVEJ AND JAMIL UDDIN
WINNSBORO, LOUISIANA
The amount of phosphorus (P) and potassium (K) fertilizer required for maximum crop yield is determined by routine soil testing. Most land- grant universities developed soil-test-based P and K fertilizer recommendations based on soil samples collected typically from late winter months (January-February) to before summer crop planting.
However, the time for soil sampling from producer fields has recently changed greatly. Nowadays, crop consultants or farm-service reps prefer to collect soil samples mostly within a few weeks after summer crop harvest (August-November, depending on the type of summer crop) due to the chances of rain in the early to late winter months (December-January), increased number of soil collection over time, and use of ATV’s or auto samplers in dry soil conditions. Since soil-test P and K concentrations are reported to fluctuate spatially and temporally, the recent change in soil sampling time may influence the current soil-test-based P and K fertilizer recommendations for row crop production.
Recent research conducted on silt loam soils in Arkansas showed that soil-test P and K concentrations changed temporally from September through March and the temporal change was affected by the previous summer crop. Researchers from University of Arkansas (Slaton et al., 2016) found that following rice harvest, soil-test P concentration remained constant across time, but soil-test K concentration increased gradually with time and peaked by 22 ppm (44 lb K/acre) in late- December to early-January (Figure 1). High residue crops such as rice and corn hold a greater amount of K than P in vegetative plant parts and eventually release more K across time after harvest compared to a low residue crop such as soybean. For example, crop residue (stover) of a 200 bushels/acre corn contains approximately 30 lb P2O5 and 200 lb K2O per acre and 200 bushels/acre rice contains approximately 15 lb
P2O5 and 100 lb K2O per acre. Since K is not a part of the cell wall or any molecular structures, it is slowly be leached from crop residues by rainfall following harvest, resulting in increased soil- test K concentration in December-January compared to September-October and lower recommended fertilizer rates for samples collected in the late winter months. Therefore, it is better to collect soil sample in late December-January following rice and corn harvest to get higher soil-test values and save fertilizer input costs. We established research trials at the Louisiana State University AgCenter – Macon Ridge Research Station and Northeast Research station to pinpoint the optimum time to collect soil sample after corn, cotton, and soybean harvest, and the preliminary results will be available by next year. ∆
RASEL PARVEJ: LSU AgCenter Soil Fertility Specialist
JAMIL UDDIN: LSU AgCenter Postdoctoral Researcher
Figure 1. Trend of soil-test K concentration following rice harvest under K fertilized (dashed line) and unfertilized (solid line) soils. Soil-test K concentration peaked after 4 months of rice harvest. (Source: Slaton et al. 2016. Soil-test phosphorus and potassium fluctuations following rice and soybean harvest yield through early spring. Wayne E. Sabbe Arkansas Soil Fertility Studies 2015. Univ. Arkansas Agri. Exp. Stn. Res. Ser. 633:42-48).