Why sample after harvest?

Some growers sample soil at or just after harvest to fine tune their nitrogen management.

The Nitrate Quick Test is inexpensive and allows sampling to monitor progress through the crop. For a leafy green crop, growers want to see a small amount of residual nitrate in the soil to ensure the crop stays looking green to meet market requirements (Figure 10).

Figure 10 Bed of lettuce with plants at rear of photo starting to yellow off just before harvest. Plants in foreground have enough nutrient.

If too much residual nitrate is present, they have wasted money and increased the risk of leaching. A cover crop or catch crop can help catch the nitrate. In this case it is helpful to sample in depths bands: say 0-15 cm, 15- 30 cm, 30 – 60 cm and below 60 cm if the soil profile is deep. This shows where the residual nitrogen is sitting and how deep roots of a following crop must go to recapture the nitrate in the lower profile. Collect multiple cores from each depth (Figure 11).

Figure 11 Set of coloured buckets to collect multiple core samples from different soil depths

REmember, even if a crop is “looking a bit yellow”, nitrogen is only one of the factors that might be involved. Other nutrients such as sulphur can also cause yellowing if in short supply, and even poor drainage or cold weather can create similar symptoms.