| Soils, Crops and Fertilizer Use |
|Chapter 7: Evaluating a soil's fertility|
A growing crop can have its stem and leaf tissue tested for N-P-K levels in the sap. This can be done either in the field with portable kits or at a lab. (Labs can also measure micronutrient levels.) The uses and limitations of tissue testing are:
• Tissue tests are best used as a supplement to soil tests and can be tricky for nonprofessionals to interpret.
• Sometimes nutrient levels in plant sap aren't well correlated to those in the soil. Weather extremes, soil compaction, poor drainage, insects, and diseases affect nutrient uptake. Deficiencies of N, for example, can stunt plant size and cause P and K to "pile up" in the sap, resulting in falsely high readings.
• The tests are usually calibrated for higher yield levels than most small farmers should be aiming for. Low to moderate fertilizer rates, rather than high ones, give the best return per dollar (see Chapter 9), yet a tissue test may indicate a deficiency at these levels.
• One advantage of tissue tests is that they can spot deficiencies in a growing crop while there's still time enough to correct them.
Portable tissue test kits cost from $25-$75, but some of the reagents need yearly replacement. Tissue testing is probably best left to trained agronomists.