Which Fields Are Candidates?
Not every field is a good candidate for topographic sampling of nutrients, points out NDSU’s Dave Franzen. This precision farming method is based on the assumption that all topographically related areas of a given field have been farmed similarly in the past. Should that not be the case, a test in one sub-area will not be reflective of nutrient levels in other parts of that area — thereby negating the value of topographic sampling.
How do you know whether certain fields are viable candidates for topographic sampling? Franzen lists these guidelines, developed at last year’s North Central Extension-Industry Soil Fertility Conference.
• Remote sensing or yield mapping reveals their landscape relationships.
• They do not have a history of manure use.
• They have low nutrient levels (particularly nonmobile nutrients).
• Mobile nutrients (nitrogen, sulfur, chloride) are especially important to the crop being planted there.
On the flip side, fields would be better candidates for grid sampling, not topographic sampling, if the following conditions exist:
• Production histories are unknown.
• Fertility is high due to buildup of applied fertilizer; or if high levels of fertilizer or limestone were recently used.
• There is a history of manure use.
• Several fields have been merged.
• Nonmobile nutrients (phosphate, potash, zinc) are important to the crop.
How does the producer determine whether his soil nutrients vary sufficiently to justify precision fertilizer application? Franzen has a couple suggestions:
First, in several fields measure nutrient levels on top of a ridge, the side of the slope, the bottom of the slope. Pick those nutrients which are likely to be of greatest importance to the crop. That would be nitrogen in most cases; perhaps phosphate; maybe even chloride or sulfur.
Second, install a yield monitor and mapping software on one’s combine. If yields do vary substantially from one field portion to another, one can then begin determining what’s causing it. It could be soil fertility variations, weed control practices and/or drainage problems.
The support of knowledgable individuals is very important, Franzen adds. Whether it’s with the help of the local fertilizer company or an outside consultant, the producer needs to know how to gather accurate data — and then interpret those data appropriately.
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