Look closely and you’ll see a small area of wheat that is thriving compared with the surrounding plants. Also, note the yellow regions in the background, a visual symptom of sulphur deficiency on the eroded hillsides.
Farmers often ask me what they can do to increase their yields and profits, often expecting a "silver bullet" answer. In most examples, though, it’s a number of weak links in the system that need to be strengthened to increase yields.
Growing conditions and available moisture obviously affect yield, but it all starts with varieties that have the genetic potential to succeed in your area. Then the seeding equipment needs to position every seed at a consistent depth to help achieve uniform emergence. The next step, and sometimes the most important, is building a sound fertility package capable of creating high yields, which includes banding at least some of the nutrients in the row at seeding time. The most important nutrient to band is phosphorus, especially in no-till systems and soils with lower soil phosphorus level soils or high or low pH.
There seems to be an increasing gap between average yields and true yield potential, even in a given set of environmental conditions. This is often confirmed with yield maps generated from calibrated yield monitors. For example, it’s not uncommon to see regions within a field that yield two to three times more or less than the overall average. The limiting factor can range from soil type and soil moisture retention to stand uniformity, especially when no-tilling into heavy residue, and fertility issues such as high or low soil pH levels. Building a sound and balanced fertility plan that incorporates as much field information as possible boosts yields and profits, even when commodity prices are lower.
Liebig’s Law of the Minimum states that crop yields are proportional to the most limiting nutrient. If the deficient nutrient is supplied and taken up by the crop, yields will be improved, but only to the point at which another nutrient becomes limiting.
Strike a balance. Fertilizer requirements obviously vary on a field-by-field or region-by-region basis, so I never recommend the same fertilizer plan across fields. To aid in the fertilizer application decision, we pull soil samples. Keep in mind: To maximize profits, it often pays to address the lowest fertility regions first, followed by those with higher levels.
The image of a leaking barrel on the next page represents Liebig’s Law of the Minimum. It states that crop yields are proportional to the most limiting nutrient. If the deficient nutrient is supplied and taken up by the crop, yields will be improved, but only to the point at which another nutrient becomes limiting.
It’s also important to add that when a farmer transitions to a no-till system, the soils will likely be cooler early in the spring, which can have a negative impact on nutrient availability. For example, nutrients such as phosphorus are not as available to the growing crop at cooler soil temperatures, compared with later in the season when the soils are warmer. So just because a soil test suggests phosphorus levels are adequate for maximum yields, the timing of the availability is often an issue and the weak link in the system.
Sometimes these nutrient deficiencies only occur in small regions of the field, perhaps in sandy or sandy loam soils, which struggle to hold on to soil-mobile nutrients such as nitrogen, sulphur and boron. In these examples, applying fertilizer using variable-rate technology is often the best way to address specific deficiencies.
Variations in fertility also occur at different elevations, which can influence soil pH. Sampling these areas separately and creating zone maps for fertilizer application can often pay dividends. Be sure to record the specific soil sample locations using differential GPS so you can return to the same areas to track how the nutrient levels change over time.
Even with a sound soil-sampling system, nutrient deficiencies within the sampling area can still occur. In this case, I prefer to pull both soil tests and tissue tests to help isolate nutrient deficiencies in growing crops. While a soil test might show adequate or higher fertility levels, certain areas of the field might not be able to access all of these nutrients because of soil pH, dry soils, wet soils or soil compaction.
Nutrient uptake might also be influenced by fertilizer imbalances; for example, high levels of phosphorus can reduce zinc availability.
By contrast, a good supply of plant-available nitrogen can improve phosphorus, boron and zinc uptake.
This field represents what every farmer wants to see—excellent plant health coupled with uniform head populations.
Advanced plant testing. When it comes to tissue sampling, I recommend taking samples two to three weeks after good spring growth resumes in winter wheat and when plants reach the 4th or 5th leaf stage in spring wheat. When sampling at early stages, I collect at least ½ pint of clean plant tissue, cut off level with the soil surface. Be sure to not get any soil on the samples as this contamination can significantly influence sample results. If sampling later in the season, at the fully emerged flag leaf stage, for example, I prefer to collect just the flag leaves because taking whole plants at this stage can produce misleading results.
For maximum accuracy, collect tissue samples when the plants are actively growing and try to avoid pulling samples following a period of dull, cloudy days because the results can often be misleading.
Also, avoid any plants that have been damaged by wheel tracks, disease or recently sprayed with a herbicide.
Once the samples are taken, be sure to next-day ship them to a quality lab to ensure the samples arrive fresh. Find a lab that can return the results by fax or email within 24 to 48 hours after receiving the samples. Fast turnaround is essential because if significant deficiencies are found, they need to be promptly addressed to minimize yield losses.
If the samples have to be stored overnight, make sure they are refrigerated before shipping them to the lab.
I often use tissue sampling to compare areas of a field that have good growth and plant health against areas that have poorer growth and plant health. Sometimes these areas might be only a few feet apart.
When you do receive the results, remember that they are a snapshot in time—specific to that area of the field and the day the samples were collected. Compare the results to published critical concentration levels for your area. If the samples are pulled early in the spring and nutrient deficiencies are found, then corrective measures must be taken to protect yield.
A global angle helps agronomist Phil Needham teach farmers how to incorporate a practical yet intensive wheat management system. Contact Phil:
- March 2014