For maximum wheat yields, grain quality and profits, all nutrients must be present in sufficient quantities. If one is lacking, it often negates the value of the others. Micronutrients are just as important to plants as primary nutrients—they’re just required in lower amounts. In fact, all micronutrients combined only account for 1% of the dry weight of most crops.
There are eight essential plant nutrient elements defined as micronutrients: zinc (Zn), copper (Cu), manganese (Mn), iron (Fe), boron (B), molyb-denum (Mo), chloride (Cl) and nickel (Ni). The primary reasons for micronutrient deficiencies include:
- Higher yielding crops demand more nutrients and micronutrients.
- Many producers have not applied enough micronutrients to keep up with crop removal for several years.
- Topsoil depths often correlate with micronutrient levels, so if topsoil depths have declined as a result of soil erosion, then deficiencies are more likely in the eroded regions. Lower organic matter soils are also more likely to exhibit micronutrient deficiencies.
- More farmers are converting their fields to no-till, which reduces soil temperatures, especially early in the spring. Most micronutrients are less available in cooler soils, plus many of the plant-immobile nutrients, such as copper and zinc, are less soluble and can be tied up for two to five years in surface residue.
- All of the micronutrients (apart from molybdenum) are less available at higher pH levels. So if your fields have regions of higher pH, then micronutrient deficiencies will be more likely in these areas.
- Most micronutrients are less available in dry soils because most move into roots by diffusion and drier soils limit root growth.
- Lastly, combinations of the above factors greatly increase the probability of a micronutrient deficiency, such as wheat grown in high pH soil, in a lower rainfall environment using no-till.
The best way to determine if micronutrients need to be applied is with a soil or tissue test. Both methods have been used to successfully isolate micronutrient deficiencies and create response curves to help make more accurate recommendations.
Before taking tissue samples, find out if the lab requires the whole
plant or upper leaves.
If using soil tests, we prefer to separate larger fields into smaller management zones, perhaps by soil type, topography or yield zone. In no-till, we recommend 4'' deep samples and in conventional tillage we recommend 6'' deep samples.
If using tissue tests, we prefer to sample an actively growing crop early in the season, around the 3 or 4 leaf stage. If required, micronutrients can then be applied to help boost grain numbers per head, before yield is determined around the 4 to 5 leaf stage. Ideally, take comparative samples in healthy areas and regions with lower standards of plant health. Often the difference in nutrient levels between the healthy and less healthy regions is more important than the actual numbers on a tissue test.
To better determine micronutrient levels, take a soil and tissue test from the same regions of the field. It is important plant material arrives at the lab promptly, so take the tissue samples early in the week, place them in a paper bag and send them by next-day service to the lab.
Before taking tissue samples, find out from the lab if they require the whole plant or upper leaves from the plant submitted. Both methods work fine, but some labs create recommendations based on one type of sample versus the other.
Once you receive the soil test, tissue test or, ideally, both test results from the lab, compare the nutrient levels to the ideal range at the growth stage submitted. Cereals are most responsive to zinc, copper and manganese when their nutrient levels are low. Boron and chloride are soil mobile and are more likely to provide a yield response when their levels are low, especially in coarser textured soils with lower cation exchange capacity and in higher rainfall regions or irrigated crops. In general, a 100 bu. per acre wheat crop will require 0.15 lb. per acre of zinc. The same crop will require 0.05 lb. of copper and 0.3 lb. of manganese in the removal of the grain. If the straw is removed, these quantities will significantly increase.
If you determine certain micronutrients are low, we suggest a foliar application of the specific nutrients as soon as possible at the recommended rates. There are products that offer a combination of micronutrients, but we often find there is not enough of the nutrients you need and too much of the ones not required. Find nutrients that can be tank-mixed with other products, such as fungicides or herbicides, when making passes across the fields. Chelates or oxide formulations are often the most compatible but check with the supplier first.
Soil-applied micronutrients are the most efficient strategy for longer term management because they prevent micronutrient deficiencies altogether. Therefore, if most soil or tissue samples consistently have low levels of a specific micronutrient, such as zinc or copper, then add those nutrients ahead of, or at, seeding.
Compound fertilizers that contain phosphorus in addition to zinc, for example, are a good strategy. Specific micronutrient products such as zinc sulphate or copper sulphate also provide a cost-effective method to increase soil micronutrient levels.
As with major nutrients, banding micronutrients in the row at seeding offers the highest standards of availability and lowest costs. Check with your supplier to make sure the sources of micronutrients are compatible with in-row placement. Some forms of micronutrients, especially those that contain boron, can cause seedling injury, particularly in dry soils or low organic matter soils.
The Importance of Tissue Sampling When the Eyes Have It
A tissue sample confirmed this wheat crop was suffering from a copper deficiency.
Some micronutrient deficiencies might be diagnosed visually, especially during periods of active growth early in the growing season, when the root system has not developed well enough to access nutrients deeper within the soil profile. If you see these symptoms, we still recommend submitting tissue samples to a lab to confirm the deficiency, as sometimes more than one deficiency is present.
Copper deficiency symptoms, shown at right, often appear early in the season as stunted and pale plants with narrow or rolled leaf tips and thin stems. Foliar disease levels are often higher when copper deficiencies are present, in addition to small and shriveled grains at harvest.
Once micronutrient deficiency symptoms become visible, it’s likely significant yield reductions have already occurred. While a foliar application of the deficient micronutrient should create a yield response, we recommend identifying the regions affected by the deficiency symptoms, so these areas can be addressed with in-row or soil applications before planting the next crop. Soil applications will ensure the crop has adequate levels of micronutrients from the beginning to maximize root development and yield potential.