Seed & Chemical
Developing micronutrient recommendations
By Ross H. McKenzie PhD P.Ag.
Crops require a number of nutrients in very small amounts called micronutrients. The most common micronutrients include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn). There are at least five other elements that are needed by specific crops (that we won’t discuss in this article).
The term micronutrient refers to the relatively small quantity of a nutrient that is required for plant growth. It does not mean that these nutrients are less important to plants than other nutrients.
Table 1 below shows the total amounts of micronutrients taken up from the soil by high yielding wheat, barley and canola. Plant growth and development may be retarded if any one of these elements is lacking in the soil. Fortunately, we do not have widespread micronutrient deficiencies in Western Canada.
Sources of micronutrients in soils
Inorganic forms of micronutrients occur naturally in soil minerals. As minerals break down over time, micronutrients are gradually released in forms available to plants. Two sources of readily available micronutrients are nutrients that are adsorbed onto soil colloids (very small soil particles) and nutrients that are in the form of salts dissolved in the soil solution.
Organic matter is often an important source of most of the micronutrients. As soil organic matter decomposes, plant available micronutrients are slowly released into soil.
Soil sampling and testing
Soil testing can be helpful as an initial screening to determine if any of your fields are potentially low or marginal in a micronutrient. Have representative 0 to 6 and 6 to 12 inch depth soil samples analyzed for micronutrients. Most soil testing labs in Western Canada determine metal micronutrients Cu, Fe, Mn and Zn, using the diethylene triamine pentaacetic acid (DTPA) method. Boron is extracted using hot water and chloride is determined using water.
The general range levels used for determining when to add micronutrients to improve crop production are shown in Table 2, below. When a soil sample tests low in a micronutrient, a potential micronutrient deficiency may occur. A crop grown in field with a low micronutrient level in the 0 to 6 inch depth may not respond to a micronutrient fertilizer if adequate levels of the micronutrient occur in the 6 to 12 inch depth.
It is important for farmers and agronomists to recognize soil testing for micronutrients is not an exact science. The DTPA method for determining metal micronutrients works reasonably well for copper and zinc. The challenge is having reliable field research to determine the critical levels at which crops will economically respond to micronutrient fertilizers in the various soil and agro-ecological regions of Western Canada. Good research information is available for copper, limited information for zinc and very limited information is available for iron and manganese. Up to now, crop response to micronutrients across the Prairies has been minimal, making it difficult to accurately determine the critical soil test levels at which micronutrient responses may occur.
There isn’t a suitable soil test for molybdenum. The present soil tests used for boron and chloride are not very effective to predict crop response to these nutrients. For example, in a study in southern Alberta, about one-third of soils tested < 0.5 ppm for boron, which is often considered deficient. However, research trials conducted with winter wheat, spring wheat, barley, canola, pea, bean and corn at a number of sites over four years showed no positive responses to boron. From this and other work it is very clear the boron test and the critical levels used to recommend boron are not very reliable.
Soil factors affect micronutrient availability
Physical and chemical characteristics of soil can influence the availability and uptake of micronutrients:
- Soils with < 2 per cent organic matter may have lower micronutrient availability; for example Gray soils.
- Medium and fine textured loam, clay loam and clay soils are less likely to be low in plant available micronutrients.
- Coarse textured sandy soils are more likely to be low in micronutrients.
- Soils that have very high levels of organic matter > 30 per cent to a depth of 30 cm often have low micronutrient availability, particularly copper.
- Soil temperature and moisture affect micronutrient availability. Cool, wet soils reduce the availability, rate and amount of micronutrients that may be taken up by crops. Cool soil temperatures can induce micronutrient deficiencies.
- As soil pH increases up to 8.0 or higher, the availability of metal micronutrients may decrease.
Boron deficiencies have been suspected in canola and alfalfa grown on sandy-textured Gray soils. Research to specifically document crop response to added boron is limited. Normally, I would not recommend boron on an entire field based only on a low B soil test, due to the limitations of the test. If soil test B is < 0.5 ppm, I would suggest trying carefully laid out field scale test strips with sensitive crops like canola or alfalfa to determine if a soil B deficiency actually exists.
Application of borate or borax fertilizers can be broadcast for alfalfa, and either broadcast and incorporated or banded for canola. Boron containing fertilizers should not come into contact with the seed at planting. Soil application rates should not exceed 1.5 lb/ac on soils with a pH less than 6.5 to avoid boron toxicity problems. Foliar applications should not exceed 0.3 lb/ac to avoid toxicity problems. For all types of applications, extreme care must be taken to avoid toxicity problems.
The soil test for chlorine is very unreliable. Therefore, I normally would not recommend chloride on an entire field based only on a low Cl soil test.
Generally, crop requirements for chlorine are satisfied by the chlorine in the soil and received in rainfall. Rainwater on the Prairies typically contains 0.5 to 1 mg/l of Cl, which is more than sufficient to meet crop requirements. Chloride is also added to soil in potash fertilizer (KCl).
North Dakota research has shown that chloride added at rates higher than required to meet nutritional needs is associated with suppression of root and leaf diseases in some cereal crops. However, western Canadian research is very limited to demonstrate this benefit in Western Canada.
Research has clearly shown cereal crops will respond to added copper when soils tests are low. Wheat and barley grown on Black or Gray soils may benefit with copper application when the soil test for Cu is < 0.5 ppm. Wheat and barley response to copper on Brown and Dark Brown soils is uncommon and copper should only be applied to these soils when soil test Cu is < 0.3 ppm.
Cereal crops grown on soils with greater than 30 per cent organic matter to a depth of 30 cm often respond to copper fertilization, when soil test levels are < 2.5 ppm.
Generally, copper deficient mineral soils tend to be either sandy or light loam soils with levels of organic matter in the range of six to 10 per cent. Copper deficient soils are sometimes associated with soils with high levels of soil phosphorus or which have received heavy applications of manure.
Broadcast and incorporated rates of 3 to 8 lb/ac of copper in the form of copper sulphate or copper oxide are recommended for deficient mineral soils. On organic soils, broadcast and incorporated rates of 10 to 15 lb/ac are necessary. Soil application rates should be effective for five to 10 years. Chelated forms of copper are also effective in the year of application but the residual effects in Prairie soils is not well known.
The benefit of copper foliar application to cereal crops grown on mineral or organic soils is not as consistent but can be used when deficiency symptoms appear. Foliar applications are required annually and are most effective at the late tillering stage. If the deficiency is severe, two applications at mid-tillering and boot stage may be necessary. Foliar application rates of between 0.2 to 0.3 lb/ac are recommended.
Iron deficiencies have rarely been observed in field crops in Western Canada. Soybean is a relatively new crop to the Prairies and is particularly sensitive to low soil iron levels.
An iron soil test below 3.0 ppm is considered very low and at 3.0 to 5.0 ppm is considered low. These critical levels need western Canadian field research to be verified. Deficiency symptoms with soybean most commonly occur in cool, wet spring conditions. However, research in the U.S. has found the DTPA test is not well correlated to iron fertilizer response. U.S. research generally has found a foliar application of 0.15 lb/ac is recommended versus a soil application for soybean. Note that in the spring as the season warms up, soil iron tends to become more available to the crop and may grow out of the deficiency.
Manganese deficiencies may occur on organic soils and high pH mineral soils. Deficiencies are rare but can potentially occur during cool, wet conditions in spring. Oats are more susceptible to a manganese deficiency than other cereal crops. Organic soils with a high pH are more likely to respond to manganese fertilizer.
Only limited information is available on manganese fertilization. As a rule, broadcast applications are less effective. For cereals, a seed placed treatment of manganese sulphate may be more effective. Foliar application can also be used if deficiency symptoms develop during the growing season.
Zinc deficiencies tend to occur on soils that are calcareous, have a high pH, are sandy in texture and/or have relatively high soil phosphorus levels. Deficiencies tend to occur in spring when conditions are cool and wet. In southern Alberta, irrigated field beans have responded to applications of zinc particularly on sandy soils. Zinc deficiencies have been suspected in some irrigated cornfields in southern Alberta, but research trials have not confirmed this. Response to added zinc may occur on eroded or machine leveled soils or soils that have had large amounts of added phosphate fertilizer.
For soils that test low in zinc where a sensitive crop such as beans, corn or wheat is grown, a band application of 2 to 5 lb/ac of zinc sulphate or 0.5 to 1.0 lb/ac of a chelated zinc is suggested. When zinc deficiencies are suspected early in the growing season, a foliar application of 0.5 lb/ac of zinc sulphate can be used. On eroded soils, a 5 lb/ac broadcast incorporated application of zinc sulphate can be tried.
It is important to keep the need for micronutrient fertilizers in perspective. Many farmers have applied micronutrients in the hope of increasing crop yields even though there is little evidence to suggest a deficiency exists.
Farmers with fields testing very low or low in a micronutrient are encouraged to apply the nutrients in carefully laid out, replicated test strips. These strip treatments must be carefully marked out for comparison to adjacent control strips. Visual comparisons and yield measurements should be made to confirm if a yield benefit actually occurred.
There is no doubt soil micronutrient levels will gradually decline as cropping continues. As soils continue to be cropped, micronutrient deficiencies may become more common as available levels of some elements are depleted. Fortunately, most Prairie soils are currently well supplied with micronutrients. Soils and crops in Western Canada that require micronutrient fertilizers are the exception, not the rule. Care must be taken to keep the need for micronutrient fertilizers in perspective and not to promote them beyond their true significance.