Anthony Bly
Watching corn and soybean development is a fascinating experience especially when it takes place over a relatively short period of time. When plant growth is rapid, changes can be seen practically from day to day. Understanding the growth and development stages are important and closely related to nutrient uptake. Much interest has been given to foliar feeding. The promoted practice is to plant, sample, and evaluate tissue analysis results for possible nutrient deficiencies and respond with foliar nutrient application. Conceptually, it actually does sound like a good idea. However, there are many considerations to make before investing much time and effort into this practice.
Very small amounts of nutrients actually enter through the plant tissue compared with total nutrient uptake attained through the roots. The leaf cuticle, which keeps plant tissues hydrated, is a strong barrier to getting water and nutrients into the leaf. Foliar nutrient application rates are limited to very small amounts due to application volumes, available nutrient materials, and possible leaf tissue burn. Plant tissue nutrient concentration is influenced by previous weather conditions and therefore can change rapidly. Plant tissue tests are a good diagnostic tool for plant growth problem diagnosis but have no standard nutrient application recommendations (nutrient rate, source or application timing). When plant tissue testing is used in combination with soil samples from good and poor areas within a field, the diagnostic process is much stronger. Plant tissue interpretation is also somewhat confusing in that many terms are used to describe the plant tissue nutrient concentration such as: sufficiency, critical level, deficiency, responsive range, adequate, and excessive. Basically, the plant has enough of given nutrient or it doesn’t. University plant tissue interpretations use a critical level approach (Table 1). This means that if a given nutrient is above the critical level, the nutrient is at a high enough concentration for optimal plant growth and development. University research has shown little if any benefit from foliarly applied nutrients.
Table 1. Critical nutrient levels* for diagnosing plant symptoms in South Dakota. Soil and Plant Analysis Interpretation Program, SDSU, 2012.
Nutrient | Unit | Corn | Corn | Corn | Soybean | Oats | Barley |
Nutrient/tissue/
time | | Whole1 | Whorl2 | Ear leaf3 | Top4 | seedling5 | seedling5 |
Nitrogen | % | 3.50 | 3.00 | 2.75 | 4.51 | 3.00 | 3.20 |
Phosphorus | % | 0.30 | 0.25 | 0.25 | 0.26 | 0.23 | 0.23 |
Potassium | % | 2.50 | 1.70 | 1.50 | 1.71 | 2.00 | 1.60 |
Calcium | % | 0.30 | 0.25 | 0.21 | 0.30 | 0.20 | 0.50 |
Magnesium | % | 0.15 | 0.13 | 0.20 | 0.26 | 0.10 | 0.25 |
Sulfur | % | 0.15 | 0.15 | 0.15 | 0.15 | 0.15 | 0.15 |
Zinc | ppm | 20 | 15 | 15 | 21 | 10 | 10 |
Iron | ppm | 50 | 10 | 21 | 51 | -- | -- |
Manganese | ppm | 20 | 15 | 20 | 21 | 20 | 20 |
Copper | ppm | 5 | 3 | -- | -- | -- | -- |
Nutrient | Unit | Wheat | Oat/Bly/Wht | Alfalfa | Sunflower | Sorghum |
Nutrient/tissue/
time | | seedling5 | heading6 | top7 | upper8 | 3rd leaf9 |
Nitrogen | % | 3.60 | 2.00 | 4.51 | 2.00 | 3.00 |
Phosphorus | % | 0.26 | 0.20 | 0.26 | 0.25 | 0.20 |
Potassium | % | 1.60 | 1.50 | 2.01 | 2.00 | 1.40 |
Calcium | % | 0.20 | 0.20
(bly=0.30) | 1.76 | 1.50 | 0.30 |
Magnesium | % | 0.15 | 0.15 | 0.31 | 0.25 | 0.20 |
Sulfur | % | 0.15 | 0.15 | 0.20 | 0.30 | -- |
Zinc | ppm | 10 | 15 | 21 | 25 | 15 |
Iron | ppm | -- | -- | 30 | 50 | 65 |
Manganese | ppm | 20 | 25 | 30 | 50 | 8 |
Copper | ppm | -- | -- | -- | 4 | -- |
Source : SDSU