Seed Chilling
Seed imbibition is the process by which dry seeds absorb water, swell, and start germinating. A corn seed will absorb 30% of its weight in water while a soybean seed absorbs 50% water by weight to start germinating. Water moves into the seed coat, to the embryo, and the endosperm which greatly expands the seed and softens the seed coat. Soon radical roots start growing which are the first seed roots.
For corn and soybeans, the soil temperature needs to be above 500F but 600F to 700F is much better. Seeds can take 18-20 days to germinate at 500F, but only 8-10 days at 600F and 1-5 days at 700F. This measurement is taken 2 inches deep in the soil. Generally, the soil temperature and moisture are a little more uniform when seeds are planted at least 2 inches deep or deeper. Corn can be planted 3 inches deep to get a more uniform emergence or at least down to moisture. Most areas have received adequate moisture this spring, but there are still areas with much less moisture due to this past years drought. Conditions can vary greatly from field to field.
A big concern now is something called seed chilling. If the soil temperature is below 40–450F during this first 6–24 hours of water absorption by the seed, it can cause imbibitional seed chilling injury. Water becomes dense the closer it gets to 390F and cold water can over stretch or damage seed cell membranes. Cold imbibition can rupture cell walls and disrupt enzyme production within the seed. This often causes corn plants to “cork screw”. For corn seed, the critical period is for up to 24-48 hours which can reduce germination by up to 30%.
For soybean seeds, this phase is closer to 24 hours and the critical temperature is below 450F which can cause dead cotyledons tissue (first leaves from the seed), twisted hypocotyls, swollen seeds without roots, and even seed death. Seed cold imbibition generally results in reduced stand establishment, uneven seed emergence, lower seedling vigor, and usually results in the need for replanting.
Ideally, try to plant when soil temperatures are above 500F and the trend is for warming weather. Avoid planting if cold rain is expected within 24 hours. Plant a little deeper to buffer soil temperature swings. Use high quality seed with intact seed coats and avoid seed with cracks or wrinkles which increase cold chilling and reduce seed germination. If possible, plant in the early afternoon to allow soils to warm up.
Soybeans are more sensitive than corn to seed chilling for several reasons. Soybeans have thinner seed coats and are often planted shallower. Soybeans can also freeze off once they emerge. Corn has a thicker seed coat, is usually planted deeper, and the growing point stays below grown until V6 (six true leaves) so the corn plant is more insulated from the cold unless it freezes deep.
Cold wet soils also limit nutrient availability, especially phosphorus (P), Zinc (Zn) and iron (Fe). P deficiency is common in corn and causes purple to bluish tints to the base of shoots. Zinc deficiency looks like yellowing between the new leaves on corn and is called interveinal chlorosis. Often corn with Zn deficiency has broad white midribs down the middle of the corn leaf on both sides of the leaf. All plants tend to be stunted, shorter, and bushier due to slow growth. Zinc deficiency is common on Ohio soils. High pH soils and high soil P tend to make Zn less plant available. Cool wet soils also slow root growth and Zn uptake. Cold compacted soils are also more susceptible.
On Fe deficiency, similar to zinc; interveinal chlorosis is noted on newly formed leaves while the veins stay green and plants remain stunted. Iron deficiency often occurs in patches. It is often mistaken for nitrogen (N) deficiency. Some plants get severely yellow, pale, even white and in severe cases the leaves turn brown and die. With Fe deficiency, Fe is needed for N metabolism and Fe is usually applied as a foliar spray using iron sulfate plus fulvic acid.
Use tissue testing to confirm early nutrient deficiencies. Applying starter P and Zn can help on cold wet or compacted soils. Foliar applications of Zn and Fe can be helpful if conditions are severe. Nutrient deficiencies are generally the worst the longer, colder, and wetter the soil remains and persists. Once the weather warms up and the soil dries out, the bacteria grow better and release plant nutrients. Unfortunately, if the seed, shoot, and root damage is severe causing short stunted crops; then replanting may be necessary.
