Dry Seedbeds & Seedling Toxicity

The most critical nutrient for early season growth is phosphorus and for quick accessibility it is best placed with or close to the seed. In some crops, seeding configurations and soil conditions, all the phosphorus a crop needs may be seed-placed. In other cases unacceptable seedling burn and germination damage may occur.

Seed-placed fertilizer rates are restricted by a number of factors:

  • Fertilizer type – and salt index and ammonia toxicity
  • Crop sensitivity
  • Fertilizer –seed placement
  • Soil conditions – texture (affecting cation exchange capacity or CEC) and moisture

The salt properties of fertilizers can draw moisture out of germinating seeds. Crops vary in their tolerance to fertilizer salts with cereals being most tolerant, followed by pulses and lastly by oilseeds. The other hazard is ammonia (NH3) toxicity. High seedling zone concentrations of ammonia are toxic to seedling roots, impairing water and nutrient uptake. The portion of free ammonia in the soil is increased with high soil pH, high levels of free lime or carbonates, low CEC and dry conditions.

Many Manitoba soils started off dry this spring. Growers should assess seedbed moisture conditions and be prepared to make changes – especially if winds continue to dry seedbeds.  Following is a list of guidelines for seed placed phosphorus from a variety of sources:

Table 1. Maximum safe rates of actual seed-placed phosphate (P2O5) fertilizer as mono ammonium phosphate (MAP 11-52-0 or 12-51-0) for Manitoba and Saskatchewan. guidelines.

Table 1 Maximum rates

*Rates are based on disk or knife openers with a 1 in. spread, 6 to 7 in. row spacing and good to excellent soil moisture.
**Rates are based on knife openers with a one-inch spread, 9-inch row spacing and good to excellent soil moisture.

Table 2. Safe limits of seed-placed phosphate fertilizer with 30” row corn based on Ontario guidelines (Ontario Ministry of Agriculture and Food, OMAF3) .

Table 2 Safe Limits of seed placed phosphate

South Dakota State University developed a calculator that takes into account the amount of stand thinning that may occur with different crops, fertilizers, sols, moisture and seeding equipment. It is available online at: http://www.ipni.net/article/IPNI-3268

As confirmation, it is important to plug a fertilizer run for 50 feet of row. Mark that row and follow-up with a stand count of the seedlings and root inspection of any missing or damaged plants.

 Submitted by: John Heard, Crop Nutrition Specialist, MAFRD
1 Manitoba Soil Fertility Guide. 2007. Manitoba Agriculture, Food and Rural Development. P. 17 or http://www.gov.mb.ca/agriculture/crops/soil-fertility/soil-fertility-guide/phosphorus.html
2 Guidelines for safe rates of fertilizer placed with the seed. 2009. Saskatchewan Ministry of Agriculture. http://publications.gov.sk.ca/documents/20/84100-e42316e3-15ea-4249-ac0e-369212b23131.pdf
3 Agronomy Guide for Field Crops. Publication 811.Ontario Ministry of Agriculture, Food and Rural Affairs. Table 9-21 http://www.omafra.gov.on.ca/english/crops/pub811/9materials.htm#table9-21
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Why is My Corn Purple?

Several farmers will be noticing the incidence of purple corn in their fields.

Phosphorous Deficient Corn

P-deficient Corn. Note the purpling of the leaves.

Purple corn syndrome results from a number of stress factors.  The purple colour results from anthocyanin production in the leaves due to the accumulation of simple sugars.  The simple sugars build up in the plant because some other factor is limiting their further transformation or translocation in the plant.  Sometimes you will see the purpling at leaf tips of corn that are kinked by damage and the leaf can no longer transport or process the accumulating sugars resulting from the photosynthesis that is still occurring there. Many associate the purpling to reduced phosphorus nutrition of the plant.

Factors that can lead to reduced uptake of phosphorus – either due to lack of phosphorus (low testing soils, inadequate sidebanded application) or reduced rooting efficiency in taking up soil P (lack of mycorrhizae following canola,  root injury by toxic fertilizer bands, cold soils temperatures, dry soils, saturated soil, compaction, herbicide injury, insect feeding, etc).  And usually this is associated with adverse growing conditions – usually cold or saturated soils.

The crop usually “grows out” of this funk once weather and soils warm up and it is rarely seen beyond the V6 stage.  Some hybrids exhibit the purpling more than others.  This does not mean the other hybrids are any better at tolerating low P or cold conditions- they just hide their hurt better.  It would be negligent to shrug and chock it up to cold stress alone – consider whether you P fertilization practices, crop rotation or soil management are contributing to the syndrome,  then plan changes for 2015.

Submitted by: John Heard, MAFRD Crop Nutrition Specialist

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Should you plant corn after canola?

Planting corn after canola can result in corn being impacted by phosphorus deficiency, or what is commonly called “corn after canola syndrome”.  Phosphorus enters the corn plant through root hairs, root tips, and the outermost layers of root cells.  Beneficial fungi, called mycorrhizae, enhance P uptake in corn early in the season because the mycorrhizae strands increase the effective rooting volume of plants.  This is extremely important for uptake of immobile nutrients, such as P.

In corn, up to 80% of early season P uptake is by mycorrhizae since the strand network may extend 8-12 inches from root.  Crucifer plants, such as canola, sugarbeets, and mustard, are not hosts for mycorrhizae so the mycorrhizae must regrow from spores.  This is why early in the growing season you might see corn suffering from P deficiency.

Although the mycorrhizae eventually regrow and colonize the roots, the damage done by lack of P early in the growing season may have already occurred.  Yield may be impacted by the early season P deficiency, maturity of the crop may be delayed and grain moisture content at harvest may be higher (leading to higher drying costs).

How Common is Grain Corn Planted After Canola in Manitoba?

In Yield Manitoba 2014, Anastasia Kubinec (Provincial Oilseed Crops Specialist) updated her crop rotation tables based on information MASC contract holders provide to MASC.  These tables provide trends that can be used to help with crop rotation choices.  If we dig into the data from the 2008 to 2012 time period, 22% of the grain corn acres were planted into canola stubble.  In fact, it was the most popular choice for producers, followed by planting corn into soybeans (16%) and into spring wheat (10%).  So the data illustrates there are other factors producers look at when planning their grain corn crop rotations, and not necessarily the influence of beneficial fungi.

What is the potential impact to yield?

The same data source provides details on the yield response of those rotations (see Table 1 below).  Grain corn  yields are lower following canola than soybeans or spring wheat.

Table 1: Relative Yield Response (per cent of 2008-2012 average) of Manitoba crops sown on previous crops (stubble >120 acres)

Previous   Crop  Crop   Planted – Grain Corn 
Hard Red Spring Wheat 100
Winter Wheat 87
Barley 99
Oat 93
Canola 95
Soybean 103
Sunflower 99
Grain Corn 87
Yield (bu/ac)  95

The initial phosphorus uptake can be an issue but is this practice manageable?

To avoid ‘corn after canola syndrome’, producers should grow a crop less dependent on mycorrhizae for P uptake after canola (corn & flax are two of the most dependent crops).

If rotation requires corn after canola, a “Plan B” is to supply high starter P levels to try and overcome any P deficiency problems that might occur.

Variable results are seen with in-crop treatments to remedy phosphorous deficiency.   If products are tried, it is recommended to leave a check strip to determine effectiveness on crop recovery and yield.

For more information, please see the complete article by Anastasia in Yield Manitoba 2014 at: http://www.mmpp.com/mmpp.nsf/ym_2014_06_crop_rotation_tables.pdf.

Submitted by:  Pam de Rocquigny, Provincial Cereal Crops Specialist

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Do I Need to Apply Zinc to my Soybeans?

Question: When we grew edible beans in the 90’s, we were advised to use zinc. My notes show it was not so much a yield factor, it was for maturity.

We have generally high organic matter (8% and up), most pH is over 7.8 , clay soils, and our phosphorus levels are reasonbe to high at 28ppm -all factors that my notes say tie up Zinc. Some fields are ok for zinc at 4 to 6 ppm but some are lower at or less then 1 ppm. So should I be putting on a zinc product as a safe guard or are soybeans totally different from edible beans?

Answer (supplied by John Heard, MAFRI Crop Nutrition Specialist):

Zinc deficiency is rare in soybeans as compared to edible beans.  Manitoba studies have documented edible beans respond well to applied zinc fertilizer when soil test levels are below 0.5 ppm DTPA extractable zinc.  Soybeans are much less sensitive to low soil zinc levels deficiencies and have  not responded to zinc fertilizer applications in North Dakota and Minnesota studies.  At 11 Minnesota sites in 2011-2012, there was no soybean response to applications of either zinc, sulphur, manganese, molybdenum or boron (see attached reference http://www.smallgrains.org/2013SGU/KaiserSoybean.pdf ).

The main nutrients to watch in soybeans are nitrogen (through proper rhizobium inoculation) and phosphorus and potassium.  Based on your high phosphorus soil tests (26 ppm P) and the high K potential associated with high clay content, your soybeans may do quite well on residual fertility.

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