Pointers On Avoiding Nutrient Deficits
Soil type has a huge impact on soil fertility, affecting everything from nutrient- and water-holding capacity to pH levels and micronutrient availability. The muck soils of the Northeast and the granitic soils of the West can be some of the most challenging soil types when it comes to nutrient management. When growing on these soils, vegetable producers need to be aware of their special nutrient requirements to avoid deficiencies.
Muck And Mineral Are Not The Same
When properly drained, muck soils are ideal for high-quality vegetable production, but it is a mistake to treat them the same as mineral soils. The most desirable soil pH range for optimum nutrient availability in muck soils is 5.2 to 5.8, much more acidic than the 6.2 to 6.8 range for mineral soils. Because of this, certain essential nutrients are less available for plant uptake when pH is not in the right range.
“Muck soils are naturally low in phosphorus, potassium, copper, manganese, and zinc,” says Darryl D. Warncke, professor emeritus at Michigan State University. “Phosphorus levels can be built up fairly easily; however, potassium levels are difficult to accrue. Muck soils do not hold potassium as tightly because of their low clay content and high levels of available calcium. As a result, potassium can leach when large amounts of water move through the soil profile.”
Because of the increased organic matter in muck soils, as much as 20% to 80%, microbial activity intensifies, resulting in a greater release of nitrogen. As a result, nitrogen rates need to be adjusted accordingly. Under heavy rainfall conditions, seasonal losses can also occur due to denitrification and leaching.
“Cover crops provide additional organic matter and are effective at taking up available nitrogen at the end of the season,” says Warncke. “They also help reduce erosion, which keeps nutrients, particularly phosphorus, in the field and out of the groundwater.”
Monitor Granitic Soils Closely
Granitic soils have reduced water- and nutrient-holding capacity due to their coarse texture. They tend to have a lower pH, which can lock up available nutrients and reduce fertility. Similar to muck soils, granitic soils are subject to high erosion rates.
“Poor fertility soils like decomposed granite can be very unforgiving. They require close monitoring as there is not much room for mistakes when it comes to nutrient management,” says Richard Smith, agricultural soil specialist for University of California Extension.
According to Smith, irrigation and nitrogen management go hand-in-hand on granitic soils because they are prone to leaching. Smaller fertilizer amounts, applied more frequently, can help keep nutrients like nitrogen and phosphorus in the uptake zone. Irrigation applications may need to be timed to release water over several short periods.
Iron deficiencies are less problematic in granitic soils, but growers should be on the lookout for potential problems with zinc, boron, and calcium deficiencies. In some cases, granitic soils can fix potassium, making it difficult for plants to use it at critical growth periods. Smith recommends monitoring potassium, phosphorus, and micronutrient levels closely through regular soil testing.
Recognize Deficiency Symptoms
The following symptoms may help identify nutrient deficiencies, but remember prevention is the best cure. By the time visual symptoms show up, the damage is done and control is much harder, if not impossible, to achieve.
- Boron: Brittle or curled leaves, rosetting, reduced flower and fruit set.
- Calcium: Distorted new leaves, stunted growth, water-soaked lesions on fruit.
- Iron: Young leaves turn yellow or white, interveinal chlorosis on younger leaves, stunted growth.
- Magnesium: Interveinal chlorosis with necrotic spotting on older leaves, leaves turn brittle and curl upwards.
- Manganese: Chlorosis on older leaves, young leaves turn yellow and curl upward, stunted growth, spotting, interveinal chlorosis.
- Nitrogen: Symptoms develop on older leaves first, leaves turn light-green to yellow, defoliation, stunted growth
- Phosphorus: Abnormally dark leaves, defoliation, poor fruit quality, stunted growth.
- Potassium: Marginal yellowing on young leaf tips and edges, necrotic spotting, necrosis of older leaves.
- Zinc: Rosetting, mottling on young leaves or uniform chlorosis, necrotic spotting.
Return To Tried And True Practices
“The foundation of any soil fertility program is soil testing,” says Warncke. “Growers need to know what their available nutrient levels are, and they need to be aware of their soil pH. On most soil types, nutrient availability issues become more problematic once pH falls below 6.
“The other part of avoiding nutrient deficiencies is maintenance fertilization,” he continues. “Growers need to know the nutrient requirements for the crops they are growing and replace what nutrients were removed at harvest to maintain optimal nutrient levels.”
Smith recommends growers use a nitrate quick test prior to making any nitrogen applications. That way they are aware of residual nitrate levels in the soil and can make adjustments if needed.
Both Warncke and Smith agree that vegetable growers will find no magic bullet for dealing with nutrient deficiencies. The best way to control them is to prevent them from happening. It all comes back to tried and true soil fertility practices that preclude over- or under-application of fertilizer, maintain optimal nutrient levels, and reduce leaching so nutrients are available when and where plants need them.