Dr. Dan Talks Agronomy
Watch High Soil pH
The soils on my family farm in northeast Nebraska are clay loams with a high soil pH (greater than 7.2) and calcareous (lime chips lie on the soil surface). I call them tight clays and generally do not consider them good soils for soybeans. Yet, if managed properly, these soils can produce good soybeans if we maintain organic levels of 2.5% to 3% and keep the crop properly fed.
Keeping soil pH in the optimal range is essential to producing high-yielding crops such as soybeans. The first step to managing soil pH is a good soil testing program, usually done in the fall to allow time to apply lime and fertilizer before planting the next crop.
Early summer is a good time to pull plant tissue samples and analyze nutrient levels. You can identify shortages before they become visually apparent and can also still apply fertilizer to correct a deficiency. Since pH affects nutrient availability, don't rely solely on a soil test level if your pH is below 6.0 and you didn't lime, or if it is above 7.2. Supplement soil testing with tissue testing to see if plants are deficient in critical nutrients.
Soybeans generally perform well at soil pH levels between 6.0 and 7.0. The optimal range is between 6.3 and 6.5; this maximizes nutrient availability and biological nitrogen fixation. When soil pH levels drop below 5.5 or exceed 7.2, nutrient deficiencies and toxicities can occur. While you can lime to raise soil pH, you can't economically lower pH.
P[L1] D[0x0] M[300x250] OOP[F] ADUNIT[] T[]
When soil pH drops below 6.0, phosphorus, potassium, sulfur, calcium, magnesium and molybdenum become less available. Let pH fall below 5.0 and metals such as aluminum and iron become more soluble and can be toxic to roots.
However, most growers aren't as aware about the impacts of alkaline soil or how soil test levels can be misleading and that only tissue testing or visual symptoms can reveal nutrient deficiencies high pH creates. Phosphorus is less available in highly alkaline soils, particularly above pH of 8 or when calcareous (calcium and phosphate form calcium phosphate, an insoluble mineral). When soil pH exceeds 7.2, key micronutrients like iron, manganese, zinc, copper and boron become less available and deficiencies can be observed in sensitive field crops like corn or soybeans.
A recent study from the University of Minnesota showed that as pH increased, both soybean yield and phosphorus and zinc availability decreased. While that fact has been previously established, the study also concluded that recommendations for phosphorus and zinc should not be based on soil test levels alone, but rates should also be adjusted for soil pH. For example, potassium recommendations are often adjusted for CEC (calcium exchange capacity).
Results of the study found adequate levels of phosphorus and zinc must be available throughout the growing season and, in high pH soils, phosphorus levels must be double (36 ppm vs. 19 ppm) that for soils with a neutral pH for P not to be limiting. Similar results were found for zinc. The authors of the study suggested producers increase phosphate and zinc rates when soil becomes more alkaline and even when soil test levels indicate an adequate level.
While their research only looked at phosphorus and zinc, this could also apply to manganese and boron. However, be careful with boron because it can easily reach toxic levels. Soybeans aren't particularly sensitive to copper shortages.
Soybeans also exhibit signs of iron deficiency chlorosis (IDC) at high soil pH levels. The most dominant factors are carbonate concentrations and salt levels. Typically, areas where water ponds and salts collect can influence iron availability in the plant rather than in the soil.
Keep field variability in mind. The pH in the top 6 to 8 inches of soil will not be the same across the field and can range from acid to neutral to alkaline in one field. Sampling soil by grid or zone reveals this variability, and you can increase fertilizer rates in alkaline soil areas. Before arbitrarily adjusting the rate for an alkaline pH, pull tissue samples this year to see if nutrients are indeed deficient in these areas.
There is little research on how much extra fertilizer to add for soil pH above 7.2. My suggestion: If your grid or zone soil samples show high pH areas and tissue tests indicate nutrients are deficient or in the low end of the sufficiency range, consider applying 1.5 or 2 times the amount of phosphorus, manganese or zinc to grids or zones with a pH greater than 7.2. I would be cautious with additional boron and not worry about copper.
(PS/GH/AG/CZ)
Copyright 2014 DTN/The Progressive Farmer. All rights reserved.
