Many of soils throughout Connecticut tend to be more acidic, formed as rocks were ground and deposited by glaciation. The pH of soils cultivated for vegetable growing typically declines (becomes more acidic) gradually from the removal of calcium, magnesium and potassium ions—by leaching and crop uptake—and from the use of acid-forming fertilizers. Testing every year gives a more complete evaluation and is also appropriate when significant changes have been made in the fertilizer program. Reviewing fertilizer labelling is essential.
Soil tests measure soil nutrients available to a plant throughout its life cycle. Soils for vegetable growing should be sampled and tested at least once every three years. This publication should be of interest to agricultural producers who have grown vegetable crops for many seasons, as well as those interested in developing or expanding their crop varieties.
Managing soil pH is one of the most effective low-cost strategies to ensure better nutrient use by plants. In general, vegetable crops thrive at soil pH 6.0 to 7, with 6.5 being optimal for most vegetables. Some vegetables do well at pH 5.5; potatoes will tolerate even greater acidity.
Vegetable crops tend to be heavy feeders as the plants maximize productivity. Typically, vegetable crops cycle through flowering, fruiting, and harvesting within a single growing season. When soil pH is adequate, the availability of both major and minor nutrients is maximized, and the accumulation of toxic metals is minimized.
Primary plant nutrients: Nitrogen, Phosphorus and Potassium
Because nitrogen levels fluctuate widely depending on environmental conditions and can also change in the sample during shipping, nitrogen is not routinely measured. A pre-side dress Nitrogen Test (PSNT) can be used by the grower to test nitrogen levels.
Apply nitrogen close to the time the crop is most active to allow for the most efficient uptake by the plants, either banding at planting or as a side dressing. Splitting applications and side dressing are important adaptation strategies especially after major rainfalls. Use plastic mulch to limit leaching and enhance mineralization from organic matter. When calculating application rates, take into account nitrogen from organic matter, cover crops, composts, manure, etc., which become available as soils warm. As soil organic matter decomposes, generally 20 pounds of N/acre/year is released for each percent of organic matter.
Phosphorus is usually tightly bound to soil particles with only small amounts in irrigation water. Phosphorus may also occur in soil organic matter. Most phosphorus loss is attributable to surface runoff and soil erosion. Techniques that help prevent nutrient loss to the environment include prevention of soil erosion and avoidance of overfertilization. Many of the soils throughout the state, especially in fields close to dairy and poultry farms (where large amounts of manure were applied) can have very high phosphorus levels.
Potassium deficiency in vegetables is often found in soils that test high in potassium due to issues with potassium uptake, such as root disfunction, soil compaction, and extremely high fruit demand. To manage potassium in vegetable crops, the keys are to maintain high levels in soils, minimize soil compaction, manage irrigation, adding additional potassium though fertigation (including fertilizer in irrigation system) or side dressing prior to flowering in fruiting vegetables.
Secondary nutrients: Calcium, Magnesium, and Sulfur
Control of calcium disorders starts with proper application of lime to the soil. Lime provides soil calcium and raises the soil pH. Additional important factors to control calcium disorders are supplying a steady rate of water (through irrigation), limiting root damage (while cultivating row middles), and reducing compaction and waterlogging. Planting at a spacing that allows for good air movement will also help, by increasing the vapor pressure deficit leading to an increase in root uptake of nutrients. In addition, choose plant varieties that are less susceptible to calcium disorders. Generally, because of the way calcium moves through the plant, varieties with very large or very long fruit are more susceptible to calcium deficiencies.
Foliar calcium applications should be considered a supplement, not a correction for calcium deficiencies and good soil and water management as calcium moves primarily through xylem. Side dressed calcium has been shown to have positive effects on root crops such as potatoes, particularly in sandy soils. Calcium nitrate or chelated calcium applied through the drip irrigation system can help alleviate calcium disorders in some drip irrigated vegetables such as tomatoes.
Magnesium is often applied from dolomitic limestone or high-magnesium limestones, especially if a pH adjustment is required. If no liming agent is required, magnesium such as Epsom salt should be applied as a fertilizer source. In an emergency, magnesium may be applied as a foliar spray, but results may not be ideal.
Most of the sulfur in the upper part of the soil is held in organic matter. Crops grown in soils with less than two percent organic matter often require additional sulfur. Soils with higher organic matter content potentially supply adequate sulfur, but environmental conditions can slow microbial activity and therefore limit sulfur availability. This is especially the case in cold soils during late fall and early spring. Manures and compost generally supply ample amounts of sulfur.
Cole crops such as cabbage, broccoli, mustard, turnip greens, and radishes remove higher amount of sulfur (30 and 45 lbs/a) than most other vegetables (10-25 lbs/a). Growing deep-rooted cover crops can be beneficial for sulfur and other nutrient availability by preventing nutrient leaching from the soil profile.
Micronutrients: Boron, Zinc, Manganese, Copper, Molybdenum, and Iron
Micronutrients are as important to plants as the primary and secondary nutrients, but in smaller amounts. Response to micronutrients is rare with soils of reasonable organic matter or manured soils having pH is in the proper range.
Micronutrient deficiencies are most likely to occur in sandy soils with low organic matter. Of all the micronutrients, boron is most likely to be needed to supplement soil levels for cauliflower, broccoli, cabbage, and beets that are most susceptible to hollow heart caused by boron deficiency. On the other hand, bean, cucumber, garlic, Jerusalem artichoke, lima bean and pea are sensitive to high levels of boron and should not be planted on fields following crops that received boron application.