Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
As soil pH drops, availability of magnesium and calcium declines while manganese availability increases, often to toxic levels. Below pH of 5.2, the chemistry of the soil changes and aluminum is released into the soil solution at increasing levels, further acidifying the soil. This free aluminum also is very harmful to plant roots because aluminum interferes with calcium, can bind with phosphorus, and can interfere with cell expansion at root tips, effectively stopping root tip development. Most of the active mineral nutrient uptake occurs in the region just behind the root tips. Without further root tip growth, nutrient uptake will become limited. Effective rooting volume is also reduced, thus placing the plant under additional stress. In severe cases, plants can die. During fruit formation, there will be increased incidence of blossom end rot in tomatoes and peppers in plastic beds with low pH.
The following are minimum pHs for various vegetable crops:
| Crop | Min. pH |
| Cucumbers, cantaloupes, squash, pumpkins | 5.8 |
| Watermelons | 5.5 |
| Tomatoes, peppers, and eggplant | 5.8 |
| Cole crops (broccoli, cabbage, cauliflower, Brussels sprouts, kale, collards) | 6.0 |
| Spinach, beets, chard | 6.0 |
| Snap beans and lima beans | 5.8 |
| Sweet corn | 5.8 |
| Peas | 6.0 |
| Potatoes (scab resistant) | 5.5 |
| Carrots | 5.5 |
| Sweet potatoes | 5.5 |
| Onions | 5.8 |
Below these pH levels, crop performance will be affected, and yields will be reduced. Lime should be applied immediately if soil pH has dropped to these values. Target pHs for vegetable crops can be found in Table B1 in the 2019 Mid-Atlantic Commercial Vegetable Production Recommendations: https://cdn.extension.udel.edu/wpcontent/uploads/2012/03/23152426/SectionB.pdf
In the eastern US, soil pH will drop naturally due to the 45+ inches of rainfall received. In addition, if ammonium and urea containing nitrogen fertilizers are used, they will also lower pH. Ammonium nitrogen is also released from organic nutrient sources. Ammonium will convert to nitrate in the soil, a process called nitrification, and will release hydrogen (H+) ions, thus dropping the pH. As a general rule, lime should be applied to raise the pH every 3 years. After very wet years such as 2018, pH will drop more than normal or dryer years.
Plastic Mulched Beds and pH
Each year we see problems with vegetable crops related to low pH in plastic mulched beds. A common scenario is a field with sandy soil (loamy sand, sandy loam) that has not been limed in the last 2 years. The starting pH of beds in this situation will usually be 5.5-6.0. Granular or liquid nitrogen fertilizers applied prior to or at bed formation and nitrogen fertilizers applied through the drip irrigation system during fertigation will commonly consist of ammonium sulfate, urea, ammonium nitrate or UAN (urea-ammonium nitrate) solutions. All of these fertilizers are acidifying because the ammonium which they contain (urea releases ammonium nitrogen as it reacts with the soil). As a result, pH in the plastic mulched beds gets progressively lower throughout the growing season. Beds with a starting pH of 5.5 can drop down into the 4s. The largest drops in pH will be in the wetted area around the drip emitter and drier areas of the bed will have a higher pH.
It is also possible to have low pH under plastic in organic production systems depending on the rate and type of organic material being applied. For example, blood meal used to supply nitrogen in organic systems is very soil acidifying.
Managing plastic mulched bed pH starts with making sure that fields are limed the fall before beds are to be made. Spring applications can also be made to the area, but full lime reaction should not be expected.
If marginal pHs are encountered after plastic is laid (below 5.8), manage fertilizer programs so that large pH drops do not occur. Consideration should be given to eliminating ammonium or urea containing fertilizers and switching to calcium nitrate and potassium nitrate sources for fertigation. Both these fertilizers cause a basic reaction in soils because plant roots excrete hydroxides and carbonates as they take up the nitrate. There are few other materials that can be used to raise the soil pH through the drip system once plastic is laid. One option is potassium carbonate which is alkaline and thus will raise the pH. It is fully soluble and can be made in liquid forms. Potassium hydroxide is another fertilizer that has a basic reaction and that can go through the drip system.
Liquid lime products with ultrafine ground limestone can also go through a drip system. Recently, a finely ground (< 0.5 micron) liquid limestone-based product (Top Flow 130; Omya, Oftringen, Switzerland) was developed for agriculture use to be injected through drip irrigation tubing. It shows some promise but does not replace liming because it only affects the area around emitters about 4 inches.