Soil pH plays an important role in plant nutrition. However, we might not be familiar with how nitrogen fertilizers influence soil pH over time. This article explains how nitrogen fertilizers influence soil pH over time and discusses some considerations in choosing N fertilizers.
Plants can take up N in two forms: ammonium and nitrate. Ammonium (NH4+) is positively charged, while nitrate (NO3–) is negatively charged. When plant roots take up a charged ion, they typically release an ion with identical charges to maintain a balanced pH in plant cells. Following this rule, plants release a hydrogen ion (H+) when taking up an ammonium ion; and release a hydroxide ion (OH–) when taking up a nitrate ion. As a result, the net effect of taking up nitrate-N is to increase soil pH around root zones; taking up ammonium-N reduces rhizosphere soil pH.
Nitrogen fertilizers contain N in the forms of ammonium, nitrate and urea. Upon application to the soil, urea-N rapidly hydrolyzes to ammonia, thus it shares similar characteristics as ammonia-based N fertilizers. When nitrate-N based fertilizers are applied to the soil, plants take up N in nitrate forms. When ammonium-based N fertilizers are applied to soil, soil bacteria convert ammonium into nitrate under aerobic conditions and with warm temperatures. Therefore, nitrate is still the primary nitrogen form taken up by plants in ideal crop production conditions. The nitrification process (bacteria convert ammonium to nitrate) releases hydrogen ions (H+), which react with hydroxide ions (OH–) released by plants during the process of taking up of nitrate. The overall effect on soil pH is close to neutral. However, in reality, we often over apply ammonium-based N fertilizers to compensate for the nitrate leached from the soil, thus soil pH is reduced over time, partly because of the accumulation of H+ released through nitrification process.
In field vegetable production, the choice of N fertilizers is often determined by price. In most cases, urea is used because of the low cost, easy application, and high N content. While in greenhouse/high tunnel production, fertilizer costs only represent a small portion of the total production cost, and fertilizer leaching normally is not a big concern. The relatively more expensive nitrate-based N fertilizers such as potassium nitrate and calcium nitrate are more commonly used. They are highly soluble and provide other essential nutrients that are in high demand by greenhouse crops such as tomatoes, peppers, cucumbers etc. However, the constant use of nitrate-N based fertilizers increases soil/substrate pH. Ammonium-N based fertilizers such as nitrogen solutions (a mixture of ammonium nitrate and urea dissolved in water) are used to maintain pH in the desirable slightly acidic range.
Plants often take up extra N and store it in plant cells for later use as we apply most nitrogen fertilizers at the beginning of the season. It is safe to store nitrate but storing too much ammonium in plant cells could lead to ammonium toxicity. Ammonium toxicity is more likely to occur when ammonium-N based fertilizers were applied in cool weather. When the temperature is below 60° F, soil bacteria can not convert ammonium into nitrate, and plants tend to take up and store too much ammonium. Ammonium toxicity is a physiological disorder that may inhibit root growth, and cause interveinal chlorosis and necrosis of young leaves. Increasing the temperature, using nitrate-N based fertilizers and leaching of the soil/substrate may alleviate the problem. Nevertheless, it should be noted that plants do take up and use ammonium without harm as long as it is not at toxic levels.