This increases nitrogen availability to many crop plants which prefer nitrate as a nitrogen source. The root nodules of legumes contain bacteria which convert nitrogen in the air into a form plants can use. The best pH for the activity of these bacteria varies according to the crop in which they live. Maximum availability of phosphorus occurs between pH 6.0 and 7.0. Outside this range the formation of insoluble phosphorus compounds reduces its availability to plants.
All crops have a range of pH at which they grow best.
Examples of some optimum pH ranges for some crops are given in Figure 2. The effectiveness and breakdown of some herbicides differ across the range of pH encountered in soils. Below pH 5.8 some triazine herbicides are held by the soil and cannot be taken up by plants. This may reduce their effectiveness against weeds. Raising the pH above 6.5 makes the herbicide available to plants and effective at weed control.