Some Thoughts About Soil pH, Fertilizers and Lime

Tim Smith, WSU extension, Wenatchee & north
(My apologies to any soil scientist in the audience.)

When you apply fertilizers that have ammonium as a portion of the product, or some fraction that will turn into ammonium, you are adding "acidity" to the soil. It doesn't matter what kind or type of fertilizer, conventional or organic source, just that it has ammonium (or ammonia) in it.

Ammonium is made up of nitrogen and hydrogen, three atoms of hydrogen for each atom of nitrogen. (NH3-). Shortly after the ammonium reaches the soil, because it has a negative charge, it binds to the positively charged soil or organic matter. Through time, it is converted to nitrate by soil bacteria. The warmer the soil, the faster the conversion. It is the nitrate form of N that is most mobile in the soil, most likely to leach into groundwater, and usually most used by the crop.

During the conversion to nitrate, the nitrogen element loses the three hydrogen and adds two oxygen. The oxygen comes from the air in the soil. The three hydrogen particles are free to float about the water in the soil and to react with various substances there. Free hydrogen very reactive, and is the "acid" in anything. (Incidentally, acids taste tart, but I don't recommend that you go around tasting soil to see if it's tart or sweet.) The H+ atoms can react with lime (calcium carbonate) in the soil, forming water and carbon dioxide and thus be neutralized, or the H+ can tie up on soil particles, react with all sorts of chemicals or organisms in the soil, or they can just drift around in the soil water, waiting to interact with (and mess up) all sorts of critical reactions. Most plants have difficulty getting the chemical nutrients they need, in the proper amounts, when soil has too many hydrogen ions in the soil water solution.

Some soils have a great ability to cope with these added H+ particles, as they have large amounts of natural "lime" to convert the H+ into water, or maybe they have lots of organic matter and clay to bind the H+, taking it (temporarily) out of the soil water solution. Other soils quickly lose the ability to "buffer" the H+ in the soil, and allow the H+ to build up in the soil water. Once a soil has become acid, it has lost most of it's ability to deactivate H+ particles, and is likely to become even more acid rapidly if more H+ is added to the soil.

When a lab does a pH test of the soil, they are measuring the balance of hydrogen particles to "bases" in the soil solution. If there are more H+ particles than bases (-OH, which taste sweet....), the soil is acid.

The pH number indicates a ratio of acid to base particles in the soil. The lower the number below 7 (neutral), the more acid the soil. Each pH number is in a multiple of 10. So a soil with a pH of 5 has 10 times as many free-floating H+ particles in the soil solution as a soil with a pH OF 6. Soil with a pH of 4 has 100 times the "acidity" of a soil with a pH of 6.

Most "native", uncropped soils in the desert areas of Eastern Washington have pH levels of about 7.5 to 8.5, Indicating that they are somewhat alkaline (sweet) to strongly alkaline. The pH test does not show you how quickly the pH will drop with the addition of ammonium fertilizers. The native soil may have small or massive amounts of natural lime, and both situations are common.

If there are massive amounts of lime and other bases in the soil, you have what people often call "alkaline", or "caleche" layered soil, which is difficult to manage. It is difficult or impossible to get all the lime out of this soil, and you will have to live with high pH, along with the drainage problems, iron deficiency, and compaction that are often production problems. If you have one of these soils, you should use the most acidifying forms of nitrogen fertilizer (such as ammonium sulfate), as you may never live long enough to develop an acidity problem. Some growers have helped the trees get critical soil nutrients by banding strong acids along the row. This may force the pH down in a narrow, shallow zone along the soil surface, alowing more iron to enter the tree through shallow roots.

If your soil is normal, and has been in production for many years, it may already have become acid since it was broken out of the sagebrush. Since acidity is a chemical reaction, it occurs at the point where the ammonium nitrogen was applied through the years. If the fertilizer was banded and concentrated under the tree, that is where the pH will be lower. The pH is often much higher under the grass between the trees where less fertilizer has been applied. Since the H+ particles react to the soil near where the fertilizer was placed, the surface soil is most effected. The pH is often higher in the second and third foot than in the surface foot in the zone beneath trees, unless you have been treating the soil acidity with lime over the past several years.

So, it is a complicated question when you are asked, "what is the pH of your orchard soil?" The pH where? I have seen soil in the same orchard that ranged from 4.2 in the top foot directly under the tree to 7.2 in the second foot in the middle of the drive row. Under these circumstances, the trees roots contact many different pH's, which is likely to balance the trees needs a bit, compared to having the entire root zone in an unfavorable pH. However, since some substances, such as manganese and aluminum, are released in toxic levels from soils as they drop into pH's below about 5.5, it is best to treat your soil with lime to start the long process of bringing the soil acidity down and the pH up.

As I mentioned earlier, lime reacts with the H+ in the soil solution to form water and carbon dioxide. A ton of lime will convert a lot of H+. Each ton of lime added can neutralize the H+ particles added by 1110 pounds of ammonium nitrate or urea. So, depending on how much fertilizer you have applied each season, each ton can compensate for about 4 to 20 years of fertilization with those acidifying products.

Most growers had built up quite a large "lime debt" by the 1980's, and needed to apply several tons to bring all their soil back to 6-7 pH. The lime should be directed to those areas in the orchard where fertilizer has been applied, generally under the trees. Trees often responded visibly the first year of application, so other nutrients (especially N) should be reduced after liming to bring the tree growth back into better ballance.

If you have low pH in your orchard, you do not need to put the total required lime on all at once. Lime works best when it is mixed with the soil so it can come in contact with a large volume of the soil water. Since lime is not very soluble, it moves into the soil very slowly when applied to the soil surface. When you apply lime in the orchard, you are probably treating only the surface 4-8" for the first few years, so there is no hurry to get the total "lime debt" applied. We generally recommend that you apply two tons the first season of treatment, let a few seasons pass, then apply two tons more. You should then wait a few more years before re-testing and possibly re-applying more lime.

The best time to take care of liming is "between" orchards, as the block is replanted. Lime should be applied while you can still identify the old rows, so the application can be concentrated along the row, where the pH is lowest and the lime needs are highest. Ripping and discing the soil prior to planting will greatly speed the correction of pH in the block.

Do NOT over-lime! Lime adjusts soil chemistry, it is not a fertilizer. A little too much can raise pH to undesirable levels and keep it there, causing serious management problems. Make certain you know how much lime is needed, then apply it over a number of seasons until your soil is back in balance.