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Soil pH & Soil Acidity with Haiying Tao

Posted by Blythe Howell | June 18, 2018

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Contact Haiying Tao via email at

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Episode Transcription:

[ Music ]

Drew Lyon: Hello, and welcome to the WSU Wheat Beat Podcast. I’m your host, Drew Lyon, and I want to thank you for joining me as we explore the world of small grains production and research at Washington State University. We have weekly discussions with researchers from WSU and the USDA-ARS to provide you with insights into the latest research on wheat and barley production. If you enjoy the WSU Wheat Beat Podcast, do us a favor and subscribe on iTunes or your favorite podcasting app. And leave us a review while you’re there so others can find the show too.

[ Music ]

Drew Lyon: My guest today is Haiying Tao, Haiying is an assistant professor in soil fertility and nutrient management in the Department of Crop and Soil Sciences. Haiying was born and grew up in inner Mongolia, she came to the US in 2002 and obtained her PhD degree in soil fertility at the University of Connecticut in 2007. She worked on two large multistate projects on nitrogen loss reduction and land application of livestock manure prior to joining WSU. She joined WSU in August 2015. Her research and extension focus at WSU is soil fertility and soil health management for dryland cropping systems. Hello Haiying.

Haiying Tao: Hello Drew.

Drew Lyon: So tell us a little bit about one issue that’s gotten a lot of press lately and that is soil pH and soil acidity.

Haiying Tao: Right, so first I would like to talk about what is soil pH and what is soil acidity. So basically, soil pH is a measurement of soil acidity, so it measures the hydrogen content in the soil solution. So when the hydrogen content in the soil solution is high, soil pH is low. So soil pH measures one component of soil acidity, which we call it active soil acidity. So in the soil there is another component of soil acidity we call it exchangeable soil acidity. So the exchangeable soil acidity mainly is the hydrogen and aluminum on the CEC. So the reason I want to emphasize there are two components of soil acidity in the soil is that, so when farmers want to apply lime to increase soil pH and if they do not know that there is another component of soil acidity in the soil they can be confused because when they get the numbers back for lime requirements for different soils, even if the start pH are different, are the same, excuse me, and the end pH are the same but different soils they will have different lime requirements. So that is mainly because the exchangeable acidity in soil buffer are different in different soils. So then the Palouse area, so where naturally soil pH is neutral, which means around sevenish. So in soil that has been acidified and I have seen soil pH as low as 4.5 in the Palouse area. But I have heard from other people that they have seen soil pH as low as low fours.

Drew Lyon: Okay and the pH scale is a logarithmic scale right, so if you move from 6 to 5 it’s actually 10 times… If I can get this right — 10 times more hydrogen, not just a little bit it’s a fairly big change moving from.

Haiying Tao: Yes. It is a huge change yes.

Drew Lyon: And you mentioned CEC earlier, can you tell our listeners what CEC means for those that may not know.

Haiying Tao: Sure. CEC is cation exchange capacity. So your soils have clay minerals, have organic matter, and those particles have net negatively charged sites. And those sites can attract or absorb positively charged ions. And those sites we call them cation exchange capacity because they can hold cations.

Drew Lyon: Okay so more of those clay particles or get the higher the CEC would be your cation exchange capacity?

Haiying Tao: Right.

Drew Lyon: Okay. So soil acidification I know you’ve had a workshop on this not too long ago, it’s kind of getting a lot of press, a lot of people talking. What’s the big deal about soil acidification?

Haiying Tao: So soil acidification can be a big deal because the pH or soil acidity is — we call it the master of soil health. So the soil pH, in fact, many characteristics of the soil. So I’d like to look at this issue in different angles. I like to look at in terms of soil and in terms of crop, and also from long-term sustainability point of view. So in terms of soil, so when your soil pH decreases the soil, say soil chemistry, soil fertility, soil biological properties will shift. So as your soil change from neutral to low pH, then for example your soil fertility. So most of the essential nutrients for crops are most available at neutral pH. So if your pH is very low then your base saturation starts to decrease, your nutrients availability starts to decrease. So and another thing is if your soil pH is low enough, say very low, lower than 4, lower than 5 or 4.7 for some soils, then you start to see aluminum. And the crops have, a lot of crops have very limited amount of tolerance to the aluminum, so you could have aluminum toxicity problems. And in terms of microbial communities, so if you — most of bacteria they have an optimum soil pH, they like neutral pH. So if pH is very low some of the microbes are not tolerant to low pH, therefore, their population decreases. And that can outbalance the good microbes in terms of in relation to the bad. Because some fungis they have more of tolerance to low pH, so when your pH is low then your good microbe soil decrease and then which leaves the unbalanced of microbe activities.

Drew Lyon: And like you said you know our soils prior to farming cultivation were somewhat neutral.

Haiying Tao: Yes.

Drew Lyon: But now we’ve been farming them for a long time and using the certain types of nitrogen fertilizer mainly that drives the pH down. We’ve been doing that long enough that we’re starting to get into this area where we’re concerned that we might, what’s the analogy I’ve heard, fall off a cliff. We’ve, you know, we haven’t seen big falls in yields yet, but we’re getting close to the point where a lot of people think maybe we will if we go much further. Is that part of the reason why there’s all of a sudden, some focus on this where there hasn’t been that much focus up until recently?

Haiying Tao: Yes, so I’ve heard that a lot and that I think that can be a big deal. The reasons I think that farmers have not been seeing a big yield reduction or not seeing big yield increase from liming. So one reason is that availability of aluminum tolerant varieties are available for wheat. Another reason is that for some of the trials that I know for liming are not done on the fields where the pH is as low as what we think. So the aluminum does not show up in most of soils until soil pH is lower than 5.2. In some soils, you may not see aluminum until, soluble aluminum excuse me, until soil pH is as low as 4.9, 4.7.

Drew Lyon: Okay.

Haiying Tao: Yeah, but if we leave it without doing anything, your soil pH will continuously decline because you will continuously apply fertilizers. And if we leave it untreated and then your soil pH continues to decline another issue that I see with that is and then you will subsoil acidification problems. Once you have subsoil acidification problems, then that will be very difficult to correct and it is very expensive to correct.

Drew Lyon: Okay, so how do farmers know if their soils have been severely acidified, is there a test they can do or something they see out in the field that tells them that might be occurring?

Haiying Tao: Right, so the way that farmers can tell if they have soil acidification problems is to test their soil pH. Soil pH is a very good indicator of soil acidity and soil acidification. So they can continuously monitor their soil pH. So when they send the soil samples to the soil testing labs they often come back with soil pH and if not, they can ask for soil pH. And there’s another thing that they can do, that farmers can do by themselves. So there are things like portable pH readers that farmers can take to the fields and walk around and test soil pH in the field. So Dr. Paul Carter published extensive publications describing how to use those type of meters and how they can do in field testing.

Drew Lyon: Okay and soil pH fairly uniform in a field or can it change quite a bit? Does it change across the landscape and by depth? I imagine there’s, so I guess how does a farmer go about checking that soil pH across a field?

Haiying Tao:  Right that’s a very good question. So soil pH can be very different from one area to another area. So that’s why I said I recommend farmers to walk around within the fields and test in different spots so that they have a good idea where their soils are more acidified than other areas. And another thing I want to emphasize is how you sample soils is very, very important when you want to monitor the changing soil pH. So if you want to test soil pH in a conventional tilled field, so do not take samples deeper than your tillage zone. So six inches is a good soil sampling depth for conventionally tilled soils. So if you are measuring soil pH for a no-tilled field I recommend you to take soils at different depths. So you will take zero to 2 inches, 2 to 4 and 4 to inch layers of soils. And then you test soil pH for each layer and that way you can see where your low pH is. So if for any reason which is possible I haven’t seen that you see low pH for all three layers, then your soil is pretty, in a pretty bad situation for soil acidification. And then you may want to go deeper because you may also have some subsoil acidification problems.

Drew Lyon: Okay. So sounds like if you’re going to use nitrogen fertilizers you’re going to be acidifying your soil. What can farmers do to slow down that process or delay how long it is before they suffer the consequences of really low soul pH and acidification?

Haiying Tao: So in terms of slowing down soil acidification process, so there are a few things we can do. So remember, the main reason of soil acidification is nitrogen applications. Actually it’s not only nitrogen, some other fertilizers like sulfur also acidify soils. But the main reason for soil acidification is nitrogen because we apply a larger amount of nitrogen fertilizers. So and since we know that is the cause and then if we can, any practices that can improve nitrogen use efficiency and practices can reduce nitrogen loss, then that will help with slowing down acidification problems. And another thing is that remember I talked about buffering capacity. So soil organic matter is a very important component in terms of soil buffering capacity. So the higher the soil organic matter is then the higher soil buffering capacity, which means more resistance to pH change. So any practices that you can increase soil organic matter that will be helpful, for example leave the residue in the field, do not harvest straw, do not burn the straw. And other things that you can they improve soil organic matter is, for example, apply soil amendment, like manure, compost, say biochar, biosolid. So I was talking about to somebody the other day asking about biosolid, if biosolid will change soil pH and so I told him well it depends. So if you have a biosolid that is treated by base, then it does increase soil pH. And if not, it could even decrease a little bit soil pH. But it does provide a large amount of organic matter which is very helpful. In terms of increasing soil pH besides the amendments I mentioned, a quick way of doing it is to apply lime materials. So in terms of applying lime materials it can be a little tricky because there are many different types of lime materials. Different lime materials have different lime scores. So for the same soil if you use different lime materials you will need different amount of lime materials. And for different soils if you want to change the pH from one level to another level, then you will require a different amount of lime materials. For example, if you have five soils and your other soils have pH say 4.5 and you want to increase the pH from 4.5 to 5 and all those five soils you will find out will require five different amounts of lime material. So it can be tricky for this calculation, so WSU extension does have a series of publications to help farmers to determine how much lime is required. So that there is a publication that has a link to a spreadsheet that they can download that spreadsheet and put required parameters in there and it will tell you how much lime you should apply.

Drew Lyon: Okay so if our listeners are interested in finding out more information where do they go to get that?

Haiying Tao: Yeah, so get on small and on the left, click soil and water conservation, and then you click publications. And you scroll down, you will see a number of publications and among those publications there is a series of three publications, one of them has a link to the spreadsheet.

Drew Lyon: Okay, very good. So our listeners who want to find these publications we’ll include a link in our show notes. So soil acidification an issue in this area. And if you want more information go to

Haiying Tao: Right.

Drew Lyon: Thank you very much Haiying.

Haiying Tao: Thank you, Drew.

Drew Lyon: Thanks for joining us and listening to the WSU Wheat Beat Podcast. If you like what you hear, you can subscribe on iTunes or your favorite podcasting app so you never miss an episode. And leave us a review while you’re there. If you have questions for us that you’d like to hear addressed on future episodes, please email me at You can find us online at You can also reach out on Facebook and Twitter @WSUSmallGrains. The WSU Wheat Beat Podcast is a production of CAHNRS communications in the College of Agricultural, Human, and Natural Resource Sciences at Washington State University. I’m Drew Lyon. We’ll see you next week.

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