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How Soluble Salts Affect N Uptake

Some of our agri-coaches are working with farm clients who are farming on soils with high soluble salts in the 12 to 24 inch depth. Soluble salts start to be a concern at values greater than 0.8 (see Table 1). The soil test results below represent a typical condition they are seeing. Their question is, “How will this affect the uptake of N from this depth and how much of the soil test N in the 12 to 24 inch depth will be available for this crop?” And a related question is, “How much will the yield potential of the crop be affected?”

Very good questions! But these are questions with complicated answers.

From the soil sample results in Table 1, you have a soluble salts (SS) value of 1.1. That puts you into the moderately saline category based on Table 2. It also means that the SS is likely greater below the 24-inch depth. The good news is the top 12 inches looks just fine. Therefore, the effects of the sub-soil salinity will depend on things like the rooting depth of the crop and its salinity tolerance.

My colleague, Doug Penney, senior agri-coach, says he hadn’t found any information that specifically addresses the questions of how this will this affect the uptake of N from this depth or how much of the soil test N in the 12 to 24 inches will be available for the crop. He says that in conjunction with the typical spatial and temporal variability in soluble salts within fields, he thinks the best we can do is make an educated guess, sometimes referred to as a “SWAG” (scientific wild-assed guess).

This is his thinking. First, it is common to see elevated soil test N in soils with elevated levels of soluble salts. This makes sense because, as soluble salt levels increase, crops can extract less and less water from the soil. The water holding capacity of soils is defined as the amount of water held between field capacity and wilting point. As the soluble salt level in a soil increases, the amount of water left in the soil at wilting point increases, so the effective water holding capacity of the soil decreases. This is the main effect when SS is in the range of 0.81 to 1.60 (moderately saline to saline). When the salinity level is above this range, other factors also come into play.

Water uptake by plant roots requires an osmotic gradient; the salt concentration of the plant sap needs to be higher than in the soil solution. As a saline soil dries, the salt concentration in the soil solution increases to a point where water uptake stops. Therefore, in a saline soil, not all of the “available water” can be used. In addition, if not all of the water can be used; not all of the nitrate N in the soil solution can be used.

Here is Penney’s SWAG: “If I assume that in the moderately saline to saline range, 20 to 30 per cent of the “available water” cannot be used, then 20 to 30 per cent of the N cannot be used (this could be as high as 40 per cent at the more saline end of the range). In the example soil test, there is only five ppm of N in the 12 to 24 inch depth, so the reduction in available N is quite small (five ppm X four = 20 lb./ac. x 25% = five lb./ ac.). However, when soil test N is higher, the reduction in available soil N becomes quite significant (i. e. 20 ppm X four = 80 lb./ac. X 25% = 20 lb./ac.).”

It is obvious from the above that when soil test N is high in the sub-soil, it is very important to know why. Is all, some or none of it available for the crop to be gown? If we can establish that all or most of it is available, we can achieve our yield goal at a substantially lower input cost. If sub-soil N is high because crop roots are not accessing it, we need to determine why. Field history, cropping history and previous soil tests all help to determine the root cause.

This article discusses the effect of sub-soil salinity on soil test N. Stay tuned for discussion of other factors such as restricted rooting cause by hard pans (natural and induced by heavy traffic) and saturated soil (high water table).

ElstonSolbergisasenioragri-coachand presidentofAgri-TrendAgrology

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Depth (in)

0-6 6-12 12-24

0-6 6-12

12-24

OM ppm

4.3 2.5

1.5

Sppm NO3-N

24

37 465

13

5 5

Phos (ppm)

Bicarb Bray-P1

19

Zn ppm

3.8

30

Mn ppm

61

Kppm Mgppm

349 22

126

Fe ppm

52

360 435

Ca ppm

2240 2460

7200

Cuppm Bppm

910 0.9

0.9

pH

7.4 8.0

8.0

Mo

ppm

0.7

CEC meq/100g

15.2 16.5

44.2

Sol Salts mmhos/cm

0.3 0.3

.1

%K %Mg %Ca %H %Na

5.9 3.1 0.7 Sat P % 13

19.8 21.9

17.2

Al ppm

305

73.9 74.4

81.5

K/Mg ratio

0.30 0.14

0.04

Cl ppm

15

0.4 0.6

0.6

Na ppm

13 22

57

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