Temperatures at the 5 cm depth (2 inch) vary tremendously under both zero and conventional tillage (Figure A1). Soil temperature at that level usually follows prevalent air temperatures. Temperatures tend to be warmer by 1-2° C under conventional than zero tillage. It has been shown that such temperature differences will not impact rate of emergence because temperature differences will not impact rate of emergence because emergence is not only a function of soil temperature, but also of soil moisture and seeding depth (Lafond et al 1992). It should be noted also that daily temperature fluctuations are greater under conventional than zero-tillage at this shallow depth. The larger temperature fluctuations under conventional tillage can lead to head canker and banding.

At the 40 cm depth (Figure 2A), temperatures tend to be warmer by a few degrees under zero-till as compared to conventional-till during the growing season. During the winter month period, these temperature differences were greatly magnified due to the insulating effects of the snow. At 80 cm (Figure A3), the differences between zero and conventional tillage were somewhat reduced but again temperatures under zero-till are slightly higher. At 120cm (Figure A4), the temperature profiles are very similar between zero and conventional tillage during the growing season. The winter of 1991-92 was very mild and the soil temperatures during the winter tended to be warmer by approximately 1° C for conventional than zero-tillage. These differences essentially disappeared by May 1 (Julian Day 122) and from that point the temperature profiles were essentially the same for both tillage systems.

Summary: Based on the results presented, tillage systems had only minor effects on soil temperature. At the 5cm depth, soil temperature tended to be warmer by a few degrees on the conventional than the zero-till system. At the lower depths, soil temperatures tended to be warmer by a few degrees under zero-till. Previous results have shown that these small differences in surface temperatures does not impact the rate of plant establishment because the rate of emergence is a function of not only soil temperature but also soil moisture and seeding depth.

The depth of rooting of a crop is a function of soil temperature and moisture as well as nutrients available for growth and stresses encountered by the shoots (above ground plant parts). The soil temperatures encountered by spring wheat roots under zero-till at 40 cm (16 inches), 80 cm (32 inches) and 120 cm (48 inches) for different crop stages are given in Figures B1-B3 for the years 1988-1992. At seeding, soil temperatures ranged from 0 to 5° C at 40cm, -1 to +2° C at 80 cm and –1 to +0.5° at 120 cm. At flowering, soil temperatures ranged from 13 to 17.5° C at 40 cm. 1.5 to 14° C at 80 cm and 8 to 11° C at 120 cm. At harvest, soil temperatures ranged from 12.5 to 18° at 40cm, 12.5 to 16° C at 80 cm and 10.5 to 13° C at 120 cm.

The optimum soil temperature range for root growth in spring wheat is generally accepted to be 18 to 21° C (Potter 1956). It has also been suggested by Wilkinson (1967) that cereal root systems tend to grow downwards following a 12° C isotherm.

Given these facts, and the soil temperature data presented above, the spring wheat rots in this study were always exposed to soil temperatures which were below the optimum. This means that the rate of root elongation will be slowed down. As well, if the assumption of the 12° C isotherm is correct, then rooting depth will always be restricted because of low soil temperatures in the 40 cm (16") to 120 cm (48") layer. As well one must also recognize the environmental conditions that the shoots are exposed to. For instance in 1988, soil temperatures were higher than the other years reported but the shoots were exposed to high temperature stresses. Soil moisture measurements for that particular year revealed only shallow extraction of water under these conditions. The coolest soil temperatures were recorded in 1992 and yet we observed more water extraction at depths then on the other years (refer to section C) because the shoots were not exposed to high temperatures and/or high evaporative demands in 1992. It should also be remembered that the discussion presented also applies to conventional tillage systems since results discussed in section (A) showed that soil temperature profiles were similar for zero and conventional tillage systems.

Summary: The soil temperature profiles for the period 1988-1992 under zero-till at 40,80 and 120 cm were such that the spring wheat roots would have encountered temperatures which were below the optimum for the entire life cycle of the plant in the 40-412 cm. If the assumption of the 12° isotherm is correct, then roots would not have elongated much past the 80 cm (32 inch depth). In the discussion of rooting depth, one must also recognize the stresses encountered by the shoots because of their direct impact on root growth.

Making use of available soil water requires proper fertilization because of its direct effect on shoot and root growth. In order to demonstrate this, results on moisture use at depth between fertilized and unfertilized spring wheat from the Indian Head Long Term Rotations are presented in Figure C1. Soil moisture measurements were done at specific growth stages and the corresponding temperatures at theses times are given in Table 2.

Close examination of soil water extraction at various depths shows the importance of proper fertilization because it allows more water to be extracted from the soil and this is apparent at all depths. The other observation to note is the patter of water extraction. At the shallow depths, the decline is almost linear except for the 15 cm level where fluctuations due to recharge from rainfall are observed. Beyond the 60cm depth, a period of no changes is observed initially because the roots have not yet reached that level. By referring back to Table 2, it can be shown that when no changes in soil moisture are observed, the soil temperature is below 12° C. Consequently, this supports the observation of Wilkinson (1967) that cereal roots grow downward following a 12° C isotherm. The other observation to note is that in the 3-30 cm soil layer, most of the available water is used but as depth increases, less and less of the water is being used by the crop. This reflects the decrease in rooting density as depth increases.

Effects of tillage systems and crops: The information presented in this section is from the tillage x crop rotation study outlined previously. The discussion pertaining to the crops spring wheat,

Winter wheat, flax and field peas are all from the third rotation involving a sequence of spring wheat-flax-winter wheat-field peas.

Each crop will be discussed separately. A listing of soil temperatures under zero and conventional tillage at different times during the growing season is given in Table 3.

Soil moisture extraction at depth for spring wheat is given in Figure C2 for both zero and conventional till. The first observation is that the pattern of water extraction is similar for zero and conventional tillage and both systems tend to draw down soil moisture to the same level. In the supper soil layers (15-75 cm), zero-till has more water available while in the lower soil layers, conventional till has more water available then zero-till. The lower levels is a reflection of previous cropping history. In this case, zero-till has out yielded the conventional till consistently which over a period of time could reduce the amount of water percolating down the profile. The other observation is the amount of available water remaining in the lower soil profile after the crop has matured.

Soil moisture extraction at depth for winter wheat is given in Figure C3 for both zero and conventional tillage. It should be remembered that winter wheat is always direct seeded into stubble. Any observed differences would be a reflection of the length that the soil was left undisturbed or untilled. In the 15 to 60 cm soil layer, the pattern and amount of water extracted was similar for both tillage systems. In the 75 to 120 cm soil layer, the plots in continuous zero-till tended to extract more water than those in conventional till. This could be a reflection of the long term benefits of using a continuous cropping – zero till production system.

Soil moisture extraction at depth for flax is given in Figure C4 for both zero and conventional tillage. The first observation is that the pattern of water extraction and the amount extracted was similar in the 0 to 60 cm soil layer. In the 75 cm soil layer, more water tended to be extracted under zero then conventional tillage. In the 90 to 120 cm soil layer, the change in soil moisture was very small during the course of the growing season. The lower soil moisture content in the 90 to 120 cm soil layer under zero-till is a reflection of previous cropping history because yields of spring wheat under zero-till tended to be better than under conventional till. The other important observation is that in the 0 to 75 cm soil layer, flax was able to withdraw more water than the other crops.

Soil moisture extraction at depth for field pea is given in Figure C5 for both zero and conventional till. The patter and extent of water extraction as similar between zero and conventional till in the 0 to 75 cm soil layer. In the 90 to 120 cm soil layer, there was essentially no extraction of water regardless of tillage system.

The effects of tillage systems and crops on soil moisture content at flowering and physiological maturity under zero-till are given in Table 4. At flowering, the soil moisture content at depth was lower for winter wheat and spring wheat than field pea or flax. The corresponding Julian days at flowering for winter wheat, spring wheat, flax and field pea were 181, 196, 189 and 196, respectively. The soil moisture contents at maturing were lowest for flax than the other crops in the 0 to 75 cm soil layer. This would explain the observation frequently made by producers that soil tends to be drier after a flax crop. When comparing crops for depth of water extraction, in all cases, most of the water used is in the 0-60 cm soil layer with very little of the available water being used in the rest of the profile. Examination of soil temperatures in Table 3 at different depths reveals that at flowering for the various crops, soil temperatures at 40 cm ranged from 14.5 to 16° C, at 80 cm from 11.8 to 12.0° C and at 120 cm from 9.0 to 9.5° C. At maturity of the crops, soil temperatures at 40 cm ranged from 12.5 to 17.° C, at 80 cm from 12.5 to 13.5° C and at 120 cm from 10.8 to 11.3° C. A close examination of Figures C2 to C5 shows that water extraction beyond 75 cm was greatly reduced and with crops like field pea and flax, essentially no water was extracted. The corresponding Julian days at maturity for winter wheat, spring wheat, flax and field peas are 226, 239, 246 and 246, respectively.

---------------Flower Stage--------------------------------------- ------------Maturity----------------------------

Summary: Water use by crops is directly related to mineral nutrition and making use of available water requires proper fertilization of the crops. The pattern and extent of water extraction was similar between zero and conventional tillage systems. The depth of water extraction was similar between zero and conventional tillage. In both tillage systems, exploitation of soil water in the lower profile was very limited. Differences in the extent of water use was different with crops. Flax and field pea extracted very little soil water beyond the 75 cm soil layer while winter wheat and spring wheat extracted more beyond the 75 cm level. However, flax was able to extract more water in the 0-60 cm level than all the other crops which explains why soil is much drier after a flax crop.