Corn (Zea mays L.) yields less with no‐till or reduced tillage than with conventional tillage on medium and fine‐textured soils in Ontario, Canada. This study was undertaken to determine whether rates of N, P, or K above those normally recommended could increase the no‐till yields. Annual rates of 120 and 240 kg N/ha, and initial rates of 95, 475, and 1,260 kg P/ha and 95, 665, and 1,330 kg K/ha were applied to a silt loam soil (Albaquic Hapludalf) in combination with four different tillage treatments (fall moldboard plow, spring disc; fall mulch plow with heavy duty cultivator, spring disc; fall ridge; no‐till). Corn was grown, harvested for grain, and stover returned to the soil over a 6‐year period. Increasing N, P or K applications gave similar response patterns on each tillage treatment. There was no indication that the higher nutrient applications could overcome yield depressions associated with reduced tillage. Adequate N, P, K levels in corn tissue further substantiated an adequate nutrient supply with each tillage treatment. Resistance to penetration measurements on soil suggested that mechanical impedance might be more critical than nutrient supply in growth performance and explain the lower yields with reduced tillage. Soil tests for available P and K, combined for increments of the upper 30 cm of soil, tended to be higher with no‐till than with the plow treatment.

A study of the interacting effects of soil bulk density and soil water matric potential on resistance to penetration and on corn root elongation showed that for any level of resistance, elongation was further restricted as potential decreased (suction increased). This was explained through the effect of matric potential on plant turgor, which in turn affects the ability of the root to overcome resistance. At negligible resistance, elongation was greater in a clay loam soil than over a range of equivalent osmotic potentials in solution, indicating that moisture transport in the soil apparently did not restrict moisture supply to the root. It is suggested that the presently accepted upper limits of resistance for corn growth may be too high.

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More precise effects of soil t e m p e r a t u r e on t h e direction of corn root g r o w t h were d e t e r m i n e d b y growing seedlings in soil for s h o r t periods in g r o w t h c a b i n e t s controlled to give a range of soil t e m p e r a t u r e conditions.The angle of g r o w t h (relative to t h e horizontal} was f o u n d to be m i n i m u m (10 °) at c o n s t a n t 17°C. A b o v e or below this t e m p e r a t u r e (I0-30°C), a more vertical direction occurred. Cyclic t e m p e r a t u r e s also influenced direction, w i t h t h e m a x i m u m of t h e cycle a p p a r e n t l y d e t e r m i n i n g the angle. The duration of t h e m a x i m u m t e m p e r a t u r e period in a cycle necessary to initiate a direction effect was n o t d e t e r m i n e d , however.The significance of these findings in t h e field is t h a t modifications of soil t e m p e r a t u r e s b y mulches, etc. p r o b a b l y influence t h e d i s t r i b u t i o n of roots in t h e rooting zone of soils. This in t u r n m e a n s t h a t p l a n t b e h a v i o u r such as moisture a n d n u t r i e n t u p t a k e can be b e t t e r explained or m a n a g e d for maxim u m performance. * Former graduate student. Present address, Ministry of the Environment,

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