Brandt, S.A.

Keys, C.H.

English

eCommons@USASK

Effect of crop rotations on soil moisture levels

University of Saskatchewan Research Archive

- 38 -Effect of Crop Rotations on Soil Moisture Levels S.A. BRANDT & C.H. KEYS On the Dark Brown Soils of Northwestern Saskatchewan, a three-year rotation of; fallow - crop - crop is typically followed, although rotations used vary from fallow- crop to continuous cropping. As more attention is focused on the problems of soil salinity (estimated to affect 10% of our available acreage) and declining soil organic matter (estimated to have de-clined 40% over the past 60 yr.) more producers are moving to extended ro-tations. Producers are also expressing greater interest in more frequent use of green manure or forage crops in grain crop rotations. Yields in crop rotation studies conducted in the past were traditionally related to factors such as weed populations, soil fertility and crop incompatability. Less attention has been focused on the effects of crop rotations on soil moisture levels and crop utilization of soil moisture. Whenrotations are extended, all cropping inputs become crucial; more soil nutrients are removed with the crop; weed control is more difficult since summerf&llowing is practiced less frequently; insect and disease problems may become more severe due to frequent cropping to susceptible crops or alternate hosts. Farmers have at their disposal a variety of inputs and management tools that can be used to help overcome these problems; commercial fertilizers, herbicides, seed protectants, fugicides and insecticides. Crop rotation can be used to provide nitrogen, by using legume crops having nitro-gen fixing rhizobia associated with their roots. Crop rotation can also be used in pest control. Weed control can be facilitated by selection of crops for which good chemical controls are available. In this manner, weed popula-tions can be reduced to allow cropping to a crop for which chemical controls are less effec.tive. By rotation of susceptible crops with non susceptible ones, varying degrees of insect and disease control can be obtained. The quantity of moisture received is one factor over which we have little control. Agronomists have examined a number of ways in which this resource can be managed after it is received. Cultural practices have been developed to increase snow trapping on stubble fields, to improve efficiency of moisture storage during summerfallowing and to conserve moisture in the seed placement zone prior to and following spring seeding. A more complete understanding of how different crops and summerfallowing frequency affect soil moisture levels and moisture utilization would add another management tool that producers could use, particularly in extended rotations. This paper is based on a long-term crop rotation study intiated by C.ll. Keys conducted at the Experimental Farm, Scott, Sask. While the study is limited by the number of crops and rotations used, it does provide information on some of the major crops and rotations used in the area and hopefully will provide a base for additional research. The test was conducted on a Dark-Brown Elstow Clay Loam Soil. Plot sizes were approximately 7 m by 30 m and treatments were replicated four times. Soil moisture determinations were made gravimetrically on soil cores taken to - 39 -a depth of 90 em. Sampling was done as near to seeding as possible in the .spring and shortly after harvest in the fall. Rotations used are shown in Table 1. During the period 1966 to 1971 wheat was used as the crop grown on summerfallow. In 1972 and in subsequent years, rap.eseed was grown in place of wheat on fallow in all rotations except where summerfallow substitutes were used. ·In 1980 the rotations were changed again to facilitate more direct comparisons between wheat and rapeseed when grown on summerfallow. During the period 1966-71 an average of 223 mm of precipitation was re-ceived between the spring and fall soil samplings, during 1972-79 average growing season precipitation was 199mm. · Table 1. Crop Rotations - Scott, Sask. Rotation I II III IV v VI VII VIII IX X Crops 1966-71 continuous wheat-fallow - wheat --fallow-wheat-wheat ....__ fallow-wheat-oats fallow-wheat-oats(alfalfa)-hay & break fallow-wheat-wheat-oats (alf)-'-hay-hay & break fallow with wheat in rows- ---. wheat fallow with oats in rows-wheat fallow with corn in rows--wheat fallow with sunflowers in rows-wheat Moisture use by Crops Crops 1972-79 continuous wheat fallow - wheat fallow-rapeseed-wheat fallow-rapeseed-oats fallow-rapeseed-oats (alf)-hay & break · fallow-,rapeseed-wheat-oats (alf)-hay-hay & break fallow with wheat in rows-wheat fallow with oats in rows-wheat fallow with corn in rows-wheat fallow with sunflowers in wheat rows-Wheat grown on summerfallow during the 1966-71 period had the highest soil moisture use, ranging from 105 to 125 mm (Table 2). This would be expect-ed since more soil moisture was available to the crop grown on summerfallow. Where wheat was grown following the summerfallow substitutes, moisture use was similar to wheat grown following conventional summerfallow. Wheat grown on stubble used more than oats grown on stubble. for stubble grown wheat. less water than wheat grown on fallow, but Wat.er use by continuous wheat was lower than Polish type rapeseed (Brassica compestris) grown on summerfallow during 1972-79.used significantly less water than did wheat following a summerfallow substitute •. Rapeseed used only slighly more water than did wheat grown on stubble. It is apparent that Polish type rapeseed is not as heavy a water user as is wheat. Some of this difference may be attributed to the reduced number of days required for the rapeseed crop to reach maturity. However, rainfall be-tween maturity-of the rapeseed crop and wheat was not sufficient to account for more than one third of the difference. - 40 -Table 2 Soil Moisture Availability and Use - Scott Rotations 1966-71. Soil moisture in m.m. available in spring used left Continuous wheat 95 58 37 Summerfallow Wheat 163 124 39 Summer fallow Wheat 154 111 43 Wheat 108 70 38 Summerfallow Wheat 164 125 39 Oats 100 50 50 Summerfallow Wheat 162 121 41 Oats (alfalfa) 109 60 49 Alfalfa hay 104 34 70 Sunnnerfallow Wheat 142 105 37 Wheat 112 74 38 Oats (alfalfa) 111 64 47 Alfalfa hay 102 49 53 Alfalfa hay 93 61 32 Wheat in rows Wheat 169 132 37 Corn in rows Wheat 157 123 34 - 41 -Table 3 Soil Moisture Availability and Use Scott Rotaticia 1972-79, · Continuous wheat Summerfallow Rapeseed Summer fallow Rapeseed Wheat Summerfallow Rapeseed Oats Sununerfallow Rapeseed Oats (alfalfa) Alfalfa hay Sununerfallow Rapeseed Wheat Oats (alfalfa Alfalfa hay Alfalfa hay Wheat in rows Wheat Corn in rows Wheat Soil moisture in m .m. available in spring 101 162 150 113 161 118 142 117 119 112 115 93 94 93 139 156 used 62 101 90 74 94 65 69 76 60 62 85 67 63 53 101 126 left 39 61 60 39 67 53 73 41 59 50 30 26 31 40 38 30 - 42 -The quantities of soil moisture rema~n~ng after harvest were related prim-arily to the crop grown. Wheat consistently resulted in lower levels of soil moisture than did oats or rapeseed. ~eat g~ c~uously resulted in similar levels of soil moisture after harvest to those w5ete~heat was grown in shorter rota~ions. The hi~levels of soil moisture remaining following rapeseed would increase the amount avaTr~g crop however where a rapeseed-fallow rotation is followed, this moisture could contribute to seepage resulting in soil salinity. Soil moisture levels in the fall, following the hay crops in the 4 to 6-year rotations were a:t or above levels following wheat. In the hay rotations some soil moisture recovery could.have occurred following breaking~ which was usually done in July after hay removal. In the 4-year rotation, soil moisture levels had risen to 59 to 70 mm in the fall, where as in the 6-year rotation soil moisture was only 32 - 40 rrnn after breaking. One factor which is not accounted for is the depletion of soil moisture below the 90 em sampling depth by the hay crop. It is well documented that alfalfa roots will penetrate be~ low this depth, particularly where the crop becomes well established such as in the six-year rotation. Soil moisture storage in summerfallow Soil moisture storage during the fallow period followed similar trends during both periods, 1966-71 and 1972-79, for all rotations except rotations II and VI. The combined data for these two periods are presented in Figure 1. Most of the soil moisture sto fallow occurred from snowmelt overwinter follo~g tn~:;;:-=~n crop. ~uantities store over he first winter period did not g eatly betw~~.!Qt.atJ.ans.,. and the differences were not statiStically significant. Storage during the summer period was quite vari-able and the differences were statistically significant (Figure 1). In the conventional grain crop rotations, storage was 20 - 30 mm. In the mixed grain ....: forage rotations, 20 mm was stored during this period in the 4-year rotation and only 5 mm in the 6-year rotation. In the 4-year rotation the alfalfa did not become well established and was readily eradicated during summerfallowing. However, in the 6-year rotation the alfalfa was established better and persisted throughout much of the fallow period and reduced moisture storage. Where wheat or oats were grown in widely spaced rows as summerfallow substitutes, there was a loss of soil moisture during the summer period. Corn or sunflowers grown in widely spaced rows resulted in near normal moisture storage during the summer perio.d. Storage of snowmelt moisture over the final winter period was highest where wheat or oats in rows were used in summerfallow substitutes, with the corn or sunflowers in rows resulting in lower snowmelt storage. The lowest total fallow moisture storage was recorded on the four and six-"year rotations. When combined with the low residual moisture prior to surrnnerfallowing in the six-year rotation, there was a significantly lower quantity of moisture avail-able to the crop grown on summerfallow in this rotation. 200 150 or F-R Figure 1 Accumulation of Available Soil Moisture in Sumrnerfallow residual soil moisture after preceeding crop residual s.m. plus snowmelt over first winter accumulated s.m. after summer period when in fallow s.m. available to crop grown on sumrnerfallow or F-R-0 or F-R..,.-O(A)-A or F~R-W-O)A)-A-A - 44 -The two rotations in which notable changes occurred between the two time periods were the fallow-wheat which was changed to a fallow-rapeseed rotation and the 6-year grain-forage rotation. At the start of the fallow period, residual soil moisture from the pre-ceeding crop was higher in the fallow-rapeseed rotation than when the rotation was fallow-wheat (figure 2). In turn, soil moisture levels the following spring were higher for this rotation. During the summer period however, more moisture was stored in the fallow-wheat rotation. This difference in storage may be partly due to differences in trash cover. The trash from the rapeseed crops disappeared rapidly in summerfallow, while wheat residues persisted longer and may have reduced surface evaporation. Another possible explanation is that the higher levels of soil moisture occurring early in the fallow period contri-buted to greater losses below the sampling depth. If such were the case, fall-owing after a rapeseed crop would likely contribute more moisture to subsoil seepage and consequently greater salinity. Soil moisture gained from snowmelt over the final winter period was higher in the fallow-rapeseed rotation than the fallow-wheat rotation, resulting in a slightly higher level of available soil moisture at the time of seeding. In the six-year rotation soil moisture accumulation during the summer period when in fallow was markedly lower during the period 1972-79 than in the earlier period. During the period 1966-71 less persistent varieties of alfalfa were used and generally less vigorous stands were obtained. Thus, during the earlier period the alfalfa was more readily eradicated during fallow, where as it was more persistent in later years contributing to greater losses of soil moisture during the fallow period. Soil moisture levels from the continuous wheat rotation are included in figure 2, as a reference point. The levels of soil moisture in this rotation changed very little during.the two time periods. ,....._ ~ '-' (!) 1-1 ::s .1-J {/) ·rl· 0 ::<:: 11) ....; ·r'-1 "<t' 0 (/). .,;;, (!) .1-J ctl ....; :l s (.) ·u <t: 200 150 100 50 F-W 1966-71 Figure 2 Accumulation of Avilable Soil Moisture in Summerfallow residual soil moisture after proceeding crop residual soil moisture moisture plus snowmelt over first winter F-R 1972-79 accumulated s.m. after summer period when in fallow soil moisture avaialbe to crop gown on summerfallow F-W-W~O(A) -A..;.A 1966-71 F-R-W-O(A) -A-A 1972-79 Cont.W 1966-71 Cont.W 1072-79 - 46 -Relationship between soil moisture levels and crop yields In this study crop yields tended to be related to levels of soil moisture at the time of seeding although other factors such as soil fertility and weed populations also affected yields. Yields of wheat grown on fallow were similar for rotations II, IV and V as were levels of available soil moisture at the time of seeding (Table 4). Soil moisture levels were lower on rotations III and VI and wheat yields also tended to be lower. Wheat yields on stubble also tended to decline with declining levels of soil moisture at seeding. Yields of rapeseed did not fallow this pattern as closely. Rapeseed yields in the straight grain rotations decreased as avaialbe soil moisture levels decreased. However, in the grain-forage rotations, yields tended to be higher per m.m. of available soil moisture at seeding. Summary Although alfalfa may be useful in improving levels of soil organic matter and supplying nitrogen to succeeding crops, the lower soil moisture levels that occur on Dark Brown Soils as a result of this practice, restricted yields of succeeding crops. Where the alfalfa became well established and persisted during the fallow year, soil moisture levels on fallow were particularly low. Studies conducted on Black and Gray Luvisolic soils in the Melfort and Loon Lake areas have shown that on those soils, forage crops do not necessarily result in lower soil moisture levels. In some cases storage of moisture has been enhanced in grain-forage rotations. Crops S'l,l~S rapesee~ which do not use as much of the available soil moistur~ as wbeat~r~)etteF adap:ted t~on on fallow, as they result in higher moisture levels for su~ in this area. Such crops are not adapted to a~n because the higher moisture levels in the fallow period could increase saline seepage. In a wheat monoculture system there is a sharp decline in the amount of soil moisture available to the first stubble crop as compared with a fallow crop (Figure 3). As recropping is extended to a second stubble crop and to continuous wheat, available moisture at seeding declines only slightly. It would appear that sufficient moisture is available to continuous wheat to main-tain yields at levels near those obtained on first crop stubble. ! Crops grownin widely spaced rows as summ. erfallow substitutes do not result n hiifmr H!.velS~!?l mois~va4-l~-t.o-1:,he succeeding crop as compared with conventionai~~erfallQ~a~~dium te~tured soils. On light textur-ed s~her~ture s~!Tei~-fallow is very low or where erosion of fallowed fields is a problem, this practice may have some merit. This practice may also be beneficial in improving moisture storage following a forage or green manure crop. - 47 -Table 4 Crop Yields and Available Soil Moisture at Seeding Rotation W - on fallow w - on stubble I Cont. W 1890 (95) II F - W 2731 (163) III F- W- W 2498 (154) 2103 (108) IV F - W - 0 2640 (164) V F - W - 0 -A 2623 (162) VI F-W-W-0-A-A 2452 (142) 2243 (112) I Cont. W 1252 (101) II F - R 1031 (162) III F - R - W 869 (150) 1736 (113) IV F ..;. R - 0 953 (161 V F - R - 0 - A 914 (142) VI F-R-W-0-A-A 897 (ll2) . 1898 (ll5) Yields are in kg/ha with available moisture in mm in brackets. - 48 -Figure 3 Effect of Extended cropping on Soil Moisture Levels Won fallow s.m. avail at seeding residual s.m. after harvest 1st crop W on stubble 2nd crop W onstubble cont. w 

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Effect of crop rotations on soil moisture levels

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