Effects of soil amendments on lettuce production

Figure 1 Part of the open-ground trial site

Figure 1 Part of the open-ground trial site

Effects of soil amendments on lettuce production: Elaine McKeogh

Organic soil amendments are used to increase soil nutrient content and to improve characteristics of the soil such as water-holding capacity, pH and drainage rate. In this study, two widely used amendments (garden compost and animal manure) were used, in addition to Bacteriosol-Bio  (a commercial product marketed to increase humus production) and soft rush.

 Rushes are frequent and widespread on wetter soils, and Jim Cronin, the well-known organic grower from Co. Clare, had already reported some value from composted soft rush.  In this study, fresh stems of soft rush were harvested and incorporated directly into the soil without prior composting.

When butterhead lettuce  was grown  in amended and unamended (control) soil (Figure 1),  composted animal manure and garden compost  both resulted in significantly higher dry matter yields  (Table 1), with manure doubling the yield; dry weights are more sensitive measures of yield than fresh weight  because they do not reflect differences due to water content .  Interestingly, lettuce in soil amended with garden compost produced yields which were not significantly different from those grown in soil amended with animal manure. The other two amendments, Bacteriosol-Bio and soft rush resulted in yields which were not significantly different from those in control soil (Table 1).         


Table 1. Effect of soil amendments on the dry matter yield of butterhead lettuce

     Soil amendment

 Median dry matter yield (g)

  Animal manure

                   922 b

  Garden compost

                   873 b


                   390 a

  Soft rush

                   346 a

  None (control)

                   459 ab

Any two medians with a common letter were not significantly different (P>0.05).

    Nutrient analysis of the different soils demonstrated that all the amendments exhibited increased available nitrogen (N) and potassium (K), while all bar soft rush had increased available phosphorus (P) (Table 2). Animal manure was the best source of nutrients, followed by Bacteriosol-Bio. All the soil amendments resulted in increased soil pH (although the received wisdom is that manures and composts lower the soil pH), while animal manure, garden compost and the humic stimulant Bacteriosol-Bio resulted in increased humic acid. (Table 2).

               Table 2. Effect of soil amendments on nutrient content and pH of soil.

Soil amendment

Available N

Available P

Available K


Humic acid (% DM)

 Animal manure






Garden compost












Soft rush














    When the water-holding capacity (WHC) of amended and unamended soils was compared, the WHC of soil amended with animal manure was highest, with that amended with garden compost being second, although neither was significantly different from the control (unamended soil) (Table 3). This reflects the higher organic matter content of these two amended soils, which increases the ability of the soil to act as a reservoir for water, as well as reducing the risk of flooding. Neither Bacteriosol-Bio nor soft rush resulted in increases in WHC (Table 3).

                               Table 3. Effect of soil amendments on soil characteristics

       Soil amendment

    Median water-holding capacity (%)

          Median drainage time (s)

         Animal manure

                 53.1 b

                      71.0 ab

        Garden compost

                 50.1 ab

                      77.0 ab


                 47.6 a

                      83.5 b

        Soft rush

                 48.0 a

                      68.0 a


                 48.8 ab

                      84.5 b

Any two medians with a common letter were not significantly different (P>0.05).

     The unamended soil was a heavy, clay-rich soil with a slow rate of drainage. Both of the bulky organic amendments, animal manure and garden compost, resulted in marked but not significant reductions in drainage time, reflecting faster drainage (Table 2). The negative charges on the organic matter cause the tiny clay particles to clump together forming larger particles (“micelles”), so that water drains more rapidly, with larger pores between the micelles.

   The most interesting result came from the rush-amended soil. After 6 months incorporation of the intact rushes, little was detectable of the plant material, but the soil texture was noticeably more friable than the corresponding control soil. This was reflected in a significantly faster drainage time in the rush-amended soil compared to the control soil (Table 2).

  To investigate the effect of soil amendments on the bacteria in the soil, the numbers of bacteria and the range of different food sources they could utilise (as a measure of bacterial diversity) were studied by Community Level Physiological Profiling using Biolog plates. Animal manure showed increased bacterial numbers, richness and diversity (Shannon Index) compared to all the other soils, while Bacteriosol-Bio also resulted in greater bacterial diversity.

  In the absence of amendments, a short comparison was also made between control plots with and without tillage. Without tillage, there were increased numbers and richness of soil bacteria compared to the tilled control, while the untilled control exhibited a much higher pH (7.56 as opposed to 7.06) and conductivity, an indirect measure of soil fertility (443.1 as opposed to 356.0 µS/m2 in the tilled control). These data underline the damaging effects of tillage on the soil’s inhabitants.

Centre for Organic Horticulture Research

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