Anne-Kristin Løes

Long-term changes in extractable soil phosphorus (P) in organic dairy farming systems

On five farms that have been managed organically for several years, all cultivated soils were sampled on two occasions. The time span between the first and second soil sampling varied from 6 to 12 years. At the first sampling the farms had been managed organically for 3, 4, 6, 11 or 53 years. The average phosphorus (P) concentrations in topsoil (0-20 cm) extracted by ammonium-acetate lactate solution (P-AL) decreased from the first to the second sampling on all farms. At the second soil sampling, the average topsoil P-AL concentrations on the five farms were 50, 64, 65, 75 and 119 mg P kg−1, which is characterised as medium (26–65 mg P kg−1) or high (66–150 mg P kg−1). The decrease occurred mostly in soils with high and very high (>150 mg P kg−1) P-AL concentrations at the first sampling. In these samples, the average value decreased from 100 to 87 and from 188 to 151 mg P kg−1, respectively. In subsoil (20–40 cm), an increase from 15 to 27 mg P kg−1 (P<0.01) in P-AL concentration was found in subsoil samples with low P-AL concentrations (0–25 mg P kg−1) at the first sampling. This indicates P transfer from topsoil to subsoil. The pattern of decrease in topsoil was fairly well explained by farm level P balances. The average topsoil concentrations of P-AL were well below values for comparable conventional farms, but still at a level acceptable for crop production. Crop yields were acceptable, but the general pattern of decrease shows that in the future, some P should be supplied from external sources to avoid a further decrease, especially on the fields with lowest P-AL concentrations.

Changes in the nutrient content of agricultural soil on conversion to organic farming in relation to farm‐level nutrient balances and soil contents of clay and organic matter

Agricultural soil on 12 farms converting to organic farming was sampled in 1989 and 1995 at the same sample points. Concentrations of plant‐available (‐AL) P, K, Ca and Mg, HNO3‐soluble K, total‐N, total‐C and pH were measured in two layers. The average K‐AL concentration and pH were reduced in both layers. The average P‐AL concentration was reduced in the topsoil. K‐AL and P‐AL increased in samples with low concentrations and decreased in samples with high and very high concentrations in 1989. KHNO3 increased in the topsoil and total‐N increased in the subsoil. Total‐N and total‐C increased in top‐ and subsoil with a low organic matter content. The net import of P, calculated by farm‐level nutrient balances, was negatively correlated to the change in kg P‐AL ha−1 in the topsoil. No such correlation was found for K.

Repeated use of green-manure catch crops in organic cereal production – grain yields and nitrogen supply

Abstract By restricted access to manure, nitrogen (N) supply in organic agriculture relies on biological N-fixation. This study compares grain yields after one full-season green manure (FSGM) to yields with repeated use of a green-manure catch crop. At two sites in south-eastern Norway, in a simple 4-year rotation (oats/wheat/oats/wheat), the repeated use of ryegrass, clover, or a mixture of ryegrass and clover as catch crops was compared with an FSGM established as a catch crop in year 1. The FSGM treatments had no subsequent catch crops. In year 5, the final residual effects were measured in barley. The yield levels were about equal for grains with no catch crop and a ryegrass catch crop. On average, the green-manure catch crops increased subsequent cereal yields close to 30%. The FSGM increased subsequent cereal yields significantly in two years, but across the rotation the yields were comparable to those of the treatments without green-manure catch crop. To achieve acceptable yields under Norwegian conditions, more than 25% of the land should be used for full-season green manure, or this method combined with green-manure catch crops. The accumulated amount of N in aboveground biomass in late autumn did not compensate for the N removed by cereal yields. To account for the deficiency, the roots of the green-manure catch crops would have to contain about 60% of the total N (tot-N) required to balance the cereal yields. Such high average values for root N are likely not realistic to achieve. However, measurement of biomass in late autumn may not reflect all N made available to concurrent or subsequent main crops.

Potassium uptake by grass from a clay and a silt soil in relation to soil tests

In a pot experiment with five harvests of ryegrass the potassium (K) uptake from soil was studied in a clay and a silt soil, with (K1) and without (K0) K fertilizer. Ammonium acetate lactate-extractable K (K-AL) was rapidly depleted, and in K0 the K-AL level stabilized at 30 and 100 mg K kg -1 in the silt and clay soil, respectively. Corrected for different minimum values, the K-AL value predicted the K uptake by ryegrass from AL extractable K very well. In the silt soil the K release from reserve K (total K release from soil minus K release from K-AL) was small, whereas in the clay soil there was a substantial release from reserve K. Part of the reserve K in the clay soil was easily releasable and contributed to luxury consumption of K in the first crop. Acid-soluble K (K-HNO 3 minus K-AL) was a good parameter by which to assess the ability of the soil to supply ryegrass with reserve K. The results were compared with the results of the field experiments from which the soils were collected. The difference in K release between silt and clay soil was larger in the pot than in the field experiments, but without K fertilization the K-AL values levelled off at the same values in field and pot experiments.

Concentrations of Soil Potassium after Long-Term Organic Dairy Production

On five long-term organic dairy farms aiming at self-sufficiency with nutrients, soil concentrations of ammonium-acetate lactate extractable potassium (K-AL) and acid-soluble K was measured twice in topsoil (0–20 cm) and subsoil (20–40 cm) over periods of 6–14 years. Organic management had occurred for >9 years at the second sampling. On average there were most probably field level K-deficits. Even so, topsoil K-AL concentrations were medium high (65–155 mg K kg–1 soil), and did not decrease during the study period. However, for three farms, topsoil K-AL was approaching a minimum level determined by soil texture, where further decrease is slow. Subsoil K-AL concentrations were generally low (<65). The soils were mostly light-textured, and reserves of K-releasing soil minerals (illite) were low, never exceeding 6% of the mineral particles <2 mm diameter. Topsoil acid-soluble K concentrations were low (<300 mg K kg–1 soil) on two farms, medium (300–800) on three farms and decreased significantly on one farm. Cation-exchange capacity increased on two farms. This may indicate increased amount of expanded clay minerals caused by K-depletion. On self-sufficient organic dairy farms, purchased nutrients will be required by low soil nutrient reserves to avoid seriously decreased yields and quality of crops.

Root development in potato and carrot crops – influences of soil compaction

Row crops such as potatoes (Solanum tuberosum L.) and carrots (Daucus carota L.) are of high economic value in the Nordic countries. Their production is becoming more and more specialized, including continuous arable cropping and heavier farm machinery, with increased risk of soil compaction. The result may be restricted root development and economic losses. Potatoes have widely branched adventitious roots, whereas carrots have taproots with fibrous roots extending from them. Under optimal soil conditions, total root length per surface area may reach more than 10 km m−2 for both species. Maximal root depth is about 140 cm for potato and more than 200 cm in carrots. Most of the root mass is usually distributed within the upper 100 cm, whereof more than 50% may be deeper than 30 cm. Soil compaction causes a dense soil with few large pores, poor drainage and reduced aeration, especially in wet soils with low organic matter content and high proportions of silt or clay. With compacted subsoil layers, roots will be concentrated more in the upper layers and thus explore a smaller soil volume. This will lead to reduced water and nutrient uptake, reduced yields and low nutrient utilization efficiency. In this review article, we describe the interactions between root development and soil conditions for potatoes and carrots, with special focus on sub-optimal conditions caused by soil compaction. We also discuss the effects of tilling strategies, organic material, irrigation and fertilization strategies and controlled traffic systems on root and yield development. To reduce subsoil compaction there is a need to implement practises such as controlled traffic farming, new techniques for ploughing, better timing of soil operations, crop rotations with more perennial crops and supplements of organic material. Moreover, there is a need for a stronger focus on the impacts of farm machinery dimensions.