Optimum values of reduced bulk density Or for soils are around 1.2 g. cm-3, but more important is the minimum value of bulk density for the restriction of root growth which is about 1.7 — 1.8 g. cm-3 for light soils and only 1.40 — 1.45 g. cm-3 for heavy-textured clay soils. Bulk density Or is a crucial parameter for the assessment of the soil compaction rate as an important negative factor of soil productivity. Bulk density of topsoil in the range of 0.95 — 1.15 g. cm-3 shows loose topsoil while the value > 1.25 g. cm-3 indicates heavily compacted topsoil.

Another important value of soil is soil aeration VZ. It is expressed in volume % as the difference between porosity Po and momentous soil moisture Wobj.

vz = Po — Wobj. (16)

Optimum aeration e. g. for grasslands is 10% by volume, for soils for barley growing it is already as much as 24% by volume. Soil porosity Po is the sum of all pores in volume per cent, in topsoils it is around 55%, in subsoil it decreases to 45 — 35%. Sandy soils have on average P = 42% by vol., out of this 30% are large pores and 5% are fine pores while heavy — textured clay soils have the average porosity of 48% by vol., out of this only 8% are large pores and 30% are fine pores. Fine pores are capillary and large pores are non-capillary ones. Cereals should be grown in soils with 60 — 70% of capillary pores out of total porosity and 30 — 40% of non-capillary pores. Forage crops and vegetables require the soils with 75 — 85% of capillary pores and only 15 — 25% of non-capillary pores out of total porosity. Ploughing resistance P is also significant. It is a specific resistance that must be overcome during cutting into and turning over the soil layer. It is expressed by the drawbar pull measured dynamometrically on the coupling hook of a tractor. It is related to the texture and moisture of soil, to its content of organic substances and ploughing depth. Ploughing resistance for light soils is 2 — 4 t. m-2, for heavy-textured soils it is 6 — 8 t. m-2. The units kp. dm-2 are also used. For sandy soils the ploughing resistance of 25 — 28 kp. dm-2 is usual, for clay soils it is 70 kp. dm-2.

Hence heavy-textured soils are more responsive to the higher reduced bulk density of soil when roots develop poorly, they need more non-capillary pores to allow for the better infiltration of precipitation water, they also need higher aeration because they are mostly too moist and many aerobic processes including the microbial activity take place with difficulty. Of course, the high ploughing resistance is not desirable either for the economics of soil cultivation or for the production process of any crop. Therefore it is necessary to improve heavy-textured soils and the question is how. Organic fertilisers are not sufficient; peat was used previously but now it is banned to use it for the reason of the peat bog conservation, and synthetic soil amendments (Krilium, Flotal etc.) are currently too costly for the agriculture sector. An excellent material for the improvement of heavy-textured soils is the solid phase of digestate if ploughed down at higher doses than those used for the application of farmyard manure or compost, i. e. 100 — 150 t. ha-1. Even though we cannot expect any great release of mineral nutrients from organic matter of the solid phase of digestate due to high stability of this material, the improvement and aeration of heavy — textured soil with better conditions for the microbial activity of soil and undisturbed root growth often bring about a higher yield effect than is the yield effect of nutrients from high — quality organic fertilisers as shown by the results of this field trial:

When we still believed that the solid phase of digestate was an organic fertiliser, we laid out an exact field trial on a heavier-textured, loamy-clay soil with medium to good reserve of available nutrients. The trial had two treatments: the one treatment was fertilisation with the solid phase of digestate only (after fugate centrifugation) and the other treatment was the application of only mineral fertilisers in the form of pure salts at such a dose that the level of these easily available nutrients to plants was the same as the amount of unavailable or little available nutrients in the treatment fertilised with digestate. We wanted to find out from the yield of the grown crop what amount of mineral nutrients would be released from the digestate in comparison with completely available nutrients in the first year and in subsequent years of the crop rotation: early potatoes — winter barley — red clover — oats. We intended to compare the digestate with other organic fertilisers, e. g. farmyard manure which in the first year mineralises about a half of its nutrients bound in organic matter. But the result we obtained was surprising: in the first year the yield of early potatoes was higher by 12% in the digestate treatment although nobody could doubt that this treatment had a lower amount of nutrients than the variant fertilised with pure salts. The only explanation is that the higher yield effect in the digestate treatment was not caused by the higher input of nutrients but by the improvement in physical properties of heavy-textured soil that surely occurred as seen in Tab. 7. The favourable effect of the heavy-textured soil improvement on yield was positively reflected in subsequent years also in other crops of the crop rotation that were fertilised in both treatments in the same way, i. e. mineral fertilisers were applied. We drew a conclusion that in practice the yield effect is often ascribed to digestate nutrients although it is caused by better soil aeration and better root growth due to soil loosening after the application of digestate.