4 September 1999

Doctors cure for nutrient lock-up

Despite stepping up soil fertility, one Lincolnshire grower is still having problems with nutrient uptake. Tom Allen-Stevens finds out what the solution could be.

A SOIL analysis can often be the best place to start when finding out what nutrients are lacking. But this only tells half the story. "We have to know what the plant can actually take up, despite what the soil test indicates, and the problems with the sheep tell us that something is wrong. Plant tissue analysis will help to identify soil nutritional problems which will also affect the arable crops," maintains Mr Fuller.

When Bob Watt arrived as manager at Hungerton Farms, they were producing only 2.5t/ha of barley from the limestone brash soils. He introduced the sheep to build soil structure, organic matter and fertility as depth of soil and lack of moisture are major limits. Nowadays yields are more like 6.25t/ha (malting barley only is grown on this soil, broken with grass, linseed and sugar beet).

"Bob has done an incredible job to build fertility and productivity. Compared to similar soils, the situation here is excellent and the need for fertiliser inputs should be low. It would appear that nutrients are not making it from the soil into the plant," remarks Mr Fuller.

As it turns out, although the soil tests showed good available and reserve levels for most nutrients and micronutrients, the plant tissue analysis tells a very different story: the crop is low on nearly all nutrients, including potash and phosphate in some cases. "The reason is fairly straightforward: because of the brash content of the soil, most of the nutrients are locked up by the calcium, which is dominating nutrient uptake," explains Mr Fuller.

The solution is not quite so simple, however. "Theres no way we can reduce the calcium, so we have to adopt a fertiliser programme that works around it." Phosphate is a nutrient that is very attracted to calcium, and so easily locked up. Mr Fuller advises that triple super phosphate (TSP) should be avoided at all costs in a calcium-rich soil. "Phosphate is needed from the word go for root development and establishment. In this soil, TSP would lock up within hours, so it would have to be placed right on the end of the root. Applying mon-ammonium phosphate at or just after drilling would be far more effective in meeting crop demands."

Potassium, on the other hand, is not needed until spring. The crop needs it for stem extension and to make carbohydrates. "Potash is very mobile in a soil with a high pH and low clay content, so will be washed out of these soils if applied in autumn. It would be better to apply a low rate of potassium sulphate, ideally with first nitrogen at GS30. However, soil levels here are generally good due to the high organic matter, so uptake should improve as soil nutrient balance improves."

The high calcium level is also causing problems with magnesium, copper and manganese availability, which will limit plant growth, photosynthesis, disease resistance and nitrogen metabolism.


Sulphur also has a crucial role to play by improving nitrogen uptake and protein building. Straight sulphur can work well, but applying many of the major nutrients in their sulphate form does two jobs in one. A sulphur deficiency is often only noticed at harvest, when grain shows distinct yellowing.

Mr Fuller also advises against relying on nitrate fertiliser, advocating the use of some ammonium sulphate instead. As the plant root takes up nitrates it puts bicarbonates (carbon molecules developed during photosynthesis) out into the soil. This makes the root zone more alkaline, reducing uptake of most micro-nutrients, except for molybdenum. This in turn makes the copper deficiency and high soil pH worse. After being taken up by the root, nitrates are pumped into the leaf cells, but will only be converted by the plant if micro-nutrients are available to drive the process.

The ammonia form of nitrogen is far less mobile as it can be held on clay particles and organic matter. Using ammonia also leads to a more efficient use of the leaf because the plant feeds on this form of nitrogen as and when it is needed. This can have an impact on the spread of foliar pathogens like rust and mildew. Ammonia is then quickly turned into amines, which are the building blocks for amino acids and proteins.

Ammonia should not be relied on alone, however. "There has to be a balancing act between ammonia and nitrate fertiliser. Amounts of each depend on soil type, crop growth, etc. In order to make it easy to manage and get the balance right, its important to know what is going on in the soil," Mr Fuller points out.

"Following this sort of regime should mean that the crop will be fed more from soil reserves, rather than out of a bag. Its not a question of applying more fertiliser, but of meeting crop demands, and selecting the right type of fertiliser can be crucial. For this you need accurate analysis, independent advice and sound management. Then its just a question of knowing what materials you need and what they can do for you," concludes Mr Fuller.


Lincolnshire farm manager Bob Watt has spent 33 years building up the fertility of the "light, miserable soils" he inherited. Nevertheless he is certain he now has severe micronutrient uptake problems. The symptoms are not apparent in his crops, but the lambing average in his sheep has gone down, despite injecting them with huge amounts of copper. Other alarming discoveries include high levels of molybdenum in his forage and increasing soil pH. If his livestock is suffering so much, how much potential is being held back from his crops?

Perhaps our troubleshooter can help. Independent agronomist and soil specialist Neil Fuller has helped many farmers who have experienced problems with nutrient lock-up, both in the UK and abroad. Often a few changes in soil management or fertiliser programmes are all that are needed to get things on to an even keel. So what will the soil doctor prescribe for a healthier crop in this situation?